Friday, February 06, 2015

A simple phenomenology on quasiparticles

Photo: 128 qubit Rainier chip from here
A simple and phenomenological insight about how to calculate the rate of tunneling nonequilibrium quasiparticles in superconducting small islands.

M. H. Ansari, Supercond. Sci. Technol. 28, 045005 (2015).

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.

This paper addresses relevant questions for many of the on-going experiments with superconducting qubits.

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.

Want to know a bit more?! Read the abstract here.
An arxiv version in here: arXiv:1303.1453

* A bit of side story:

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.

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!

Wednesday, August 20, 2014

Quantum entropy flows

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