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).
DOI:10.1088/0953-2048/28/4/045005

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.
UPDATE:
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!


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