Tuesday, September 15, 2015

Let's move to physical quantities: Keldysh Green functions!

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!

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.

Excerpts from:

Keldysh formalism for multiple parallel worlds
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.

Mohammad Ansari and Yuli V. Nazarov

(Submitted on 14 Sep 2015)

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.

Wednesday, August 19, 2015

Early look at CQI sessions in APS March meeting 2016

I'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.

Focus Topics (see descriptions below)
Towards Scalable Quantum Computers
Hybrid Quantum Systems
Adiabatic Quantum Computation and Quantum Annealing
Finite-size Quantum Information Theory
Quantum Characterization, Validation, and Verification
Quantum Information and Thermodynamics
Gravity and Quantum Information

Regular Sorting Topics
Superconducting quantum information
Semiconducting quantum information
Atomic, molecular and optical (AMO) quantum information
Topological quantum information
Algorithms and architecture for quantum information
Quantum information theory and quantum foundations


Towards Scalable Quantum Computers
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.

Hybrid Quantum Systems
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.
Organizer: Guido Burkard, University of Konstanz

Adiabatic Quantum Computation and Quantum Annealing
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.
Organizer: Daniel Lidar, University of Southern California

Finite-size Quantum Information Theory 
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.
Organizer: Mark Wilde, Louisiana State University

Quantum Characterization, Validation, and Verification
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.
Organizer: Charles Tahan, Laboratory for Physical Sciences, University of Maryland

Quantum Information and Thermodynamics
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.
Organizer: Mohammad Ansari, TU Delft

Gravity and Quantum Information
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.

Monday, August 17, 2015

My memory of Jacob Bekenstein

I heard today a sad news that Jacob Bekenstein passed away at age 68.  I know him mostly from reading his papers on gravity and information theory, but I also remember him through some personal communications. Below I will write it but before this, let me comment on his pioneering works out of the box a bit.

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. 
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. 

Since then there have been many attempts to calculate the black hole entropy.  For example, in one of the most recent attempts Ashtekar, 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.  

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 (this and this) 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.    

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. 

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:

"Dear Dr. Ansari,

Your paper on loop quantum gravity's predictions for the black hole area spectrum is interesting...
Although you find another series of levels that are not equidistant, you correctly argue that the
transitions between equidistant ones will prevail in the radiated spectrum. From an "observational" point of view, the equidistant levels are all that matters.

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. 

In a few days I responded to him, cc-ed to  Mukhanov: 

"Dear Professor Bekenstein,

It is a pleasure that I read your valuable comments. Thank
you for your message. I was not aware of the history of this line of
research to this depth. Previously, I enjoyed a discussion with Prof. Mukhanov
on this topic in a talk I gave a while ago at the Perimeter
Institute. I will read and study the references you mentiond and also
in the next version of the paper I will give the information to
readers and put enough stress on the origins of the methods.

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: 

"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."

In response he wrote:

"... Regarding the observability of the lines (in whatever scheme, yours, or ours) the problem will
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."

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. 

Mohammad H. Ansari

Wednesday, July 15, 2015

Stimulated quantum phase slips

Stimulated quantum phase slips from weak electromagnetic radiations in superconducting nanowires,
Amir Jafari-Salim, Amin Eftekharian, A. Hamed Majedi, Mohammad H. Ansari

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.

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.

Wednesday, June 03, 2015

Fluctuations induced by quasiparticles

In 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.

M. Bal, M. H. Ansari, J.-L. Orgiazzi, R. M. Lutchyn, and A. Lupascu
Phys. Rev. B 91, 195434 – Published 22 May 2015


TSFs are a generic type of noise, observed in many mesoscopic systems, with examples including charge, flux, and critical current fluctuators.

 In most of these experiments, TSFs are characterized using classical detectors, such as single-electron transistors or SQUIDs.

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.

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.

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.

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.