Creating and sharing knowledge for telecommunications

Spin chain probe for quantum refrigeration and thermometry

on 20-04-2016

... Hamed Mohammady (Physics of Information and Quantum Technologies Group, Instituto de Telecomunicações)

20/04/2016, 11:00
Room P3.10, Mathematics Building, IST

We investigate the prospects for using a spin chain of length N as a probe for refrigeration and thermometry on a collection of identical qubits, all prepared in a thermal state (T) with temperature T. This requires a minimal degree of control -– one need only perform a swap operation between a single spin of the probe and the thermal qubits. Each swap operation either cools the thermal qubit, or leaves it the same, and the state of the probe incrementally gets closer to (T) N. It is this latter property, which we call pseudo-thermalisation, that allows the probe to be used for thermometry. By measuring each spin with the observable for the thermal qubits' Hamiltonian, the statistics can be used to infer T up to some arbitrary accuracy. We then propose an algorithm which, provided prior knowledge of T and the dynamics of the probe, will allow for optimising the cooling process. We show that the presence of local dephasing on the spins, or an imperfection of the swap operation, reduces the entropy reduction possible and slows the rate of pseudo-thermalisation. However, the thermal qubits always remain thermal, with a temperature no greater than T.

Quantum Computation and Information Seminar

Support: Phys-Info (IT), SQIG (IT), CFIF and CAMGSD, with support from FCT, FEDER and EU FP7, namely via the Doctoral Programme in the Physics and Mathematics of Information (DP-PMI), and projects PEst-OE/EEI/LA0008/2013, QuSim, QUTE-EUROPE (GA 600788), Landauer (318287) and PAPETS (323901).

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Breathers and metastable states in the Discrete Nonlinear Schroedinger Equation

on 19-04-2016

... Stefano Iubini (CNRS Orléans)

Date & time: 19/04/2016 at 11:30.

Location: Room P3.10, Mathematics Building, Instituto Superior Técnico, Lisbon.
Sometimes it is more convenient to store excess energy in a small region of space rather than spreading it over all the available volume. This may happen when the evolution of a system is characterized by additional dynamic constraints that promote energy localization for entropic reasons. In this seminar I will discuss the role of peculiar nonlinear excitations (discrete breathers) for the localization process in networks of coupled oscillators. Particular attention will be devoted to the dynamics of a Discrete Nonlinear Schroedinger Equation which exhibits a metastable phase with a finite density of breathers and partial energy localization. Such state persists over very long (astronomical) times as a consequence of the extremely low interaction of these excitations with the surrounding background.

Physics of Information Seminar

Supported with funding from FCT, FEDER and EU FP7, namely via projects PEst-OE/EEI/LA0008/2013, UID/EEA/50008/2013, IT/QuSim, CQVibes, Landauer (318287) and PAPETS (323901).

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