Matteo Bina

Drawbacks of the use of fidelity to assess quantum resources

Fidelity is a figure of merit widely employed in quantum technology in order to quantify similarity between quantum states and, in turn, to assess quantum resources or reconstruction techniques. Fidelities higher than, say, 0.9 or 0.99, are usually considered as a piece of evidence to say that two states are very close in the Hilbert space. On the other hand, on the basis of several examples for qubits and continuous variable systems, we show that such high fidelities may be achieved by pairs of states with considerably different physical properties, including separable and entangled states or classical and nonclassical ones. We conclude that fidelity as a tool to assess quantum resources should be employed with caution, possibly combined with additional constraints restricting the pool of achievable states, or only as a mere summary of a full tomographic reconstruction.

Exact results on decoherence and entanglement in a system of N driven atoms and a dissipative cavity mode

We solve the dynamics of an open quantum system where N strongly driven two-level atoms are equally coupled on resonance to a dissipative cavity mode. Analytical results are derived on decoherence, entanglement, purity, atomic correlations and cavity field mean photon number. We predict decoherencefree subspaces for the whole system and the N-qubit subsystem, the monitoring of quantum coherence and purity decay by atomic populations measurements, the conditional generation of atomic multi-partite entangled states and of cavity cat-like states. We show that the dynamics of atoms prepared in states invariant under permutation of any two components remains restricted within the subspace spanned by the completely symmetric Dicke states. We discuss examples and applications in the cases N = 3, 4.

Phase-reference monitoring in coherent-state discrimination assisted by a photon-number resolving detector.

Phase estimation represents a crucial challenge in many fields of Physics, ranging from Quantum Metrology to Quantum Information Processing. This task is usually pursued by means of interferometric schemes, in which the choice of the input states and of the detection apparatus is aimed at minimizing the uncertainty in the estimation of the relative phase between the inputs. State discrimination protocols in communication channels with coherent states also require the monitoring of the optical phase. Therefore, the problem of phase estimation is relevant to face the issue of coherent states discrimination. Here we consider a quasi-optimal Kennedy-like receiver, based on the interference of two coherent signals, to be discriminated, with a reference local oscillator. By means of the Bayesian processing of a small amount of data drawn from the outputs of the shot-by-shot discrimination protocol, we demonstrate the achievement of the minimum uncertainty in phase estimation, also in the presence of uniform phase noise. Moreover, we show that the use of photon-number resolving detectors in the receiver improves the phase-estimation strategy, especially with respect to the usually employed on/off detectors. From the experimental point of view, this comparison is realized by employing hybrid photodetectors.Matteo Bina,1, 2 Alessia Allevi,3, 4 Maria Bondani,5, 4 and Stefano Olivares1, 2, ∗ Dipartimento di Fisica, Università degli Studi di Milano, 20133 Milano, Italy CNISM UdR Milano Statale, via Celoria 16, 20133 Milano, Italy Dipartimento di Scienza e Alta Tecnologia, Università degli Studi dell’Insubria, Via Valleggio 11, 22100 Como, Italy CNISM UdR Como, via Valleggio 11, 22100 Como, Italy Istituto di Fotonica e Nanotecnologie, CNR, Via Valleggio 11, 22100 Como, Italy (Dated: August 4, 2014)

Collapse and revival of quantum coherence for a harmonic oscillator interacting with a classical fluctuating environment

We address the dynamics of nonclassicality for a quantum system interacting with a noisy fluctuating environment described by a classical stochastic field. As a paradigmatic example, we consider a harmonic oscillator initially prepared in a maximally nonclassical state, e.g. a Fock number state or a Schroedinger cat-like state, and then coupled to either resonant or non-resonant external field. Stochastic modeling allows us to describe the decoherence dynamics without resorting to approximated quantum master equations, and to introduce non- Markovian effects in a controlled way. A detailed comparison among different nonclassicality criteria and a thorough analysis of the decoherence time reveal a rich phenomenology whose main features may be summarized as follows: i) classical memory effects increase the survival time of quantum coherence; ii) a detuning between the natural frequency of the system and the central frequency of the classical field induces revivals of quantum coherence.

ENTANGLEMENT TRANSFER IN A MULTIPARTITE CAVITY QED OPEN SYSTEM

We describe the dynamics of tripartite state mapping and entanglement transfer from qubit-like radiation states to two-level atoms via optical cavity modes. When the entangled radiation is carried to the cavities by single-mode fibers, optimal pure and mixed state transfer is predicted for perfect mirror transmittance, and entanglement sudden death (and birth) is demonstrated for Werner input states. The general case of multi-mode fiber coupling is also discussed. The dynamics is finally investigated under various dissipative effects.

Homodyne-like detection for coherent state-discrimination in the presence of phase noise

We propose a homodyne-like detection scheme involving photon-number-resolving detectors to discriminate between two coherent states affected by either uniform or gaussian phase noise. A proof-of-principle experiment is performed employing two hybrid photodetectors, whose outputs are used in post processing to calculate the shot-by-shot photon-number differences. The performance of the strategy is quantified in terms of the error probability in discriminating the noisy coherent signals as a function of the characteristic noise parameters.

Assessing the significance of fidelity as a figure of merit in quantum state reconstruction of discrete and continuous-variable systems

We experimentally address the significance of fidelity as a figure of merit in quantum state reconstruction of discrete (DV) and continuous variable (CV) quantum optical systems. In particular, we analyze the use of fidelity in quantum homodyne tomography of CV states and maximum-likelihood polarization tomography of DV ones, focussing attention on nonclassicality, entanglement and quantum discord as a function of fidelity to a target state. Our findings show that high values of fidelity, despite well quantifying geometrical proximity in the Hilbert space, may be obtained for states displaying opposite physical properties, e.g. quantum or semiclassical features. In particular, we analyze in details the quantum-to-classical transition for squeezed thermal states of a single-mode optical system and for Werner states of a two-photon polarization qubit system.

Tripartite quantum state mapping and discontinuous entanglement transfer in a cavity QED open system

We describe the transfer of quantum information and entanglement from three propagating (radiation) to three localized (atomic) qubits via cavity modes resonantly coupled to atoms in the presence of a common reservoir. Upon addressing the full dynamics of the resulting nine-qubit open system, we find that once the cavities are fed, fidelity and transferred entanglement are optimal, while their peak values exponentially decrease due to dissipative processes. The external radiation is then turned off and quantum correlations oscillate between atomic and cavity qubits. For a class of mixtures of W and GHZ input states, we deal with a discontinuous exchange of entanglement among the subsystems, facing the still open problem of entanglement sudden death and birth in a multipartite system.ISI Foundation, I-10133, Torino, Italy(Dated: October 12, 2009)We describe the transfer of quantum information and correlations from an entangled tripartitebosonic system to three separate qubits through their local environments also in the presence ofvarious dissipative effects. Optimal state mapping and entanglement transfer are shown in theframework of optical cavity quantum electrodynamics involving qubit-like radiation states and two-level atoms via the mediation of cavity modes. For an input GHZ state mixed with white noise weshow the occurrence of sudden death and birth of entanglement, that is discontinuously exchangedamong the tripartite subsystems.

Probing the diamagnetic term in light–matter interaction

We address the quantum estimation of the diamagnetic, or

About the use of fidelity in continuous variable systems

A^2