We generalize the operational quasiprobability involving sequential measurements proposed by Ryu {\em et al.} [Phys. Rev. A {\bf 88}, 052123] to a continuous-variable system. The quasiprobabilities in quantum optics are incommensurate, i.e., they represent a given physical observation in different mathematical forms from their classical counterparts, making it difficult to operationally interpret their negative values. Our operational quasiprobability is {\em commensurate}, enabling one to compare quantum and classical statistics on the same footing. We show that the operational quasiprobability can be negative against the hypothesis of macrorealism for various states of light. Quadrature variables of light are our examples of continuous variables. We also compare our approach to the Glauber-Sudarshan $\mathcal{P}$ function. In addition, we suggest an experimental scheme to sequentially measure the quadrature variables of light.

The first detection of a quantum particle on a graph has been shown to depend sensitively on the sampling time {\tau} . Here we use the recently introduced quantum renewal equation to investigate the statistics of first detection on an infinite line, using a tight-binding lattice Hamiltonian with nearest- neighbor hops. Universal features of the first detection probability are uncovered and simple limiting cases are analyzed. These include the small {\tau} limit and the power law decay with attempt number of the detection probability over which quantum oscillations are superimposed. When the sampling time is equal to the inverse of the energy band width, non-analytical behaviors arise, accompanied by a transition in the statistics. The maximum total detection probability is found to occur for {\tau} close to this transition point. When the initial location of the particle is far from the detection node we find that the total detection probability attains a finite value which is distance independent.

Any experimental realization of Bose-Einstein condensation (BEC) deals with composite bosons (cobosons), which are quasi-particles made of fermion pairs that suffer Pauli blocking. We evidence that the prime consequence of the Pauli exclusion principle is to prevent cobosons from having their centers of mass located at the same position within their spatial extension. We moreover predict a many-body effect induced by Pauli blocking that leads to high interference wave modes resulting from fermion exchanges between two colliding coboson condensates. These two effects can be detected by measuring spatial correlation functions. We here propose an original procedure to exactly derive such correlation functions for Frenkel-like cobosons characterized by just their momentum. We physically extend the obtained results to more complex Wannier-like cobosons that have an additional relative motion index. In addition to revealing the physics involved in results previously obtained for elementary boson condensates, we use Shiva diagrams to show how it is changed for cobosons. The predictions we here present, established for matter bosons, are directly applicable to experiments.

We derive the leading asymptotic approximation, for low angle {\theta}, of the Wigner rotation matrix elements $d^j_{m_1m_2}(\theta)$, uniform in $j,m_1$ and $m_2$. The result is in terms of a Bessel function of integer order. We numerically investigate the error for a variety of cases and find that the approximation can be useful over a significant range of angles. This approximation has application in the partial wave analysis of wavepacket scattering.

We introduce a set of Bell inequalities for a three-qubit system. Each inequality within this set is violated by all generalized GHZ states. More entangled a generalized GHZ state is, more will be the violation. This establishes a relation between nonlocality and entanglement for this class of states. Certain inequalities within this set are violated by pure biseparable states. We also provide numerical evidence that at least one of these Bell inequalities is violated by a pure genuinely entangled state. These Bell inequalities can distinguish between separable, biseparable and genuinely entangled pure three-qubit states. We also generalize this set to n-qubit systems and may be suitable to characterize the entanglement of n-qubit pure states.

According to the geometric characterization of measurement assemblages and local hidden state (LHS) models, we propose a steering criterion which is both necessary and sufficient for two-qubit states under arbitrary measurement sets. A quantity is introduced to describe the required local resources to reconstruct a measurement assemblage for two-qubit states. We show that the quantity can be regarded as a quantification of steerability and be used to find out optimal LHS models. Finally we propose a method to generate unsteerable states, and construct some two-qubit states which are entangled but unsteerable under all projective measurements.

Adopting the geometric description of steering assemblages and local hidden states (LHS) model, we propose a geometric LHS model for some two-qubit states under continuous projective measurements of the steering side. We show that the model is the optimal LHS model for these states, and obtain a sufficient steering criterion for all two-qubit states. Then we demonstrate asymmetric steering using the results we get.

We consider the Heisenberg-Euler action for an electromagnetic field in vacuum, which includes quantum corrections to the Maxwell equations induced by photon-photon scattering. We show that, in some configurations, the plane monochromatic waves become unstable, due to the appearance of secularities in the dynamical equations. These secularities can be treated using a multiscale approach, introducing a slow time variable. The amplitudes of the plane electromagnetic waves satisfy a system of ordinary differential nonlinear equations in the slow time. The analysis of this system shows that, due to the effect of photon-photon scattering, in the unstable configurations the electromagnetic waves oscillate periodically between left-hand-sided and right-hand-sided polarizations. Finally, we discuss the physical implications of this finding, and the possibility of disclosing traces of this effect in optical experiments.

Matrix Product States (MPS) are a particular type of one dimensional tensor network states, that have been applied to the study of numerous quantum many body problems. One of their key features is the possibility to describe and encode symmetries on the level of a single building block (tensor), and hence they provide a natural playground for the study of symmetric systems. In particular, recent works have proposed to use MPS (and higher dimensional tensor networks) for the study of systems with local symmetry that appear in the context of gauge theories. In this work we classify MPS which exhibit local invariance under arbitrary gauge groups. We study the respective tensors and their structure, revealing known constructions that follow known gauging procedures, as well as different, other types of possible gauge invariant states.

So-called average subsystem entropies are defined by first taking partial traces over some pure state to define density matrices, then calculating the subsystem entropies, and finally averaging over the pure states to define the average subsystem entropies. These quantities are standard tools in quantum information theory, most typically applied in bipartite systems. We shall first present some extensions to the usual bipartite analysis, (including a calculation of the average tangle, and a bound on the average concurrence), follow this with some useful results for tripartite systems, and finally extend the discussion to arbitrary multi-partite systems. A particularly nice feature of tri-partite and multi-partite analyses is that this framework allows one to introduce an "environment" for small subsystems to couple to.

This Letter discusses topological quantum computation with gapped boundaries of two-dimensional topological phases. Systematic methods are presented to encode quantum information topologically using gapped boundaries, and to perform topologically protected operations on this encoding. In particular, we introduce a new and general computational primitive of topological charge measurement and present a symmetry-protected implementation of this primitive. Throughout the Letter, a concrete physical example, the $\mathbb{Z}_3$ toric code ($\mathfrak{D}(\mathbb{Z}_3)$), is discussed. For this example, we have a qutrit encoding and an abstract universal gate set. Physically, gapped boundaries of $\mathfrak{D}(\mathbb{Z}_3)$ can be realized in bilayer fractional quantum Hall $1/3$ systems. If a practical implementation is found for the required topological charge measurement, these boundaries will give rise to a direct physical realization of a universal quantum computer based on a purely abelian topological phase.

Author(s): Yehonatan Gilead and Yaron Silberberg

We investigate a quasi-one-dimensional periodic array of coupled waveguides, with one extended and one bound dimension, incorporating both first- and second-order coupling. We study the evolution of optical fields in this system, and measure quantum correlations when path-entangled photon pairs are ...

[Phys. Rev. A 96, 053803] Published Thu Nov 02, 2017

Author(s): Imran M. Mirza, Jeremy G. Hoskins, and John C. Schotland

Architectures based on waveguide quantum electrodynamics have emerged as promising candidates for quantum networks. In this paper, we analyze the propagation of single photons in disordered many-atom waveguides. We pay special attention to the influence of chirality (directionality of photon transpo...

[Phys. Rev. A 96, 053804] Published Thu Nov 02, 2017

A special type of quantum vortex formed in a Bose-Einstein condensate precesses like a spinning top.

[Physics] Published Thu Nov 02, 2017

Categories: Physics