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.

Monday, June 01, 2015

RF/FCS correspondence published

Our paper on "Exact correspondence between Renyi entropy flows and physical flows" has been published in

What is RF/FCS correspondence?

A new correspondence, similar to the fluctuation-dissipation theorem in spirit, that provides an exact relation between the flows of quantum entropy and full counting statistics of energy transfers.

Thursday, May 21, 2015

A novel correspondence between entropy and statistical physics

Exact correspondences between seemingly different concepts play important role in all fields of physics. An example is the fluctuation-dissipation theorem, which states that the linear response of a system to externally applied forces corresponds to the system fluctuations. 

In the last decade, the fluctuation-dissipation theorem has initiated important developments in quantum transport, quantum computation, and other similar phenomenological theories. This theorem can be extended to nonlinear responses and to full counting statistics, giving more extended sets of similar relations. 

The Shannon entropy in quantum physics is considered unphysical, or non-observable, due to its nonlinear dependence on density matrix. So is the generalized Shannon entropy, called Renyi entropies --named after the French Mathematician and Physicist Alfred Renyi

An interesting and non-trivial question is: 
Is there any relation between the flows of entropy and the physical flows? 

The answer to the question, is YES! 

A novel relation is presented in the following research article that is similar to the fluctuation-dissipation theorem in spirit and provides an exact correspondence between the flows of quantum entropy and full counting statistics of energy transfers.

By: Mohammad H. Ansari 


1- Exact correspondence between Renyi quantum entropy flows and physical flows, 
M. H. Ansari and Yu. V. Nazarov,  
To appear in Phys. Rev. B., arXiv:1502.08020

2- Renyi entropy flows from quantum heat engines, 
M. H. Ansari and Yu. V. Nazarov, Phys. Rev. B 91, 104303 (2015) arxiv:1408.3910

Monday, March 16, 2015

We understood how to measure it

A novel correspondence between entropy flow and  statistics of energy exchange was reported in this paper. This is an extension of earlier result by Levitov and Klich on how to measure entropy in systems with charge transfers.
There seems to be an exact correspondence
between Renyi entropy flow
and statistics of energy exchange. 

The situation is fairly similar to the fluctuation-dissipation theorem however the new correspondence links between the Renyi (and Shannon) entropy and physical and measurable quantities.

What is Renyi entropy?
It was proposed by Alfred Renyi in '60s as novel information measures. Shannon entropy is determined from them as an example.

Why measuring entropy? 
Information content in Renyi entropy is a key concept to establish fundamental laws of thermodynamics in small scales. It helps to understand  how to communicate information with quantum devices.

Now between two systems A and B that exchange energy, we understood how to measure the Renyi entropies.

This correspondence can serve as a fundamental approach to study a second law of thermodynamics in quantum scales (if there is any.)