Wednesday, August 20, 2014

Quantum entropy flows - updated

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. 

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.

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. 

Rényi entropy flows from quantum heat engines
Mohammad H. Ansari, Yuli V. Nazarov

An update is that paper has been published in Phys. Rev. B 91, 104303 - doi

Tuesday, July 01, 2014

another contribution to quasiparticle poisoning

just appeared on arxiv:1406.7350 in a collaboration that connect people in Sanata Barabra, Waterloo, Kocaeli,  and Delft.

In a flux qubit, the energy spectrum versus magnetic flux must be single hyperbolic, but what is observed usually in practice is double lines.

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?

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.

Previously I made 2 more contributions to the theory of qusiparticle tunneling, here arXiv:1211.4745 (published) and arxiv:1303.1453 (recently submitted for publication).

The new preprint is an experimental evidence to the problem.

Thursday, June 26, 2014

Critical current as a function of magnetic field

Another theoretical collaboration with a great experimentalist group.

Josephson Interference due to Orbital States in a Nanowire Proximity Effect Junction,
K. Gharavi, G. W. Holloway, C. M. Haapamaki, M. H. Ansari, M. Muhammad, R. R. LaPierre, and J. Baugh,  arXiv:1405.7455

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.

Also our recent paper published in New J. Phys. : Toward tripartite hybrid entanglement in quantum dot molecules by M Khoshnegar, A Jafari-Salim, M H Ansari, and A H Majedi, became available on arxiv:1406.6933 .

Friday, April 25, 2014

2 lioned physicists

This 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!

In this blog I have posted one of the earliest reviews on Majorana fermions, here and here and here 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.

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!

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!

Congrats Leo and Cees!
Cees Dekker on mic and Leo Kouwenhoven with flowers in hands. Photo by: MHA

Sunday, March 23, 2014

review on majorana quasiparticles

My review on Majorana fermions in the Physics 13 is now available online on the University of Waterloo website (link)

Tuesday, February 18, 2014

Toward tripartite hybrid entanglement in quantum dot molecules

Our latest paper appeared on New Journal of Physics...

M Khoshnegar, A Jafari-Salim, M H Ansari, and A H Majedi

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. 

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. 

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. 

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. 

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. 

We particularly analyze whether the level broadening of favorable spin configurations could be manipulated to eliminate the distinguishability of photons.