Preprints

We address in this paper the notion of relative phase shift for mixed quantum systems. We study the Pancharatnam-Sjoeqvist phase shift for metaplectic isotopies acting on Gaussian mixed states. We complete and generalize previous results obtained by one of us while giving rigorous proofs. This gives us the opportunity to review and complement the theory of the Conley-Zehnder index which plays an essential role in the determination of phase shifts.

GCQIQ - CBPF

Quantum walks via quantum cellular automata

Pedro C. S. Costa, Renato Portugal, Fernando de Melo

Quantum Inf Process 17, 226 (2018)

ArXiv: https://arxiv.org/abs/1803.02176

Abstract

Very much as its classical counterpart, quantum cellular automata are expected to be a great tool for simulating complex quantum systems. Here we introduce a partitioned model of quantum cellular automata and show how it can simulate, with the same amount of resources (in terms of effective Hilbert space dimension), various models of quantum walks. All the algorithms developed within quantum walk models are thus directly inherited by the quantum cellular automata. The latter, however, has its structure based on local interactions between qubits, and as such it can be more suitable for present (and future) experimental implementations.

GOIQ - UFRJ

GOQIQ - UFMG

Steady State Entanglement beyond Thermal Limits

F. Tacchino, A. Auffèves, M. F. Santos, D. Gerace

Phys. Rev. Lett. 120, 063604 (2018)

ArXiv: https://arxiv.org/abs/1710.01377

Abstract

Classical engines turn thermal resources into work, which is maximized for reversible operations. The quantum realm has expanded the range of useful operations beyond energy conversion, and incoherent resources beyond thermal reservoirs. This is the case of entanglement generation in a driven-dissipative protocol, which we hereby analyze as a continuous quantum machine. We show that for such machines the more irreversible the process, the larger the concurrence. Maximal concurrence and entropy production are reached for the hot reservoir being at negative effective temperature, beating the limits set by classic thermal operations on an equivalent system.

GIQ-UFABC

GCQIQ - CBPF

Reversing the thermodynamic arrow of time using quantum correlations

Kaonan Micadei, John P. S. Peterson, Alexandre M. Souza, Roberto S. Sarthour, Ivan S. Oliveira, Gabriel T. Landi, Tiago B. Batalhão, Roberto M. Serra, Eric Lutz

arXiv:1711.03323 (2017)

ArXiv: https://arxiv.org/abs/1711.03323

Abstract

The second law permits the prediction of the direction of natural processes, thus defining a thermodynamic arrow of time. However, standard thermodynamics presupposes the absence of initial correlations between interacting systems. We here experimentally demonstrate the reversal of the arrow of time for two initially quantum correlated spins-1/2, prepared in local thermal states at different temperatures, employing a Nuclear Magnetic Resonance setup. We observe a spontaneous heat flow from the cold to the hot system. This process is enabled by a trade off between correlations and entropy that we quantify with information-theoretical quantities.

GOQIQ - UFMG

GOIQ - UFRJ

Coarse graining a non-Markovian collisional model

Nadja K. Bernardes, Andre R. R. Carvalho, C. H. Monken, Marcelo F. Santos

Phys. Rev. A 95, 032117 (2017)

ArXiv: https://arxiv.org/abs/1701.06419

GOIQ - UFRJ

Unified framework to determine Gaussian states in continuous-variable systems

Fernando Nicacio, Andrea Valdés-Hernández, Ana P. Majtey, Fabricio Toscano

Phys. Rev. A 96, 042341 (2017)

ArXiv: https://arxiv.org/pdf/1707.01966.pdf

GOIQ - UFRJ

GOQIQ - UFMG

GMCT - UFRJ

Photonic Counterparts of Cooper Pairs

André Saraiva, Filomeno S. de Aguiar Júnior, Reinaldo de Melo e Souza, Arthur Patrocínio Pena, Carlos H. Monken, Marcelo F. Santos, Belita Koiller, Ado Jorio

Phys. Rev. Lett. 119, 193603 (2017)

ArXiv: https://arxiv.org/abs/1709.04520

Abstract

The microscopic theory of superconductivity raised the disruptive idea that electrons couple through the elusive exchange of virtual phonons, overcoming the strong Coulomb repulsion to form Cooper pairs. Light is also known to interact with atomic vibrations, as, for example, in the Raman effect. We show that photon pairs exchange virtual vibrations in transparent media, leading to an effective photon-photon interaction identical to that for electrons in the BCS theory of superconductivity, in spite of the fact that photons are bosons. In this scenario, photons may exchange energy without matching a quantum of vibration of the medium. As a result, pair correlations for photons scattered away from the Raman resonances are expected to be enhanced. An experimental demonstration of this effect is provided here by time-correlated Raman measurements in different media. The experimental data confirm our theoretical interpretation of a photonic Cooper pairing, without the need for any fitting parameters.

GIQ-UFABC

GOIQ - UFRJ