tag:blogger.com,1999:blog-40708552017-06-26T14:03:12.222-07:00Gauge Invarianceمحمدnoreply@blogger.comBlogger82125tag:blogger.com,1999:blog-4070855.post-11236541066294542252017-05-20T08:17:00.003-07:002017-05-20T08:21:30.197-07:00Entropy production in a photovoltaic cell<div class="separator" style="clear: both; text-align: center;"><a href="https://4.bp.blogspot.com/-advoRMZ6vy8/WSBeA2RSI1I/AAAAAAAAqvE/M4vID8oZaHE4M0eCjmDD2eTG_P-SNtWdwCLcB/s1600/Screen%2BShot%2B2017-05-19%2Bat%2B10.44.28.png" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"><img border="0" height="151" src="https://4.bp.blogspot.com/-advoRMZ6vy8/WSBeA2RSI1I/AAAAAAAAqvE/M4vID8oZaHE4M0eCjmDD2eTG_P-SNtWdwCLcB/s200/Screen%2BShot%2B2017-05-19%2Bat%2B10.44.28.png" width="200" /></a></div>In the everyday world, the amount of disorder, or entropy, in an isolated system can only increase over time. This relation is described by the second law of thermodynamics. This indicates that not all of the energy we provide for a system can be converted into engineered work.<br /><br />In quantum physics, however, the existence of such a law is obscure. One of the main reasons is that entanglement can dramatically alter the notions of disorder and equilibrium state. Instead of an analogue law, recently a correspondence has been found between entropy production and the statistics of energy transfers in a quantum system.<br /><br />In my paper published recently at Physical Rev. B I studied how entropy is produced in a quantum heat engine. This helps us to explain how heat is dissipated in the engine. The engine is modelled by four electronic levels resonantly-coupled to thermal heat baths kept at different temperatures. Results show that quantum coherence that is induced by environment can significantly and nonlinearly modify entropy production in the cells. Consequently, the nonlinear entropy production can take place much slower or faster.<br /><br />This determines under what conditions information in these cells can take a reversal flow from a cold to hot bath.<br /><br />Ref: Mohammad H. Ansari, Phys. Rev. B 95, 174302 (2017)<br />It is available online at <a href="https://doi.org/10.1103/PhysRevB.95.174302">https://doi.org/10.1103/PhysRevB.95.174302</a><br /><div><br /></div><blockquote class="tr_bq"><b>Abstract: </b><blockquote class="tr_bq"> I evaluate entropy production in a photovoltaic cell that is modeled by four electronic levels resonantly coupled to thermally populated field modes at different temperatures. We use a formalism recently proposed, the so-called multiple parallel worlds, to consistently address the nonlinearity of entropy in terms of density matrix. Our result shows that entropy production is the difference between two flows: a semiclassical flow that linearly depends on occupational probabilities, and another flow that depends nonlinearly on quantum coherence and has no semiclassical analog. We show that entropy production in the cells depends on environmentally induced decoherence time and energy detuning. We characterize regimes where reversal flow of information takes place from a cold to hot bath. Interestingly, we identify a lower bound on entropy production, which sets limitations on the statistics of dissipated heat in the cells.</blockquote></blockquote>محمدnoreply@blogger.com0tag:blogger.com,1999:blog-4070855.post-54955970117763751752017-04-15T21:13:00.002-07:002017-04-15T21:21:58.312-07:00Another PhD position is availableJob type: PhD<br />Application deadline: Monday, June 5, 2017<br />Employer: Peter Gruenberg Institute (PGI-2)<br /><br /><a href="https://www.quantiki.org/position/phd-position-quantum-computing-and-physical-devices-germany" target="_blank">Another PhD position</a> is available to work at Peter Gruenberg Institute (PGI-2) and Juelich-Aachen Research Alliance Institute (JARA) in Forschungszentrum Juelich in Germany. The degree will be granted by RWTH University in Aachen. The student will work with Dr. Mohammad H. Ansari and the project can be started in the Summer or Fall 2017.<br /><br />The purpose of the project is to develop relations between quantum computing models and physical devices. The project requires that you have basic knowledge of quantum physics and information, e.g. density matrix, decoherence, Bloch equation, correlations, nonequilibrium statistics, quantum information measures, Keldysh techniques, etc. We will collaborate with some theoretical and experimental research groups, such as the research group of Prof. D. DiVincenzo at PGI-2 and Prof. Y. Nazarov in Delft University of Technology.<br /><br />For full consideration, please apply **as soon as possible** by sending your documents in *ONE pdf file* to "m.ansari AT fz-juelich.de", including:<br /><br />1. your academic CV,<br />2. your academic transcripts and the list of publications,<br />3. a short essay relating your knowledge to recent papers written by Dr. Ansari (no more than 300 words ~ two paragraphs)<br />4. the names, affiliation, and email addresses of 2 or 3 referees, (make sure they are willing to send letters on time)<br /><br />Please make sure to choose the following subject for your email: "Ph.D. position at PGI-2"<br /><br />More updates about the position can be found at: https://sites.google.com/site/mansariمحمدnoreply@blogger.com0tag:blogger.com,1999:blog-4070855.post-72658155763587803152017-03-27T15:10:00.002-07:002017-03-31T04:34:12.522-07:00My talks on Entropy-Noise Correspondence<div class="separator" style="clear: both; text-align: center;"><a href="https://2.bp.blogspot.com/-0fPWUtCCtaE/WNmIlDKP8mI/AAAAAAAAm38/YkggVH4Sh0QR4k0N67Q5f922Tf5b3mY_ACLcB/s1600/huckleberry_finn_015_jpg_fedu.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="218" src="https://2.bp.blogspot.com/-0fPWUtCCtaE/WNmIlDKP8mI/AAAAAAAAm38/YkggVH4Sh0QR4k0N67Q5f922Tf5b3mY_ACLcB/s320/huckleberry_finn_015_jpg_fedu.jpg" width="320" /></a></div>I am back from a loooong long trip into several cities in the US and Canada, where I gave a series of talks about my research. <br><br> In my trip to New Orleans, for the first time I saw real palm trees! The city has its own character within its cheesy streets full of excited visitors and free storytelling stations! I was also excited to see Mississippi river, which reminded me one of the cartoons I was watching when I was almost ten: the Adventures of Huckleberry Finn! At APS March meeting I enjoyed meeting some old friends from Japan, the US, Canada, Germany, Holland, Australia, etc, gave a short talk about "information contents of physical interactions" and enjoyed learning new things. <br><br> Later I traveled to Canada to give a series of talk in different places and to visit a research group for future collaborations. My schedule was very rich! I started with the University of Toronto, then went to Waterloo to visit and give a talk at the Institute for Quantum Computation (<a href="http://iqc.ca">IQC</a>) on how to measure entropy in quantum systems. <a href="https://uwaterloo.ca/institute-for-quantum-computing/events/seminar-mohammad-ansari">+</a> <br><br> As planned I also visited the Perimeter Institute in Waterloo too, where I got my PhD degree 9 years ago from within its old building, the post office building. The "new" building has changed a lot and now has a wired wing full of offices to somehow compensate its original land-wasting floor map. There were too many new faces there and almost nobody recognised me except a few of present faculty members. There I gave a talk in the quantum foundations seminars about the new entropy-noise correspondence we found last year <a href="http://www.perimeterinstitute.ca/seminar/entropy-measurement-quantum-systems">+</a> <br><br>These talks turned out to fall into the interest of two main communities: condensed matter theorists and quantum information theorists. <br><br> To provide equal opportunities to those not present in the cities yet like to form and express a judgement, I uploaded <a href="https://www.dropbox.com/s/8zelb8kz2vkuwqp/MAnsariPublic1.pdf?dl=0">a short summary of my presentations here to be accessible to public</a>. Please feel free to review. <br><br> M.H. Ansari محمدnoreply@blogger.com0tag:blogger.com,1999:blog-4070855.post-55590746484996063952017-03-05T23:26:00.002-08:002017-03-05T23:26:28.084-08:00Talks<br />If you happen to be near Aachen this week, come to hear news about entropy. (<a href="http://www.quantuminfo.physik.rwth-aachen.de/go/id/fwck/lidx/1" target="_blank">this link</a>) <div><br /></div><div><div><div>***********************************************************<br />Quantum information seminar, Mar 09 11:00, Physikzentrum MBP2 116<br />***********************************************************</div><div>SPEAKER: Mohammad Ansari (FZ Jülich)<br />TITLE: Entropy measurement in quantum systems<br /><br />ABSTRACT: Entropy is an important measure of information. Being nonlinear in density matrix, its consistent evaluation for a quantum system requires a formalism that allows simultaneous evolution of more-than-one-copy of density matrix. Recently in [MHA and Y. Nazarov, Phys. Rev. B 91, 104303 (2015)] a formalism for such evolutions has been proposed and [MHA and Y. Nazarov, Phys. Rev. B 91, 174307 (2015)] shows that such entropy correspond to physical quantities. Interestingly this correspondence is not equivalent to the second law of thermodynamics. In this talk I describe how to measure entropy flow in a quantum heat engine.<br /><br />************************************************************</div></div><div><br /></div><div>Also next week I am in New Orleans. If you are attending APS March meeting I am going to give a talk in <a href="http://meetings.aps.org/Meeting/MAR17/Session/L52">Session L52: Statistics of Ensemble Quantum Systems</a> Wednesday at 11:50 AM in Room: 399. </div><div><div></div></div><div><br /></div></div>محمدnoreply@blogger.com0tag:blogger.com,1999:blog-4070855.post-73033660178899532342017-02-01T03:27:00.000-08:002017-02-03T05:12:37.535-08:00A postdoc/PhD student positionA fully-funded <a href="https://quantiki.org/position/postdocphd-student-position-germany" target="_blank">postdoc/PhD student position</a> is available to work at Peter Gruenberg Institute (PGI-2) in Forschungszentrum Juelich, near Aachen in Germany. For students the degree will be granted by RWTH university in Aachen. The researcher will work with Dr. Mohammad H. Ansari. The project can be started in the Summer or Fall 2017.<br /><br />The purpose of the project is to develop the relations between quantum information theory and physics. For this aim we recently developed a new formalism called multiple parallel world technique. Our findings will be used to explore new phenomena in quantum computing, thermodynamics, and photosynthetic complexes in biology. The project requires that you have basic knowledge about quantum physics and information, e.g. density matrix, decoherence, Bloch equation, correlations, nonequilibrium statistics, quantum information measures, Keldysh techniques, etc.<br /><br />We will collaborate with some theoretical and experimental research groups, such as the research group of Prof. D. DiVincenzo in PGI-2 and Prof. Y. Nazarov in Delft University of Technology, the Netherlands, et. al. The employed researcher will have the opportunity to visit our collaborators.<br /><br />Everyone (Masters and PhD students and Postdocs) from all around the world are welcome to apply.<br /><br />For full consideration, please apply as soon as possible, by sending your documents in *ONE pdf file* to "mansari AT gmail DOT com", including:<br /><br />1. your academic CV,<br />2. your list of publications,<br />3. research interests on what makes your past research experience related to quantum thermodynamics, (no more than 300 words ~ two paragraphs)<br />4. the names, affiliation, and email addresses of 2 or 3 referees, (make sure they are willing to send letters on time)<br /><br />More information about the research details as well as announcements (e.g. whether the position is still available or not) can be found at:<br /><a href="https://sites.google.com/site/mansari">https://sites.google.com/site/mansari</a><br /><div><br /></div>محمدnoreply@blogger.com0tag:blogger.com,1999:blog-4070855.post-46292582598394817652016-11-30T06:27:00.002-08:002016-12-05T08:33:50.500-08:00A newly-published course book by Manoukian<div class="separator" style="clear: both; text-align: center;"><a href="https://3.bp.blogspot.com/-vk7NOscHkXs/WD7lUofJikI/AAAAAAAAhKQ/EPsJs_Z-xg4OAZPLzFX-BzfTAGthyS89wCLcB/s1600/41gC90fwbvL._SX330_BO1%252C204%252C203%252C200_.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="320" src="https://3.bp.blogspot.com/-vk7NOscHkXs/WD7lUofJikI/AAAAAAAAhKQ/EPsJs_Z-xg4OAZPLzFX-BzfTAGthyS89wCLcB/s320/41gC90fwbvL._SX330_BO1%252C204%252C203%252C200_.jpg" width="212" /></a></div><div class="separator" style="clear: both; text-align: center;"><br /></div><div>December this year is almost the 10th anniversary of finding new degeneracy in area spectrum and finding heir application. (<a href="https://arxiv.org/abs/gr-qc/0603121" target="_blank">1</a>,<a href="https://arxiv.org/abs/hep-th/0607081" target="_blank">2</a>)<br /><br />Yesterday I was informed in an email from Springer that some of those results about area quantization have found their way into a newly-published graduate textbook. Although this is not the first time that somebody writes about what I have done (see footnotes) but this time feels differently because my physics is now a part of a graduate course book.<br /><div></div><br /></div><div><a href="https://www.amazon.com/Quantum-Field-Theory-Introductions-Supersymmetry/dp/331933851X/ref=sr_1_1?s=books&ie=UTF8&qid=1480516424&sr=1-1" target="_blank">Here</a> is the book: "Quantum Field Theory II: Introductions to Quantum Gravity, Supersymmetry and String Theory (Graduate Texts in Physics.) written by E.B. Manoukian and published by Springer on September 2016).</div><div><br /></div><div>I read some parts of it, yet not all, and to my point of view it is quite decent book in the field, well-written and organized, and clear. It provides interesting insights about recent developments in quantum field theory. More importantly it teaches how modern physicists think about quantum foundations, overall a must-read book for graduate students working on quantum field theory.<br /><br /></div><div>This is pretty exciting to see that the degeneracy of quantum area spectrum and its physical applications ten years after its discovery are now exercises for young students!<br /><br /></div><div>-----<br />Footnotes:<br /><br /></div><div>* D. Oriti, Approaches to Quantum Gravity: Toward a New Understanding of Space, Time and Matter (Cambridge University Press 2009) highlighted my research on black hole and area quantization.<br /> <img src="file:///page2image34552" /> <img src="file:///page2image34712" /> <img src="file:///page2image34872" /> <img src="file:///page2image35032" /> <img src="file:///page2image35192" /> <img src="file:///page2image35352" /> <img src="file:///page2image35512" /> <img src="file:///page2image35672" /> <img src="file:///page2image35832" /> </div><div>* M. Aschwanden, Self-Organized Criticality in Astrophysics: The Statistics of Nonlinear Processes in the Universe (Springer London 2011) highlighted my research on cellular automata, self-organized criticality and nonequilibrium phenomena.<br /><div><br /></div><div>* A section 2.46.3 of the book: The Cosmic Compendium: Black Holes by R. W. Anderson has devoted only to explain my results on black hole.<br /><br /></div></div><div>* The article by L. Smolin, in Physics Today 59 Nov 44 (2006) highlighted some of my results on black holes <a href="http://dx.doi.org/10.1063/1.2435646">http://dx.doi.org/10.1063/1.2435646</a> , etc.</div><div><br /></div><div><br /></div>محمدnoreply@blogger.com0tag:blogger.com,1999:blog-4070855.post-16561268265113082082016-05-17T13:11:00.001-07:002016-06-09T02:00:42.218-07:00News about entropy<div class="separator" style="clear: both; text-align: center;"><a href="https://3.bp.blogspot.com/-FBIwPX-ZwNg/V1ktmguRTiI/AAAAAAAAYqI/jWUGr5tordwxmLMt9iTDgDRtaYm0iUmXACLcB/s1600/Shannon_Entropy.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://3.bp.blogspot.com/-FBIwPX-ZwNg/V1ktmguRTiI/AAAAAAAAYqI/jWUGr5tordwxmLMt9iTDgDRtaYm0iUmXACLcB/s1600/Shannon_Entropy.jpg" /></a></div><br /><br />Did you know that you have been calculating entropy incorrectly during your whole life! This is at least what I claim to be true in a recently published paper titled: "A consistent flow of entropy."<br /><div><br /><div>A common approach to evaluate entropy in quantum systems is to solve a master-Bloch equation to determine density matrix and substitute it in entropy definition. However, this method has been recently understood to lack many energy correlators. The new correlators make entropy evaluation to be different from the substitution method described above. The reason for such complexity lies in the nonlinearity of entropy. In this paper we present a pedagogical approach to evaluate the new correlators and explain their contribution in the analysis. We show that the inherent nonlinearity in entropy makes the second law of thermodynamics to carry new terms associated to the new correlators. Our results show important new remarks on quantum black holes. Our formalism reveals that the notion of degeneracy of states at the event horizon makes an indispensable deviation from black hole entropy in the leading order.<br /><br />This is a my contribution to special issue in Fortschritte der Physik for the Frontiers of Quantum and Mesoscopic Thermodynamics Conference. Link: <a href="http://arxiv.org/abs/1605.04620">arXiv:1605.04620</a><br /><br />By: Mohammad H. Ansari</div></div>محمدnoreply@blogger.com0tag:blogger.com,1999:blog-4070855.post-51719547922438654142016-02-11T14:16:00.000-08:002016-02-17T15:23:55.226-08:00Hearing a sound from a billion years ago!<div class="separator" style="clear: both; text-align: center;"><a href="https://3.bp.blogspot.com/-jxoFsc4ewA8/Vr0HyPvHDdI/AAAAAAAAT3Q/XaGJopfhLSs/s1600/2-black-holes-gravitational-waves.jpg" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"><img border="0" height="200" src="https://3.bp.blogspot.com/-jxoFsc4ewA8/Vr0HyPvHDdI/AAAAAAAAT3Q/XaGJopfhLSs/s200/2-black-holes-gravitational-waves.jpg" width="200" /></a></div><div><br /></div>Today is a day for cheering. After many years gravitational waves have been finally reported to have been detected. In two separate detectors, in west and east coasts of the US, almost simultaneously it was detected that earth got shrunk a tiny bit from a gravitational curving pulse. Many researchers have been sweating for decades to make these detectors happen.<br /><div><br /></div><div>Let me explain what has been discovered. An early approval for the theory of general relativity in 60's was based on the following fact: the presence of a heavy object, like our sun, will causes its surrounding spacetime to be curved. This has been observed from the following experiment: the straight path on which light travels in the universe from A to B, if passes adjacent to a heavy object, will bend a bit toward the object, thus the path will not remain a straight path anymore.<br /><br />Now, consider two heavy objects crazily rotate around one another with a crazy speed (a half of speed of light). Right before they merge they make a lot of ups and downs in the spacetime in between. This turbulence in the very spacetime fibres propagate to others places, similar to how tsunami waves propagate in the ocean. In other words, gravitational field is self-interacting; this means that it sources itself. It curves the space around itself because it carries mass/energy by itself much like a star does.<br /><br />If the objects are many light years away, the waves passing through the Earth today makes a bit of stretches and squeezes in the radius of earth, something of the size of less than the radius of proton! This has been detected last year at LIGO detectors and today was reported, and peer reviewed already in the <a href="https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.061102" target="_blank">PRL</a>.<br /><br />From what they received we assume that the waves came from two 30-solar-mass black holes curling up around one another in a death spiral, a billion light-years away from us. This is a better-than-expected source for LIGO and sometimes the universe is nice to us! In fact a billion years ago two heavy stars fell in love with one another and played hand in hand like kids, and today we got a bit of <a href="https://www.youtube.com/watch?v=TWqhUANNFXw" target="_blank">their happy sound</a>! Doing such an amazing science is similar walking a tightrope between surly curmudgeon and starry-eyed cheerleader. </div><div> </div>The last thing to notice: If you are sad you do not get cited more often, apparently Einstein’s prediction of gravitational waves has been cited <a href="http://adsabs.harvard.edu/abs/1916SPAW.......688E" target="_blank">only a few times</a>! <br /><div><br /></div><div>By: Mohammad H. Ansari</div>محمدnoreply@blogger.com0tag:blogger.com,1999:blog-4070855.post-86096170472049237092016-01-25T08:30:00.003-08:002016-06-09T01:52:48.970-07:00APS March Meeting 2016<div class="separator" style="clear: both; text-align: center;"><a href="http://4.bp.blogspot.com/-QjgewuEc_Vw/VqZORuOSk3I/AAAAAAAATIY/9XJkFIkqAcA/s1600/Screen%2BShot%2B2016-01-25%2Bat%2B17.31.53.png" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"><img border="0" height="139" src="https://4.bp.blogspot.com/-QjgewuEc_Vw/VqZORuOSk3I/AAAAAAAATIY/9XJkFIkqAcA/s200/Screen%2BShot%2B2016-01-25%2Bat%2B17.31.53.png" width="200" /></a></div><br />I'll be chairing a session in the upcoming American Physics Society March Meeting 2016 in Baltimore, USA. The session is focused on Quantum Information and Thermodynamics.<br /><br />Below you can find the session information. I invited some notable young scientists whose contribution to quantum physics have been impressive in the last year.<br /><br />Looking forward to run an impressive focus session!<br /><br /> <a href="http://meetings.aps.org/Meeting/MAR16/Session/R44">http://meetings.aps.org/Meeting/MAR16/Session/R44</a><br /><br /> Mohammad H Ansariمحمدnoreply@blogger.com0tag:blogger.com,1999:blog-4070855.post-89412449745914003442015-12-24T02:53:00.003-08:002015-12-25T19:12:24.495-08:00Abdolali Ansari 1931-2015<a href="http://4.bp.blogspot.com/-Wad-4cd8ikE/VnvKoUXlO5I/AAAAAAAAR5o/LxOxWXkD4dc/s1600/IMG_4891.JPG" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"><img border="0" height="240" src="http://4.bp.blogspot.com/-Wad-4cd8ikE/VnvKoUXlO5I/AAAAAAAAR5o/LxOxWXkD4dc/s320/IMG_4891.JPG" width="320"></a><br>My father passed away yesterday 22 Dec 2015 after a rather short period of age-related illness. He was 84. My father, Abdolali, was a college president and teacher in Kermanshah. He spent most of his efforts to boost up the education quality of the college Daneshsaraa-ye Aali of Kermanshah (college for teacher training). Sometimes, only for joy of it, he contributed to teach geometry, mathematics, and literature. He loved to share his knowledge with his pupils.<br><br>Before joining the college he has experienced many other activities such as the business of Giveh, a traditional type of footwear woven by silk and cotton for daily use in western Iran. Nowadays this business is in the hand of my uncles. At age 20, not satisfying from business, he passed an exam and was accepted to enter medical school in Tehran. After a short time he decided to change the field into science educational school for which he and his family of wife and two daughters moved to Tehran. Four years later he received the degree from one of the best universities in tehran and moved back to Kermanshah to help establishing a modern college named Daneshsaraa-ye Aali of Kermanshah, he was president of the college for more than 20 years.<br><br>On weekends of my intermediate school days, one of my best hobbies was to go beyond the geometry of school books. I had a wonderful teacher at home playing with the roses and other flowers of our house yard. I was going to him asking whether he likes to teach me and his response was always with a flame of joy in his eyes. Sometimes we were working more than 5 hours together on different aspects of mathematics, I was sitting next to him working on paper with compass, set square, protractor, drawing lines and curves for proving theorems he was assigning to me. On top of methods, such as how to partition an angle into three with compass, the joy of discovery was what I learned the most from him.<br><br>After retiring from the college, he started a new career at Razi University in Kermanshah shortly before our entire family move to Tehran. In Tehran, he started another careers in finance, working everyday until the age of 75, not that he needed, only because he loved to work. At this age he started to reduce his activities and stayed mostly at home, yet he was still the source of hope for us. In all ups and downs when I was sharing what is in my chest to him, he could sparkle my mind with the burst of a wise word. This was what I needed to be cheered up. <br><br>Abdolali Ansari left behind thousands of students some of whom are nowadays the most influential people in their profession.<br><br>God bless you daddy. I am happy that you are eternally home. God bless you...<br><br> ---------------------------------------------------------------------- <div id="fb-root"></div><script>(function(d, s, id) { var js, fjs = d.getElementsByTagName(s)[0]; if (d.getElementById(id)) return; js = d.createElement(s); js.id = id; js.src = "//connect.facebook.net/en_US/sdk.js#xfbml=1&version=v2.3"; fjs.parentNode.insertBefore(js, fjs);}(document, 'script', 'facebook-jssdk'));</script><div class="fb-post" data-href="https://www.facebook.com/mhhansari/posts/10153905844176995" data-width="500"><div class="fb-xfbml-parse-ignore"><blockquote cite="https://www.facebook.com/mhhansari/posts/10153905844176995">Posted by <a href="#" role="button">Mohammad H. Ansari</a> on <a href="https://www.facebook.com/mhhansari/posts/10153905844176995">Thursday, December 24, 2015</a></blockquote></div></div>محمدnoreply@blogger.com0tag:blogger.com,1999:blog-4070855.post-33621339690447727642015-09-15T02:59:00.000-07:002016-06-09T01:54:25.181-07:00Let's move to physical quantities: Keldysh Green functions!<div><div class="separator" style="clear: both; text-align: center;"><a href="https://4.bp.blogspot.com/-UGCglYLcL4E/V1kurr1aTpI/AAAAAAAAYqU/XcTmremkvYcrCehX09T9HFph43lkixGvgCLcB/s1600/Nazarov_405.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://4.bp.blogspot.com/-UGCglYLcL4E/V1kurr1aTpI/AAAAAAAAYqU/XcTmremkvYcrCehX09T9HFph43lkixGvgCLcB/s1600/Nazarov_405.jpg" /></a></div><br />Yuli Nazarov: [I] remember a talk given by a high-class theorist with a taste for abstract models, young experimentalists being his primary audience. Somewhere in the middle of the talk he said: "Now let us move to physical quantities, namely, Keldysh Green functions". A burst of laugh lasted for more than five minutes!</div><div><br /></div>For many years Keldysh formalism was considered too much complicated for a practical researcher and hardly applied beyond several specific fields. "Keldysh approach" sounded as a synonym of unnecessary theorization and an antonym to clear physical reasoning.<br /><div><br />Excerpts from:</div><div><br /><b>Keldysh formalism for multiple parallel worlds</b></div><div>Abstract: <br />We present here a compact and self-contained review of recently developed Keldysh formalism for multiple parallel worlds. The formalism has been applied to consistent quantum evaluation of the flows of informational quantities, in particular, to evaluation of Renyi and Shannon entropy flows. We start with the formulation of standard and extended Keldysh technique in single world in a form convenient for our presentation. We explain the use of Keldysh contours encompassing multiple parallel worlds In the end, we shortly summarize the concrete results obtained with the method.<br /><br />Mohammad Ansari and Yuli V. Nazarov<br /><a href="http://arxiv.org/abs/1509.04253">arXiv/1509.04253</a></div><div><br />(Submitted on 14 Sep 2015)<br /><br />It is a big honour for us to present these results in a special issue celebrating numerous scientific merits of Leonid Veniaminovich Keldysh. We gladly appreciate his pioneering research that provided a powerful and indispensable tool for many generations of quantum physicists, us including, and wish him many happy returns of the day.</div>محمدnoreply@blogger.com0tag:blogger.com,1999:blog-4070855.post-82034074322709882932015-08-19T22:56:00.001-07:002016-03-01T11:28:49.325-08:00Early look at CQI sessions in APS March meeting 2016I'll be organizing a focus session for CQI on Quantum Information and Thermodynamics in the APS March meeting 2016 in Baltimore. The submission of abstract will becomes available in September. Experimentalists and theorists please consider to submit your abstracts to the session, or introduce it to your colleagues. For any question please feel free to contact me.<br /><br /><b>Focus Topics (see descriptions below) </b><br /><div>Towards Scalable Quantum Computers <br />Hybrid Quantum Systems <br />Adiabatic Quantum Computation and Quantum Annealing <br />Finite-size Quantum Information Theory <br />Quantum Characterization, Validation, and Verification <br />Quantum Information and Thermodynamics <br />Gravity and Quantum Information <br /><br /><b>Regular Sorting Topics </b><br /><div>Superconducting quantum information <br />Semiconducting quantum information <br />Atomic, molecular and optical (AMO) quantum information <br />Topological quantum information <br />Algorithms and architecture for quantum information <br />Quantum information theory and quantum foundations <br /><br /><br /><b>Abstracts:</b></div><div><br /><b>Towards Scalable Quantum Computers</b><br />This focus topic will examine recent advances towards scalable quantum information devices. The topic will include experimental talks on both solid state and AMO qubit technologies with an emphasis on improved gate fidelities and the development of integrated systems. It will also include theoretical talks on improvements in quantum error correction, quantum control, and proposals for scalable architectures.<br /><br /><b>Hybrid Quantum Systems</b><br />This focus topic will examine recent experimental and theoretical developments in hybrid quantum systems that combine quantum system of multiple types. Examples range from quantum dots coupled to microwave cavities to trapped ions coupled to micromechanical resonators.<br /><i>Organizer: Guido Burkard, University of Konstanz </i><br /><b><br /></b><b>Adiabatic Quantum Computation and Quantum Annealing</b><br />Adiabatic models of quantum computation and quantum annealing perform computational tasks by evolving the system under a slowly changing Hamiltonian. This topic will focus on the theory and applications of adiabatic quantum computers and quantum annealers, and challenges for error suppression and correction on these devices.<br /><i>Organizer: Daniel Lidar, University of Southern California</i><br /><b><br /></b><b>Finite-size Quantum Information Theory </b><br />A growing topic of interest in quantum information theory is to understand what the capabilities are for a finite number of quantum systems. Traditionally, the focus has been on asymptotics and there has been a disconnect between the theory and what is possible in practice. In the past three years, the theoretical tools have sharpened significantly and we can answer questions such as "How many qubits can I send with 100 channel uses if I desire an error probability no larger than 10-6 ?" Answers to such questions place fundamental limitations on small quantum computers and are the focus of this session.<br /><i>Organizer: Mark Wilde, Louisiana State University</i><br /><b><br /></b><b>Quantum Characterization, Validation, and Verification</b><br />As reported errors in quantum gates approach fault-tolerance thresholds, it becomes more important to confirm the methods by which errors are assessed and gate functions are determined. This topic will include recent advances in tomography and benchmarking methods, tests for detecting coherent errors, and appropriate error bounds for quantum error correction.<br /><i>Organizer: Charles Tahan, Laboratory for Physical Sciences, University of Maryland</i><br /><b><br /></b><b>Quantum Information and Thermodynamics</b><br />It is increasingly apparent that quantum entanglement offers a powerful tool to describe physics. This is necessary to develop realistic proposals for measuring entanglement as well as other quantum information quantities from physical quantities. In the past decade, owing to the control of small-scale devices such as quantum heat engines and electronic circuits, thermodynamics has become part of the bedrock to understand how to measure information quantities in the physical world. Establishing thermodynamics in quantum scales requires a quantum description for exchange of physical quantities such as energy, charge, spin, etc. This requires generalization of information correlations that sometimes goes beyond standard definitions for entanglement. These correlations in condensed matter and information theory have been realized and are the driving force behind recent developments.<br /><i>Organizer: Mohammad Ansari, TU Delft</i><br /><b><br /></b><b>Gravity and Quantum Information</b><br />Quantum information is providing a fresh look at the gravity-quantum interface. Experiments range from high-precision measurements of the gravitational field using quantum systems all the way to actual large quantum superposition states of clocks or increasingly massive objects, where experiments may be in reach in the near future. In addition, the relevance of quantum information concepts for studying fundamental properties of space-time.<br /><br /></div></div>محمدnoreply@blogger.com0tag:blogger.com,1999:blog-4070855.post-29980737125172008332015-08-17T16:14:00.000-07:002016-03-02T10:10:39.136-08:00My memory of Jacob Bekenstein<div><br /></div>Jacob Bekenstein passed away at age 68. I know Jacob mostly from reading his papers on gravity and information theory, but I also have had an interaction with him about physics.<br /><div><br /></div><div>He was a brilliant scientist who will be missed in the community of quantum gravity. He had great contribution to the thermodynamics of black holes and its entropy, known as the Bekenstein entropy. </div><div>The original thought he had relies on an assumption that the information may be organized into binary digits on the black hole horizon where information exchange with black hole interior region (in his view) stops. This idea although is very heuristic, but motivated further investigates, without which it would take long time before thermodynamics of black holes is found. </div><div><br /></div><div><div class="p1"><span class="s1">Since then there have been many attempts to calculate the black hole entropy. For example, in one of the most recent attempts Ashtekar, et.al showed that from writing the action of spacetime dynamic in terms of gauge fields, the associated action to a black hole horizon becomes the Chern-Simons action. Quantization will end up in some punctures on the horizon, which carry degenerate states. Other arguments for similar derivation has been made much earlier in string theory by Vafa et. al. </span></div><div class="p1"><br /></div></div><div>Now, let me go back to the story between me and Jacobe Bekenstein. In 2007 I was a third year PhD student at the Perimeter Institute in Canada working on black holes. I have just published two solo-author long papers in Nuclear Physics B (<a href="https://scholar.google.ca/citations?view_op=view_citation&hl=en&user=Wlg2a9AAAAAJ&citation_for_view=Wlg2a9AAAAAJ:u5HHmVD_uO8C" target="_blank">this</a> and <a href="https://scholar.google.ca/citations?view_op=view_citation&hl=en&user=Wlg2a9AAAAAJ&citation_for_view=Wlg2a9AAAAAJ:u-x6o8ySG0sC" target="_blank">this</a>) where in I discussed the full spectrum of area operator in loop quantum gravity carries an internal degeneracy. The longer paper was first discussing an explicit mathematical proof to show the full spectrum of area no matter in what gauge representation is made of infinite number of equidistant subsets. Moreover, Jacobe Bekenstein and Viatcheslav Mukhanov (BM) have earlier argued based on the heuristic area quantization that black holes must radiate an evenly spaced spectrum of photons on top of Hawking radiation. The explicit quantization of area modified this result. Although it supports that a black hole radiates photons of discrete energies, however it predicts that, despite what BM predicts, the energy spectrum is not evenly spaced and that if one can detect at least three of the frequency lines it determines the internal gauge symmetry of the universe. </div><div><br /></div><div>For these works I received the John Brodie prize and was nominated for a fellowship at Harvard which was almost hopeless to get in the presence of a nasty war in the blog sphere flaming between string theorists and those who take different approach, similar to that of Bekenstein's. Students were the first victims of this unnecessary hurlyburly. </div><div><br /></div>Anyways, let me go back to the story. On a cold Suday morning of Canada in Nov 2007 when I woke up into the warm radiation of sun spreading in front of my little window in Waterloo, I checked my emails and to my absolute surprise I saw an email from "Jacobe Bekenstein" among my messages, which first I thought it must be fake. When I opened it up I saw it is from himself cc-ed to Viatcheslav Mukhanov. I read it and was shocked in the honour. Jacobe has already read my papers and prepared some comments on it. He started his rather long letter with:<br /><div><br /></div>"Dear Dr. Ansari,<br /><br /><div>Your paper on loop quantum gravity's predictions for the black hole area spectrum is interesting...<br />Although you find another series of levels that are not equidistant, you correctly argue that the<br />transitions between equidistant ones will prevail in the radiated spectrum. From an "observational" point of view, the equidistant levels are all that matters.<br />..."<br /><div><br /></div><div>He continued his letter with 4 important comments that helped to improve my papers and the history of proper references. I was a bit in shock to see that one of my heros in quantum information and gravity initiated writing such a descriptive letter to a random PhD student with such an exquisite accuracy of equation numbers and pages. </div><div><br /></div><div>In a few days I responded to him, cc-ed to Mukhanov: </div><div><br /></div>"Dear Professor Bekenstein,<br /><br />It is a pleasure that I read your valuable comments. Thank<br />you for your message. I was not aware of the history of this line of<br />research to this depth. Previously, I enjoyed a discussion with Prof. Mukhanov<br />on this topic in a talk I gave a while ago at the Perimeter<br />Institute. I will read and study the references you mentiond and also<br />in the next version of the paper I will give the information to<br />readers and put enough stress on the origins of the methods.<br />..."<br /><div><div><br /></div><div>Then I explained some technical details about the work and explaining how different is the spectrum of loop quantum gravity from that of his. At the end, I asked him the following question: </div><div><br /></div>"I am very interested to see the possibility of observing these lines (evenly-spaced, or unevenly-spaced ones). Do you have any estimate about the possibility of their detections. Some recent work suggest that they should potentially been detectable in INTEGRAL observations etc." <br /><br /><div>In response he wrote:</div><br />"... Regarding the observability of the lines (in whatever scheme, yours, or ours) the problem will<br />always be astrophysical radiation background. Usually lines of interest can be picked up if one knows a priori their frequencies. Here one does not because the mass of the hole could be anything."<br /><br /><div><div>Although I did not see Jacobe and do not know him in person, but these short communication helped me to see that one can be a great mind and at the same time not reluctant to learn from a nobody junior scientist. </div><div><br /></div><div>Mohammad H. Ansari<br /><br /></div></div></div></div>محمدnoreply@blogger.com0tag:blogger.com,1999:blog-4070855.post-6050574069927026772015-07-15T00:41:00.003-07:002015-07-15T01:15:56.052-07:00Stimulated quantum phase slips<div>Stimulated quantum phase slips from weak electromagnetic radiations in superconducting nanowires,<br /><a href="http://arxiv.org/abs/1507.02725">arxiv.org:1507.02725</a><br />Amir Jafari-Salim, Amin Eftekharian, A. Hamed Majedi, Mohammad H. Ansari<br /><br /><div><br /></div>This paper is on radiation-assisted quantum phase slip. We study the rate of quantum phase slips in an ultranarrow superconducting nanowire exposed to weak electromagnetic radiations. The superconductor is in the dirty limit close to the superconducting-insulating transition, where fluxoids move in strong dissipation.<br /><br />We use a semiclassical approach and show that external weak radiation exposed to an ultranarrow superconducting nanowire at low temperature stimulates a significant enhancement in the probability of quantum phase slips in the wire. This can help to outline a new type of detector for microwave to submillimetre radiations based on stimulated quantum phase slip phenomenon.</div><br /><br />محمدnoreply@blogger.com0tag:blogger.com,1999:blog-4070855.post-80724928744399265052015-06-03T02:25:00.001-07:002015-06-03T03:41:06.608-07:00Fluctuations induced by quasiparticlesIn this paper, we present experiments in which we probe the dynamics of a two-state fluctuator (TSF) coupled to a superconducting flux qubit. Our results provide new insight into the decoherence of flux-type superconducting qubits.<br /><div><br /></div><div>M. Bal, M. H. Ansari, J.-L. Orgiazzi, R. M. Lutchyn, and A. Lupascu<br />Phys. Rev. B 91, 195434 – Published 22 May 2015<br /><a href="http://dx.doi.org/10.1103/PhysRevB.91.195434">http://dx.doi.org/10.1103/PhysRevB.91.195434</a></div><div><br /><div style="text-align: center;"> <a href="http://2.bp.blogspot.com/-nHde_ChmGVY/VW7GpfapJlI/AAAAAAAAMDk/V_B1ziWba8w/s1600/medium%2B%25281%2529.png" style="text-align: center;"><img border="0" src="http://2.bp.blogspot.com/-nHde_ChmGVY/VW7GpfapJlI/AAAAAAAAMDk/V_B1ziWba8w/s320/medium%2B%25281%2529.png" /></a></div><br />TSFs are a generic type of noise, observed in many mesoscopic systems, with examples including charge, flux, and critical current fluctuators.<br /><br /> In most of these experiments, TSFs are characterized using classical detectors, such as single-electron transistors or SQUIDs. <br /><br />In this paper, we present a method to determine the time scales of a TSF which relies on conditional excitation and measurement of a qubit. Based on the parametric change of the qubit frequency and the measurement of the TSF time scales, we conclude that the TSF origin is tunneling of quasiparticles through the Josephson junctions forming the qubit. <br /><br /><div style="text-align: center;"><a href="http://2.bp.blogspot.com/-0z293qmYFKI/VW7GzsGTHuI/AAAAAAAAMDs/WggqMw3jN18/s1600/medium%2B%25282%2529.png"><img border="0" src="http://2.bp.blogspot.com/-0z293qmYFKI/VW7GzsGTHuI/AAAAAAAAMDs/WggqMw3jN18/s320/medium%2B%25282%2529.png" /></a></div><br />We present experiments on the dynamics of a two-state parametric fluctuator in a superconducting flux qubit. In spectroscopic measurements, the fluctuator manifests itself as a doublet line. <br /><br />When the qubit is excited in resonance with one of the two doublet lines, the correlation of readout results exhibits an exponential time decay which provides a measure of the fluctuator transition rate. The rate increases with temperature in the interval 40 to 158 mK. <br /><br />Based on the magnitude of the transition rate and the doublet line splitting, we conclude that the fluctuation is induced by quasiparticle tunneling. These results demonstrate the importance of considering quasiparticles as a source of decoherence in flux qubits.</div><div><br /></div><div><br /><div><div><br /></div></div></div>محمدnoreply@blogger.com0tag:blogger.com,1999:blog-4070855.post-66234934147059794322015-06-01T04:32:00.001-07:002015-06-01T04:33:00.303-07:00RF/FCS correspondence publishedOur paper on "Exact correspondence between Renyi entropy flows and physical flows" has been published in<br /><div><a href="http://dx.doi.org/10.1103/PhysRevB.91.174307">http://dx.doi.org/10.1103/PhysRevB.91.174307</a><br /><div><br /><div class="separator" style="clear: both; text-align: center;"><a href="http://1.bp.blogspot.com/-2myLCGYKa54/VWrwBIcp4HI/AAAAAAAAMDU/iBlEqFwyx2Q/s1600/medium.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="160" src="http://1.bp.blogspot.com/-2myLCGYKa54/VWrwBIcp4HI/AAAAAAAAMDU/iBlEqFwyx2Q/s320/medium.png" width="320" /></a></div><div><br /><br />What is RF/FCS correspondence?<br /><div><br />A new correspondence, similar to the fluctuation-dissipation theorem in spirit, that provides an exact relation between the flows of quantum entropy and full counting statistics of energy transfers.</div><div><br /></div><div>Also read: <a href="http://gaugeinvariance.blogspot.nl/2015/05/a-novel-correspondence-between-entropy.html">A novel correspondence between entropy and statistical physics</a>.</div><div><br /><div><span style="background-color: white; color: #141823; font-family: helvetica, arial, 'lucida grande', sans-serif; font-size: 14px; line-height: 19.3199996948242px;"><br /></span></div></div></div></div></div>محمدnoreply@blogger.com0tag:blogger.com,1999:blog-4070855.post-28593010196279098982015-05-21T02:57:00.001-07:002015-05-21T02:57:22.876-07:00A novel correspondence between entropy and statistical physics<div class="p1">Exact correspondences between seemingly different concepts play important role in all fields of physics. An example is the <a href="http://en.wikipedia.org/wiki/Fluctuation-dissipation_theorem">fluctuation-dissipation theorem</a>, which states that the linear response of a system to externally applied forces corresponds to the system fluctuations. </div><div class="p1"><br /></div><div class="p1">In the last decade, the fluctuation-dissipation theorem has initiated important developments in quantum transport, quantum computation, and other similar phenomenological theories. This theorem can be extended to nonlinear responses and to full counting statistics, giving more extended sets of similar relations. </div><div class="p2"><span class="s1"></span></div><div class="p2"><span class="s1"></span><br /></div><br /><div class="p1"><span class="s1">The Shannon entropy in quantum physics is considered unphysical, or non-observable, due to its nonlinear dependence on density matrix. So is the generalized Shannon entropy, called Renyi entropies --named after the French Mathematician and Physicist <a href="http://en.wikipedia.org/wiki/Alfr%C3%A9d_R%C3%A9nyi">Alfred Renyi</a>. </span></div><div class="p1"><span class="s1"><br /></span></div><div class="p1"><span class="s1">An interesting and non-trivial question is: </span></div><blockquote class="tr_bq"><span class="s1">Is there any relation between the flows of entropy and the physical flows? </span></blockquote><div class="p1"><span class="s1"><br /></span></div><div class="p1"><span class="s1">The answer to the question, is YES! </span></div><div class="p1"><span class="s1"><br /></span></div><div class="p1"><span class="s1">A novel relation is presented in the following research article that is similar to the fluctuation-dissipation theorem in spirit and provides an exact correspondence between the flows of quantum entropy and full counting statistics of energy transfers.</span></div><div class="p1"><span class="s1"><br /></span></div><div class="p1"><span class="s1">By: Mohammad H. Ansari </span></div><div class="p1"><span class="s1"><br /></span></div><div class="p1"><span class="s1"><br /></span></div><div class="p1"><span class="s1"><b>References:</b></span></div><div class="p1"><span class="s1"><br /></span></div><div class="p1"><span class="s1">1- Exact correspondence between Renyi quantum entropy flows and physical flows, </span></div><div class="p1"><span class="s1">M. H. Ansari and Yu. V. Nazarov, </span></div><div class="p1"><span class="s1">To appear in Phys. Rev. B., </span><a href="http://arxiv.org/abs/1502.08020">arXiv:1502.08020</a></div><div class="p2"><span class="s1"></span><br /></div><div class="p1"><span class="s1">2- Renyi entropy flows from quantum heat engines, </span></div><div class="p1"><span class="s1"> </span></div><div class="p1"><span class="s1">M. H. Ansari and Yu. V. Nazarov, Phys. Rev. B 91, 104303 (2015) </span><a href="http://arxiv.org/abs/1408.3910">arxiv:1408.3910</a></div><div class="p1"><br /></div><div class="p1"><br /></div><div class="p1"><span class="s1"><br /></span></div><div class="p1"><span class="s1"><br /></span></div><div class="p1"><span class="s1"><br /></span></div>محمدnoreply@blogger.com0tag:blogger.com,1999:blog-4070855.post-19556877982262101622015-03-16T02:59:00.001-07:002015-03-17T06:28:59.794-07:00We understood how to measure it<div dir="ltr" style="text-align: left;" trbidi="on">A novel correspondence between entropy flow and statistics of energy exchange was reported in <a href="http://arxiv.org/abs/1502.08020">this paper</a>. This is an extension of <a href="http://arxiv.org/abs/0804.1377">earlier result</a> by Levitov and Klich on how to measure entropy in systems with charge transfers.<br /><table cellpadding="0" cellspacing="0" class="tr-caption-container" style="float: right; margin-left: 1em; text-align: right;"><tbody><tr><td style="text-align: center;"><a href="http://4.bp.blogspot.com/-orhF4kjbv_g/VQcJe0-XWbI/AAAAAAAALuc/mAJiLFmRCkI/s1600/duality.gif" imageanchor="1" style="clear: right; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" src="http://4.bp.blogspot.com/-orhF4kjbv_g/VQcJe0-XWbI/AAAAAAAALuc/mAJiLFmRCkI/s1600/duality.gif" height="200" width="200" /></a></td></tr><tr><td class="tr-caption" style="text-align: center;">There seems to be an exact correspondence<br />between Renyi entropy flow<br />and statistics of energy exchange. </td></tr></tbody></table><br />The situation is fairly similar to the fluctuation-dissipation theorem however the new correspondence links between the Renyi (and Shannon) entropy and physical and measurable quantities. <br /><br /><b>What is Renyi entropy?</b><br /><div><div>It was proposed by Alfred Renyi in '60s as novel information measures. Shannon entropy is determined from them as an example.<br /><br /><b>Why measuring entropy? </b></div><div>Information content in Renyi entropy is a key concept to establish fundamental laws of thermodynamics in small scales. It helps to understand how to communicate information with quantum devices.</div><div><br /></div><div>Now between two systems A and B that exchange energy, we understood how to measure the Renyi entropies.<br /><br />This correspondence can serve as a fundamental approach to study a second law of thermodynamics in quantum scales (if there is any.) </div></div></div>محمدnoreply@blogger.com0tag:blogger.com,1999:blog-4070855.post-41105034479479951992015-02-06T20:20:00.002-08:002015-02-09T22:15:23.010-08:00A simple phenomenology on quasiparticles<div dir="ltr" style="text-align: left;" trbidi="on"><table cellpadding="0" cellspacing="0" class="tr-caption-container" style="float: left; margin-right: 1em; text-align: left;"><tbody><tr><td style="text-align: center;"><a href="http://1.bp.blogspot.com/--liGLB0w0zo/VNWQqiAH6VI/AAAAAAAALnQ/km2BLnT1Q7Q/s1600/rainierc4_2592x1944.jpg" imageanchor="1" style="clear: right; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" src="http://1.bp.blogspot.com/--liGLB0w0zo/VNWQqiAH6VI/AAAAAAAALnQ/km2BLnT1Q7Q/s1600/rainierc4_2592x1944.jpg" height="150" width="200" /></a></td></tr><tr><td class="tr-caption"><div style="text-align: left;"><span style="font-size: 12.8000001907349px;">Photo: 128 qubit Rainier chip from <a href="https://dwave.wordpress.com/2009/04/13/first-view-128-qubit-rainier-chip/">here</a>. </span></div></td></tr></tbody></table>A simple and phenomenological insight about how to calculate the rate of tunneling nonequilibrium quasiparticles in superconducting small islands.<br /><br /><div>M. H. Ansari, Supercond. Sci. Technol. 28, 045005 (2015).<br />DOI:<a href="http://dx.doi.org/10.1088/0953-2048/28/4/045005">10.1088/0953-2048/28/4/045005</a><br /><br />One always blame quasiparticles for all kinds of experimental noise and poor sample properties, but there is not yet a common understanding how exactly nonequilibrium quasiparticles affect a qubit. This is partly due to lack of experimental resolution, and partly due to lack of theoretical model. <br /><br />This paper addresses relevant questions for many of the on-going experiments with superconducting qubits. <br /><br />The main result of this work is summarized in Fig.1c, where a "non-monotonic" behavior of the relaxation rate as function of temperature is presented. This is a consequence of the assumed phenomenological model for non-equilibrium, where a fixed non-equilibrium quasiparticle density leads to a temperature-dependent chemical potential shift, see Eq.(1). The simplicity of the model point to the possible generality of the predicted non-monotonicity.<br /><br />Want to know a bit more?! Read the abstract <a href="http://iopscience.iop.org/0953-2048/28/4/045005/">here</a>.<br />UPDATE:<br />An arxiv version in <a href="http://arxiv.org/abs/1303.1453">here</a>: arXiv:1303.1453<br /><br />* <b>A bit of side story</b>:<br /><br />I remember that the core idea of this work came to me when I was sitting in a ViaRail train in a cold typical Canadian Friday evening of 2013. Inside the train I did simple calculations and surprisingly saw that experimental expectations can be satisfied from simple ideas. A few weeks later the model has become ready. There was, however, a rather long delay in publishing it, which partly comes from strange situations in life. Finally I could manage an update and sent the paper to a professional journal about superconductivity on Sept 2014.<br /><br />In response I received three reviews that not only helped to improve the text, but also helped to get confidence on my shaking knees when I stand up alone. Thanks Canada!<br /><br /><br /></div></div>محمدnoreply@blogger.com0tag:blogger.com,1999:blog-4070855.post-165272689334443752014-08-20T03:51:00.001-07:002015-03-20T05:05:54.688-07:00Quantum entropy flows - updated<div dir="ltr" style="text-align: left;" trbidi="on"><div class="separator" style="clear: both; text-align: center;"></div><div><br /></div><div><div class="separator" style="clear: both; text-align: center;"><a href="http://1.bp.blogspot.com/-qM8Ogt5RMZ8/VQwLhFaw3BI/AAAAAAAALu8/nqC_kRVEQIk/s1600/medium.png" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"><img border="0" src="http://1.bp.blogspot.com/-qM8Ogt5RMZ8/VQwLhFaw3BI/AAAAAAAALu8/nqC_kRVEQIk/s1600/medium.png" height="99" width="200" /></a></div>Non-equilibrium quantum thermodynamics is a quite new fields in physics that surprisingly left less explored in the last century. Recently this field is becoming active both experimentally and theoretically. </div><div><br /></div>When interaction occurs between two systems there is a flow of some conserved quantities, such as electric charge, energy etc. between the two. Shannon entropy (as well as its generalized Renyi entropy) is a conserved quantity in a world made of subsystems A and B. Owing to this conservation there are finite flows of entropy between A and B.<br /><div><br /><div><div><div class="page" title="Page 3"><div class="layoutArea"><div class="column"></div></div></div><div>For the first time we present a consistent derivation of the flows of Shannon and Renyi entropies for a generic quantum heat engine to a probe environment kept in thermal equilibrium. The flows consist of heat flow and fictitious dissipation originating from quantum coherence. </div><div><br /></div><div>Source: <a href="http://arxiv.org/abs/1408.3910">http://arxiv.org/abs/1408.3910</a></div><div><br />Rényi entropy flows from quantum heat engines<br />Mohammad H. Ansari, Yuli V. Nazarov<br /><br />UPDATE:<br />An update is that paper has been published in Phys. Rev. B 91, 104303 - <a href="http://dx.doi.org/10.1103/PhysRevB.91.104303">doi</a></div><div><br /></div><br /></div></div></div></div>محمدnoreply@blogger.com0tag:blogger.com,1999:blog-4070855.post-69867530912603028302014-07-01T02:54:00.001-07:002015-03-20T05:09:34.302-07:00another contribution to quasiparticle poisoning<div dir="ltr" style="text-align: left;" trbidi="on"><div dir="ltr" style="text-align: left;" trbidi="on"><div class="separator" style="clear: both; text-align: center;"><br /></div><div>just appeared on <a href="http://arxiv.org/abs/1406.7350" target="_blank">arxiv:1406.7350</a> in a collaboration that connect people in Sanata Barabra, Waterloo, Kocaeli, <span style="font-size: 9px;"> </span>and Delft.</div><br />In a flux qubit, the energy spectrum versus magnetic flux must be single hyperbolic, but what is observed usually in practice is double lines.<br /><br />Cooling down does not help to remove the second line but it helps only a little bit to reduce the gap between the two. Why is this so?<br /><br />This paper explains that the reason is quasiparticle poisoning in the junction. These quasiparticles have nonequilibrium nature, which at higher temperature turns to the equilibrium one. We propose a detailed theory and exactly extracted the gap from a quasiparticle tunneling theory.<br /><br />Previously I made 2 more contributions to the theory of qusiparticle tunneling, here <a href="http://arxiv.org/abs/1211.4745" target="_blank">arXiv:1211.4745</a> (published) and <a href="http://arxiv.org/abs/1303.1453">arxiv:1303.1453</a> (recently submitted for publication).<br /><br />The new preprint is an experimental evidence to the problem. <br /><br /></div><div class="separator" style="clear: both; text-align: center;"><br /></div></div>محمدnoreply@blogger.com1tag:blogger.com,1999:blog-4070855.post-25810874242880892172014-06-26T02:43:00.004-07:002015-03-20T05:10:59.283-07:00Critical current as a function of magnetic field<div dir="ltr" style="text-align: left;" trbidi="on">Another theoretical collaboration with a great experimentalist group.<br /><br />Josephson Interference due to Orbital States in a Nanowire Proximity Effect Junction,<br />K. Gharavi, G. W. Holloway, C. M. Haapamaki, M. H. Ansari, M. Muhammad, R. R. LaPierre, and J. Baugh, <a href="http://arxiv.org/abs/1405.7455" target="_blank">arXiv:1405.7455</a><br /><br />On the finding of critical current as a function of magnetic field in a semiconductor nanowire with two superconducting metal contacts; unexpected results based on standard models of Josephson junctions.<br /><br />----------------------------------------------<br />Also our recent paper published in New J. Phys. : <a href="http://iopscience.iop.org/1367-2630/16/2/023019/">Toward tripartite hybrid entanglement in quantum dot molecules</a> by M Khoshnegar, A Jafari-Salim, M H Ansari, and A H Majedi, became available on <a href="http://arxiv.org/abs/1406.6933" target="_blank">arxiv:1406.6933</a> .</div>محمدnoreply@blogger.com0tag:blogger.com,1999:blog-4070855.post-8144207727610959842014-04-25T09:58:00.001-07:002014-04-25T18:54:08.860-07:002 lioned physicists<div dir="ltr" style="text-align: left;" trbidi="on"><div dir="ltr"><b>This</b> morning we at the Applied Physics of TUDelft heard the wonderful news that two of our physics faculty members have become knighted with the title "Knight in the Order of the Netherlands Lion." Congrats Cees Dekker and Leo Kouwenhoven!</div><div dir="ltr"><br /></div><div dir="ltr">In this blog I have posted one of the earliest reviews on Majorana fermions, <a href="http://gaugeinvariance.blogspot.ca/2012/04/2-observations-of-majorana-fermions-in.html?m=1">here</a> and <a href="https://uwaterloo.ca/phys13news/sites/ca.phys13news/files/uploads/files/142%20Spring%202012.pdf">here</a> and <a href="http://gaugeinvariance.blogspot.com/2012/04/another-experiments-on-majorana.html?m=1">here</a> right after Leo's presentation at March Meeting 2012. I remember that day when in Boston Conference Center in one of its parallel sessions, in a small room I was sitting on the floor in front of the first row of chairs, and was listening to Leo's presentation. The room was full of audience.</div><div dir="ltr"><br /></div><div dir="ltr">Below is the photo I took from the small party we had this afternoon in our department. Cakes were decorated by their photos! The speaker indicated that they received their medals this morning in Den Haag and added they are allowed to wear their Lion medals only today until the midnight!</div><div dir="ltr"><br /></div><div dir="ltr">One funny thing happened was that at the middle of a friendly speech Leo wanted to say something in praise of the audience and he put words in a funny way and refer to us as non-Lion people!</div><div dir="ltr"><br /></div><div dir="ltr">Congrats Leo and Cees!</div><table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody><tr><td style="text-align: center;"><img border="0" src="http://lh5.ggpht.com/-q1IhMYdpB6A/U1qUO16bCTI/AAAAAAAAHl8/mt4diE3evmE/s640/IMG_20140425_185644.jpg" height="379" style="margin-left: auto; margin-right: auto;" width="640" /></td></tr><tr><td class="tr-caption" style="text-align: center;"><span style="font-size: small; text-align: left;">Cees Dekker on mic and Leo Kouwenhoven with flowers in hands. Photo by: MHA</span></td></tr></tbody></table><div class="separator" style="clear: both; text-align: center;"><a href="http://lh5.ggpht.com/-q1IhMYdpB6A/U1qUO16bCTI/AAAAAAAAHl8/mt4diE3evmE/s1600/IMG_20140425_185644.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"> </a> </div></div>محمدnoreply@blogger.com2Delft, Delft52.011578 4.3570676tag:blogger.com,1999:blog-4070855.post-21729540718989119152014-03-23T01:48:00.000-07:002014-03-23T01:48:03.035-07:00review on majorana quasiparticles<div dir="ltr" style="text-align: left;" trbidi="on">My review on Majorana fermions in the Physics 13 is now available online on the University of Waterloo website (<a href="https://uwaterloo.ca/phys13news/sites/ca.phys13news/files/uploads/files/142%20Spring%202012.pdf" target="_blank">link</a>)</div>محمدnoreply@blogger.com0tag:blogger.com,1999:blog-4070855.post-54536921596029953802014-02-18T15:12:00.000-08:002014-02-18T15:13:37.328-08:00Toward tripartite hybrid entanglement in quantum dot molecules<div dir="ltr" style="text-align: left;" trbidi="on"><br />Our latest paper appeared on New Journal of Physics...<br /><div><br /></div><div><br /></div><div><a href="http://iopscience.iop.org/1367-2630/16/2/023019/" target="_blank">Toward tripartite hybrid entanglement in quantum dot molecules</a></div><div><br /></div><div>M Khoshnegar, A Jafari-Salim, M H Ansari, and A H Majedi</div><div><br /></div><div><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="http://1.bp.blogspot.com/-rf3toWMtEN8/UwPoL-8EP1I/AAAAAAAAHig/q8ihIswuoC0/s1600/2014-02-19+12.03.29+am.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="http://1.bp.blogspot.com/-rf3toWMtEN8/UwPoL-8EP1I/AAAAAAAAHig/q8ihIswuoC0/s1600/2014-02-19+12.03.29+am.png" height="116" width="400" /></a></div><div><br />Establishing the hybrid entanglement among a growing amount of matter and photonic quantum bits is necessary for scalable quantum computation and long-distance quantum communication. </div><div><br /></div><div>Here we demonstrate that charged excitonic complexes forming in strongly correlated quantum dot molecules are able to generate tripartite hybrid entanglement under proper carrier quantization. The mixed orbitals of the molecule construct multi-level ground states with sub-meV hole tunneling energy and relatively large electron hybridization energy. </div><div><br /></div><div>We show that appropriate size and interdot spacing keeps the electron particle weakly localized, opening extra recombination channels by correlating ground-state excitons. This allows for creation of higher order entangled states. </div><div><br /></div><div>Nontrivial hole tunneling energy, renormalized by multi-particle interactions, facilitates the realization of the energy coincidence among only certain components of the molecule optical spectrum. </div><div><br /></div><div>This translates to the emergence of favorable spectral components in a multi-body excitonic complex which sustain principal oscillator strengths throughout the electric field-induced hole tunneling process. </div><div><br /></div><div>We particularly analyze whether the level broadening of favorable spin configurations could be manipulated to eliminate the distinguishability of photons.</div></div>محمدnoreply@blogger.com0