Entropy production, a quantity associated with the emergence of the arrow of time, has been successfully measured in a microscopic quantum system. Irreversibility is one of the most intriguing concepts in physics. While microscopic physical laws are perfectly reversible, macroscopic average behavior has a preferred direction of time. According to the second law of thermodynamics, this arrow of time is associated with a positive mean entropy production. Using a nuclear magnetic resonance setup, we measure the nonequilibrium entropy produced in an isolated spin-1/2 system following fast quenches of an external magnetic field and experimentally demonstrate that it is equal to the entropic dista

Within the broad research scenario of quantum secure communication, two-way quantum-key-distribution (TWQKD) is a relatively new proposal for sharing secret keys that is not fully explored yet. We analyse the security of TWQKD schemes that use qubits prepared in non-orthogonal states to transmit the key. Investigating protocols that employ an arbitrary number of bases for the channel preparation, we show, in particular, that the security of the LM05 protocol can not be improved by the use of more than two preparation bases. We also provide a new proof of unconditional security for a deterministic TWQKD protocol recently proposed [Phys. Rev. A 88, 062302 (2013)]. In addition, we introduce a n

It is well known that a quantum correlated probe can yield better precision in estimating an unknown parameter than classically possible. However, how such a quantum probe should be measured remains somewhat elusive. We examine the role of measurements in quantum metrology by considering two types of readout strategies: coherent, where all probes are measured simultaneously in an entangled basis; and adaptive, where probes are measured sequentially, with each measurement one way conditioned on the prior outcomes. Here we firstly show that for classically correlated probes the two readout strategies yield the same precision. Secondly, we construct an example of a noisy multipartite quantum sy

Highlighted as Editors’ Suggestion. Research on the out-of-equilibrium dynamics of quantum systems has so far produced important statements on the thermodynamics of small systems undergoing quantum mechanical evolutions. Key examples are provided by the Crooks and Jarzynski relations: taking into account fluctuations in non-equilibrium dynamics, such relations connect equilibrium properties of thermodynamic relevance with explicit non-equilibrium features. Although the experimental verification of such fundamental relations in the classical domain has encountered considerable success, their quantum mechanical version requires the assessment of the statistics of work performed by or onto an e

Our former PhD student, Dr Jonas Maziero was awarded a couple of prizes: (1) the best thesis Grand Prize in Natural Sciences and Engineering awarded by the Coordination for the Improvement of Higher Education Personnel (CAPES) and the Conrado Wessel Foundation (CWF); (2) the Best Physics Thesis Prize awarded by CAPES; (3) and his work also received honor mention in the José Leite Lopes Prize of the Brazilian Physical Society (SBF). Dr. Maziero’s thesis was recognized for its great quality and originality, including some pioneer theoretical results and experiments on quantum discord. The award ceremony in Brasilia was attended by the Ministers of Education and Science & Technology,

A new issue on Quantum information processing (QIP) in Nuclear Magnetic Resonance (NMR) was published in Philosophical Transactions of the Royal Society. This was compiled and edited by Ivan S. Oliveira and Roberto M. Serra. This special issue gathers the state-of-art of NMR-QIP. It covers new experimental techniques, which combine NMR and EPR, perspectives for NMR-QIP in Solid-State and implementation of quantum algorithms and protocols in liquid-state samples, including quantum simulation through NMR-QIP. On the theoretical side, particularly important are the recent developments on the QIP in the presence of noise decoherence, the role of entanglement for quantum protocols and its charac

One of the milestones of quantum mechanics is Bohr's complementarity principle. It states that a single quantum can exhibit a particlelike or a wavelike behavior, but never both at the same time. These are mutually exclusive and complementary aspects of the quantum system. This means that we need distinct experimental arrangements in order to measure the particle or the wave nature of a physical system. One of the most known representations of this principle is the single-photon Mach-Zehnder interferometer. When the interferometer is closed an interference pattern is observed (wave aspect of the quantum) while if it is open, the quantum behaves like a particle. Using a molecular quantum info

The perception that quantum correlations can still appear in separable states has opened exciting new possibilities regarding their use as a resource in quantum information science. Quantifying such quantum correlations involves the complete knowledge of the system’s state and numerical optimization procedures. Thus, it is natural to seek methods involving fewer measurements that indicate the nature of the correlations in a system. Here we propose a classicality witness that can be accurately estimated via statistics from a single measurement and perform an experiment to explore the utility of this witness for quantum states with different types of correlations. For more information: Phys. R

The quantification of quantum correlations (other than entanglement) usually entails labored numerical optimization procedures also demanding quantum state tomographic methods. Thus it is interesting to have a laboratory friendly witness for the nature of correlations. We report recently a direct experimental implementation of such a witness in a room temperature nuclear magnetic resonance (NMR) system. In our experiment the nature of correlations is revealed by performing only few local magnetization measurements. For more information: Phys. Rev. Lett. 107, 070501 (2011) | arXiv | See also the optical implementation of the witness in: Phys. Rev. Lett. 108, 063601 (2012) Coverage in the popu

We introduce an approach for quantum computing in continuous time based on the Lewis–Riesenfeld dynamic invariants. This approach allows, under certain conditions, for the design of quantum algorithms running on a nonadiabatic regime. We show that the relaxation of adiabaticity can be achieved by processing information in the eigenlevels of a time dependent observable, namely, the dynamic invariant operator. Moreover, we derive the conditions for which the computation can be implemented by time independent as well as by adiabatically varying Hamiltonians. We illustrate our results by providing the implementation of both Deutsch–Jozsa and Grover algorithms via dynamic invariants. For more inf

Nonclassical correlations play a crucial role in the development of quantum information science. The recent discovery that nonclassical correlations can be present even in separable (nonentangled) states has broadened this scenario. This generalized quantum correlation has been increasing in relevance in several fields, among them quantum communication, quantum computation, quantum phase transitions, and biological systems. We demonstrate here the occurrence of the sudden-change phenomenon and immunity against some sources of noise for the quantum discord and its classical counterpart, in a room temperature nuclear magnetic resonance setup. The experiment is performed in a decohering environ