Curtis J. Broadbent

Experimental violation of two-party Leggett-Garg inequalities with semiweak measurements.

We generalize the derivation of Leggett-Garg inequalities to systematically treat a larger class of experimental situations by allowing multiparticle correlations, invasive detection, and ambiguous detector results. Furthermore, we show how many such inequalities may be tested simultaneously with a single setup. As a proof of principle, we violate several such two-particle inequalities with data obtained from a polarization-entangled biphoton state and a semiweak polarization measurement based on Fresnel reflection. We also point out a nontrivial connection between specific two-party Leggett-Garg inequality violations and convex sums of strange weak values.

All-optical delay of images using slow light

Two-dimensional images are delayed in a cesium vapor cell. The transverse phase and amplitude profiles of the images are shown to be preserved, even at very low light levels.

Einstein-Podolsky-Rosen steering inequalities from entropic uncertainty relations

We use entropic uncertainty relations to formulate inequalities that witness Einstein-PodolskyRosen (EPR) steering correlations in diverse quantum systems. We then use these inequalities to formulate symmetric EPR-steering inequalities using the mutual information. We explore the diering natures of the correlations captured by one-way and symmetric steering inequalities, and examine the possibility of exclusive one-way steerability in two-qubit states. Furthermore, we show that steering inequalities can be extended to generalized positive operator valued measures (POVMs), and we also derive hybrid-steering inequalities between alternate degrees of freedom.

Violation of continuous-variable Einstein-Podolsky-Rosen steering with discrete measurements.

We create a stronger EPR-steering inequality for continuous variables using entropic uncertainty. We explore the asymmetry in this inequality and develop a new symmetric inequality. We also violate these inequalities in experiment.

Genuinely multipartite concurrence of N-qubit X matrices

We find an algebraic formula for the N-partite concurrence of N qubits in an X matrix. X matrices are density matrices whose only nonzero elements are diagonal or antidiagonal when written in an orthonormal basis. We use our formula to study the dynamics of the N-partite entanglement of N remote qubits in generalized N-party Greenberger-Horne-Zeilinger(GHZ)states.Westudythecaseinwhicheachqubitinteractswithalocalamplitude damping channel. It is shown that only one type of GHZ state loses its entanglement in finite time; for the rest, N-partite entanglement dies out asymptotically. Algebraic formulas for the entanglement dynamics are given in both cases. We directly confirm that the half-life of the entanglement is proportional to the inverse of N .W hen entanglement vanishes in finite time, the time at which entanglement vanishes can decrease or increase with N depending on the initial state. In the macroscopic limit, this time is independent of the initial entanglement.

Improving Einstein–Podolsky–Rosen steering inequalities with state information

We discuss the relationship between entropic Einstein–Podolsky–Rosen (EPR)-steering inequalities and their underlying uncertainty relations along with the hypothesis that improved uncertainty relations lead to tighter EPR-steering inequalities. In particular, we discuss how using information about the state of a quantum system affects oneʼs ability to witness EPR-steering. As an example, we consider the recent improvement to the entropic uncertainty relation between pairs of discrete observables (Berta et al., 2010 [10]). By considering the assumptions that enter into the development of a steering inequality, we derive correct steering inequalities from these improved uncertainty relations and find that they are identical to ones already developed (Schneeloch et al., 2013 [9]). In addition, we consider how one can use state information to improve our ability to witness EPR-steering, and develop a new continuous variable symmetric EPR-steering inequality as a result.

Null Values and Quantum State Discrìmination

We present a measurement protocol for discriminating between two different quantum states of a qubit with high fidelity. The protocol, called null value, is comprised of a projective measurement performed on the system with a small probability (also known as partial collapse), followed by a tuned postselection. We report on an optical experimental implementation of the scheme. We show that our protocol leads to an amplified signal-to-noise ratio (as compared with a straightforward strong measurement) when discerning between the two quantum states.

80 years of steering and the Einstein–Podolsky–Rosen paradox: introduction

The year 2015 marks 80 years since Einstein, Podolsky, and Rosen argued for the incompleteness of quantum mechanics, in the process describing one of the most counterintuitive characteristics of quantum mechanics: distant measurements can influence the character of local quantum states. This led to reactions from Schrodinger, Bohr, and John Bell. This issue shows that these developments are still well under way, with new foundational insights on the history of steering and Bell-nonlocality, new directions in the mathematical characterization of EPR-steering, demonstrations in quantum optical experiments, and further connections with quantum information processing.

Solving non-Markovian open quantum systems with multi-channel reservoir coupling

Abstract We extend the non-Markovian quantum state diffusion (QSD) equation to open quantum systems which exhibit multi-channel coupling to a harmonic oscillator reservoir. Open quantum systems which have multi-channel reservoir coupling are those in which canonical transformation of reservoir modes cannot reduce the number of reservoir operators appearing in the interaction Hamiltonian to one. We show that the non-Markovian QSD equation for multi-channel reservoir coupling can, in some cases, lead to an exact master equation which we derive. We then derive the exact master equation for the three-level system in a vee-type configuration which has multi-channel reservoir coupling and give the analytical solution. Finally, we examine the evolution of the three-level vee-type system with generalized Ornstein–Uhlenbeck reservoir correlations numerically.

Bell violation for unknown continuous-variable states

We describe a new Bell test for two-particle entangled systems that engages an unbounded continuous variable. The continuous variable state is allowed to be arbitrary and inaccessible to direct measurements. A systematic method is introduced to perform the required measurements indirectly. Our results provide new perspectives on both the study of local realistic theory for continuous-variable systems and on the non-local control theory of quantum information.