Saturday, May 20, 2017

Entropy production in a photovoltaic cell

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.

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.

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.

This determines under what conditions information in these cells can take a reversal flow from a cold to hot bath.

Ref:  Mohammad H. Ansari, Phys. Rev. B 95, 174302 (2017)
It is available online at https://doi.org/10.1103/PhysRevB.95.174302

Abstract: 
 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.

Saturday, April 15, 2017

Another PhD position is available

Job type: PhD
Application deadline: Monday, June 5, 2017
Employer: Peter Gruenberg Institute (PGI-2)

Another PhD position 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.

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.

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:

1. your academic CV,
2. your academic transcripts and the list of publications,
3. a short essay relating your knowledge to recent papers written by Dr. Ansari (no more than 300 words ~ two paragraphs)
4. the names, affiliation, and email addresses of 2 or 3 referees, (make sure they are willing to send letters on time)

Please make sure to choose the following subject for your email: "Ph.D. position at PGI-2"

More updates about the position can be found at: https://sites.google.com/site/mansari

Monday, March 27, 2017

My talks on Entropy-Noise Correspondence

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.

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.

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 (IQC) on how to measure entropy in quantum systems. +

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 +

These talks turned out to fall into the interest of two main communities: condensed matter theorists and quantum information theorists.

To provide equal opportunities to those not present in the cities yet like to form and express a judgement, I uploaded a short summary of my presentations here to be accessible to public. Please feel free to review.

M.H. Ansari

Sunday, March 05, 2017

Talks


If you happen to be near Aachen this week, come to hear news about entropy. (this link

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Quantum information seminar, Mar 09 11:00, Physikzentrum MBP2 116
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SPEAKER: Mohammad Ansari (FZ J├╝lich)
TITLE: Entropy measurement in quantum systems

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.

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Also next week I am in New Orleans. If you are attending APS March meeting I am going to give a talk in Session L52: Statistics of Ensemble Quantum Systems Wednesday at 11:50 AM in  Room: 399. 

Wednesday, February 01, 2017

A postdoc/PhD student position

A fully-funded postdoc/PhD student position 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.

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.

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.

Everyone (Masters and PhD students and Postdocs) from all around the world are welcome to apply.

For full consideration, please apply as soon as possible, by sending your documents in *ONE pdf file* to "mansari AT gmail DOT com", including:

1. your academic CV,
2. your list of publications,
3. research interests on what makes your past research experience related to quantum thermodynamics, (no more than 300 words ~ two paragraphs)
4. the names, affiliation, and email addresses of 2 or 3 referees, (make sure they are willing to send letters on time)

More information about the research details as well as announcements (e.g. whether the position is still available or not) can be found at:
https://sites.google.com/site/mansari