Sunday, July 08, 2018

Exact quantization of superconducting circuits

Another paper on transmon qubits is out. In this paper:

We present a theoretical description for circuits consisting of weak anharmonic qubits coupled to cavity multimodes. We obtain a unitary transformation that diagonalizes harmonic sector of the circuit. Weak anharmonicity does not alter the normal mode basis, however it can modify energy levels. We study two examples of a transmon and two transmons coupled to bus resonator, and we determine dressed frequencies and Kerr nonlinearities in closed form formulas. Our results are valid for arbitrary frequency detuning and coupling within and beyond dispersive regime. (M.H. Ansari)


Monday, April 09, 2018

Peter Grünberg

Our research institute is named after a 2007 Nobel Laureate: Prof. Peter Grünberg, yes, Peter Grünberg Institute, or briefly at PGI. Sadly today I heard that Peter Grünberg passed away today.

Peter Grünberg in 1988 discovered giant magnetoresistance (GMR), for which in 2007 together with Albert Fert received the Nobel prize.

Their discovery allowed the storage capacity of hard drives to be significantly increased and made the miniaturization of storage devices. GMR is the sudden change in electrical resistance of a material, made of alternating ferromagnetic and non-magnetic metal layers, when exposed to large magnetic field. If magnetization of neighbouring layers is parallel, electrical resistance heavily drops, however if antiparallel the resistance goes much much higher.  So this makes a large potential barrier between spin up and spin down, which can be used to code information on such a device.  

Thursday, January 11, 2018

Quantum Thermodynamics at APS March Meeting (2018)

American Physics Society organises another March Meeting for a week this year again. The venue will be at Los Angeles. There is aninvited session on quantum thermodynamics and information that contains 5 interesting talk by experimental and theoretical physicists. Highly recommended!

On Thursday morning 8-11AM, the following 5 invited speakers will give their talks:

Session R42: Progress in Quantum Thermodynamics
Sponsoring Units: DQI GSNP
Chair: Mohammad Ansari, Forschungszentrum Julich
Room: LACC 502B

8:00AM - 8:36AM
R42.00001: Progress in Thermodynamics of Superconducting and Hybrid Circuits
Invited Speaker: Jukka Pekola , Jonne Koski , Bayan Karimi , Alberto Ronzani , Jorden Senior , Olli Saira

8:36AM - 9:12AM
R42.00002: Fluctuation Theorem for Many-Body Pure Quantum States
Invited Speaker: Takahiro Sagawa

9:12AM - 9:48AM
R42.00003: Strong coupling quantum thermodynamics and beyond
Invited Speaker: Q. Jens Eisert

9:48AM - 10:24AM
R42.00004: Thermoelectrics of interacting nanosystems - Exploiting fermion-parity superselection instead of time-reversal symmetry

Invited Speaker: Janine Splettstoesser , Jens Schulenborg , Joren Vanherck , Angelo Di Marco , Maarten Wegewijs

10:24AM - 11:00AM
R42.00005: Quantum and Information Thermodynamics: A Unifying Framework Based on Repeated Interactions
Invited Speaker: Massimiliano Esposito

* By the way my own talk will be on the same day, an hour after this session is over, in another room.  I'll give a talk about a new formalism to deal with Transmon-like qubits. More info can be found below:

Session S39: Superconducting Circuits: Modeling
11:15 AM–2:15 PM, Thursday, March 8, 2018
LACC Room: 501B
Sponsoring Unit: DQI
Chair: Antonio Corcoles, IBM T J Watson Res Ctr
12:15 PM–12:27 PM

Abstract: S39.00006 : Effective Hamiltonian in superconducting qubits
Mohammad Ansari 
(Forschungszentrum Juelich)

Qubits with more than two energy levels, such as superconducting transmons, usually are externally driven in order to engineer one and two qubit gates. However due to the presence of higher excited levels the fidelity of the gates requires improvement. Such a system carries a large Hilbert space and recognizing effective qubits requires to use perturbation theory. This puts a lage limitation on the system parameters and interactions. We discuss a method that allows to go beyond regular perturbative limitations and separates classical effects from quantum fluctuations in the Hamiltonian of a weakly-anharmonic qubits. We compare results taken from applying Schrieffer-Wolff transformation, Least action principle, and our method. Our results will become practical tools for experimental efforts in circuit QED.

I may prepare a nice poster too to post on the wall, so please feel free to find my poster about a different topic:


Monday, October 16, 2017

Multiple PhD positions – JARA Institute for Quantum Information (RWTH Aachen & Forschungszentrum Jülich)

The theory groups at the Institute for Quantum Information of the Jülich-Aachen Reasearch Alliance (JARA) are looking for highly motivated candidates to fill multiple PhD positions over the next few months (a DFG-funded position could start as soon as possible).

The doctoral projects will focus on theoretical studies of physical implementations of quantum information processing, such as superconducting and semiconducting qubits, and related enabling quantum technologies. The projects will be supervised by Dr. Ansari, Dr. Catelani, and/or Prof. Hassler, and may involve collaborations with leading experimental groups (IBM, Yale, Aalto, TU Delft).

Master’s degree (or equivalent) in theoretical condensed matter physics or a related field and proficiency in English are required. Preference will be given to candidates with experience in quantum transport, superconductivity, and related topics, but strong candidates from other fields are encouraged to apply.

The application material should include:

  • short letter of motivation ( 
  • CV & transcripts 
  • contact details (names & emails) of two possible referees 

For further information, or to submit an application, please contact one of us by email. Review of applications will start immediately and continue until all the positions are filled.

Mohammad Ansari (
Gianluigi Catelani (
Fabian Hassler (


Saturday, May 20, 2017

Entropy production in a photovoltaic cell

In the everyday world, the amount of disorder, or entropy, in an isolated system can only increase over time. This relation is described by the second law of thermodynamics. This indicates that not all of the energy we provide for a system can be converted into engineered work.

In quantum physics, however, the existence of such a law is obscure. One of the main reasons is that entanglement can dramatically alter the notions of disorder and equilibrium state. Instead of an analogue law, recently a correspondence has been found between entropy production and the statistics of energy transfers in a quantum system.

In my paper published recently at Physical Rev. B I studied how entropy is produced in a quantum heat engine. This helps us to explain how heat is dissipated in the engine. The engine is modelled by four electronic levels resonantly-coupled to thermal heat baths kept at different temperatures. Results show that quantum coherence that is induced by environment can significantly and nonlinearly modify entropy production in the cells. Consequently, the nonlinear entropy production can take place much slower or faster.

This determines under what conditions information in these cells can take a reversal flow from a cold to hot bath.

Ref:  Mohammad H. Ansari, Phys. Rev. B 95, 174302 (2017)
It is available online at

 I evaluate entropy production in a photovoltaic cell that is modeled by four electronic levels resonantly coupled to thermally populated field modes at different temperatures. We use a formalism recently proposed, the so-called multiple parallel worlds, to consistently address the nonlinearity of entropy in terms of density matrix. Our result shows that entropy production is the difference between two flows: a semiclassical flow that linearly depends on occupational probabilities, and another flow that depends nonlinearly on quantum coherence and has no semiclassical analog. We show that entropy production in the cells depends on environmentally induced decoherence time and energy detuning. We characterize regimes where reversal flow of information takes place from a cold to hot bath. Interestingly, we identify a lower bound on entropy production, which sets limitations on the statistics of dissipated heat in the cells.