Author(s): Davud Hebri and Saifollah Rasouli

We report a family of solutions of the homogeneous free-space scalar wave equation. These solutions are determined by linear combinations of the half-integer order Bessel functions. We call these beams “combined half-integer Bessel-like beams.” It is shown that, by selecting suitable combinations of...

[Phys. Rev. A 98, 043826] Published Fri Oct 12, 2018

Author(s): David Lindley

Computer simulations of the diffusion and aggregation of harmful proteins in the brain reproduce the pattern of damage seen in several neurodegenerative diseases.

[Physics 11, 104] Published Fri Oct 12, 2018

Categories: Physics

Author(s): Markus Hauru and Guifre Vidal

Given two states |ψ〉 and |ϕ〉 of a quantum many-body system, one may use the overlap or fidelity |〈ψ|ϕ〉| to quantify how similar they are. To further resolve the similarity of |ψ〉 and |ϕ〉 in space, one can consider their reduced density matrices ρ and σ on various regions of the system and compute th...

[Phys. Rev. A 98, 042316] Published Fri Oct 12, 2018

Author(s): Ming-Xing Luo

The multipartite correlations derived from local measurements on some composite quantum systems are inconsistent with those reproduced classically. This inconsistency is known as the quantum nonlocality and shows a milestone in the foundations of quantum theory. Still, it is NP hard to decide a nonl...

[Phys. Rev. A 98, 042317] Published Fri Oct 12, 2018

Author(s): Adam C. Keith, Charles H. Baldwin, Scott Glancy, and E. Knill

Estimation of quantum states and measurements is crucial for the implementation of quantum information protocols. The standard method for each is quantum tomography. However, quantum tomography suffers from systematic errors caused by imperfect knowledge of the system. We present a procedure to simu...

[Phys. Rev. A 98, 042318] Published Fri Oct 12, 2018

The performance enhancements observed in various models of continuous quantum thermal machines have been linked to the buildup of coherences in a preferred basis. But, is this connection always an evidence of 'quantum-thermodynamic supremacy'? By force of example, we show that this is not the case. In particular, we compare a power-driven three-level quantum refrigerator with a four-level combined cycle, partly driven by power and partly by heat. We focus on the weak driving regime and find the four-level model to be superior since it can operate in parameter regimes in which the three-level model cannot, it may exhibit a larger cooling rate, and, simultaneously, a better coefficient of performance. Furthermore, we find that the improvement in the cooling rate matches the increase in the stationary quantum coherences exactly. Crucially, though, we also show that the thermodynamic variables for both models follow from a classical representation based on graph theory. This implies that we can build incoherent stochastic-thermodynamic models with the same steady-state operation or, equivalently, that both coherent refrigerators can be simulated classically. More generally, we prove this for any $ N $-level weakly driven device with a 'cyclic' pattern of transitions. Therefore, even if coherence is present in a thermal machine, it is often unnecessary for the underlying energy conversion process.

We report the suppression of static ZZ crosstalk in a two-qubit, two-coupler superconducting circuit, where the ZZ interaction between the two qubits can be tuned to near zero. Characterization of qubit crosstalk is performed using randomized benchmarking and a two-qubit iSWAP gate is implemented using parametric modulation. We observe the dependence of single-qubit gate fidelity on ZZ interaction strength and identify effective thermalization of the tunable coupler as a crucial prerequisite for high fidelity two-qubit gates.

Two-dimensional magnetic insulators exhibit a plethora of competing ground states, such as ordered (anti)ferromagnets, exotic quantum spin liquid states with topological order and anyonic excitations, and random singlet phases emerging in highly disordered frustrated magnets. Here we show how single spin qubits, which interact directly with the low-energy excitations of magnetic insulators, can be used as a diagnostic of magnetic ground states. Experimentally tunable parameters, such as qubit level splitting, sample temperature, and qubit-sample distance, can be used to measure spin correlations with energy and wavevector resolution. Such resolution can be exploited, for instance, to distinguish between fractionalized excitations in spin liquids and spin waves in magnetically ordered states, or to detect anyonic statistics in gapped systems.

Recent work on Ising-coupled double-quantum-dot spin qubits in GaAs with voltage-controlled exchange interaction has shown improved two-qubit gate fidelities from the application of oscillating exchange along with a strong magnetic field gradient between adjacent dots. By examining how noise propagates in the time-evolution operator of the system, we find an optimal set of parameters that provide passive stroboscopic circumvention of errors in two-qubit gates to first order. We predict over 99% two-qubit gate fidelities in the presence of quasistatic and 1/$\textit{f}$ noise, which is an order of magnitude improvement over the typical unoptimized implementation.

The design and implementation of quantum technologies necessitates the understanding of thermodynamic processes in the quantum domain. In stark contrast to macroscopic thermodynamics, at the quantum scale processes generically operate far from equilibrium and are governed by fluctuations. Thus, experimental insight and empirical findings are indispensable in developing a comprehensive framework. To this end, we theoretically propose an experimentally realistic quantum engine, that utilizes transmon qubits as working substance. We solve the dynamics analytically and calculate its efficiency, that reaches a maximum value of $35\%$.

The generation of certifiable randomness is the most fundamental information-theoretic task that meaningfully separates quantum devices from their classical counterparts. We propose a protocol for exponential certified randomness expansion using a single quantum device. The protocol calls for the device to implement a simple quantum circuit of constant depth on a 2D lattice of qubits. The output of the circuit can be verified classically in linear time, and contains a polynomial number of certified random bits under the sole physical assumption that the device used to generate the output operated using a (classical or quantum) circuit of sub-logarithmic depth. This assumption contrasts with the locality assumption used for randomness certification based on Bell inequality violation and more recent proposals for randomness certification based on computational assumptions. Our procedure is inspired by recent work of Bravyi et al. (arXiv:1704.00690), who designed a relation problem that can be solved by a constant-depth quantum circuit, but provably cannot be solved by any classical circuit of sub-logarithmic depth. We expand the discovery of Bravyi et al. into a framework for robust randomness expansion. Furthermore, to demonstrate randomness generation it is sufficient for a device to sample from the ideal output distribution within constant statistical distance. Our proposal can thus be interpreted as a proposal for demonstrated quantum advantage that is more noise-tolerant than most other existing proposals that can only tolerate multiplicative error, or require additional conjectures from complexity theory. Our separation does not require any conjectures, but assumes that the adversarial device implements a circuit of sub-logarithmic depth.

In this review paper I present two geometric constructions of distinguished nature, one is over the field of complex numbers $\mathbb{C}$ and the other one is over the two elements field $\mathbb{F}_2$. Both constructions have been employed in the past fifteen years to describe two quantum paradoxes or two resources of quantum information: entanglement of pure multipartite systems on one side and contextuality on the other. Both geometric constructions are linked to representation of semi-simple Lie groups/algebras. To emphasize this aspect one explains on one hand how well-known results in representation theory allows one to see all the classification of entanglement classes of various tripartite quantum systems ($3$ qubits, $3$ fermions, $3$ bosonic qubits...) in a unified picture. On the other hand, one also shows how some weight diagrams of simple Lie groups are encapsulated in the geometry which deals with the commutation relations of the generalized $N$-Pauli group.

We study the relations between solitons of nonlinear Schr\"{o}dinger equation described systems and eigen-states of linear Schr\"{o}dinger equation with some quantum wells. Many different non-degenerated solitons are re-derived from the eigen-states in the quantum wells. We show that the vector solitons for coupled system with attractive interactions correspond to the identical eigen-states with the ones of coupled systems with repulsive interactions. The energy eigenvalues of them seem to be different, but they can be reduced to identical ones in the same quantum wells. The non-degenerated solitons for multi-component systems can be used to construct much abundant degenerated solitons in more components coupled cases. On the other hand, we demonstrate soliton solutions in nonlinear systems can be also used to solve the eigen-problems of quantum wells. As an example, we present eigenvalue and eigen-state in a complicated quantum well for which the Hamiltonian belongs to the non-Hermitian Hamiltonian having Parity-Time symmetry. We further present the ground state and the first exited state in an asymmetric quantum double-well from asymmetric solitons. Based on these results, we expect that many nonlinear physical systems can be used to observe the quantum states evolution of quantum wells, such as water wave tank, nonlinear fiber, Bose-Einstein condensate, and even plasma, although some of them are classical physical systems. These relations provide another different way to understand the stability of solitons in nonlinear Schr\"{o}dinger equation described systems, in contrast to the balance between dispersion and nonlinearity.

We give a structured decomposition for reversible Boolean functions. Specifically, an arbitrary n-bit reversible Boolean function is decomposed to 7 blocks of (n-1)-bit Boolean function, where 7 is a constant independent of n; and the positions of those blocks have large degree of freedom. Our result improves Selinger's work by reducing the constant from 9 to 7 and providing more candidates when choosing positions of blocks.

Authors develop new nonparametric methods for verification and monitoring for quantum randomness based on the ranged correlation function (RCF) and a sequence of the ranged amplitudes (SRA). We carried out RCF-analysis of different topology subsamples from raw data of the prototype of a quantum random number generator on homodyne detection. It is shown that in the real system there are weak local regression relations, for which it is possible to introduce a robust criterion of significance, and also precise SRA-identification of long samples statistics is made. The obtained results extend the traditional entropy methods of the useful randomness analysis and open the way for creation of new strict quality quantum standards and defense for physical random numbers generators.

Vacuum fluctuations are polarized by electric fields somewhat similarly to the way that ordinary matter is polarized. As a consequence, the permittivity $\epsilon_0$ of the vacuum can be calculated similarly to the way that the permittivity $\epsilon$ of a dielectric is calculated. Retaining only leading terms, the resulting approximate formula for the permittivity $\epsilon_0$ of the vacuum is $\epsilon_0 \cong (6\mu_0)/\pi)(8e^2/\hbar)^2= 9.10\times 10^{-12}$ C/(Vm). The experimental value for $\epsilon_0$ is 2.8 \% less than the value calculated here. The absence of dispersion in the vacuum is discussed and explained.

The control landscape of a quantum system $A$ interacting with another quantum system $B$ is studied. Only system $A$ is accessible through time dependent controls, while system B is not accessible. The objective is to find controls that implement a desired unitary transformation on $A$, regardless of the evolution on $B$, at a sufficiently large final time. The freedom in the evolution on $B$ is used to define an \emph{extended control landscape} on which the critical points are investigated in terms of kinematic and dynamic gradients. A spectral decomposition of the corresponding extended unitary system simplifies the landscape analysis which provides: (i) a sufficient condition on the rank of the dynamic gradient of the extended landscape that guarantees a trap free search for the final time unitary matrix of system $A$, and (ii) a detailed decomposition of the components of the overall dynamic gradient matrix. Consequently, if the rank condition is satisfied, a gradient algorithm will find the controls that implements the target unitary on system $A$. It is shown that even if the dynamic gradient with respect to the controls alone is not full rank, the additional flexibility due to the parameters that define the extended landscape still can allow for the rank condition of the extended landscape to hold. Moreover, satisfaction of the latter rank condition subsumes any assumptions about controllability, reachability and control resources. Here satisfaction of the rank condition is taken as an assumption. The conditions which ensure that it holds remain an open research question. We lend some numerical support with two common examples for which the rank condition holds.

A scalable on-chip single-photon source at telecommunications wavelengths is an essential component of quantum communication networks. In this work, we numerically construct a pulse-regulated single-photon source based on an optical parametric amplifier in a nanocavity. Under the condition of pulsed excitation, we study the photon statistics of the source using the Monte Carlo wave-function method. The results show that there exits an optimum excitation pulse width for generating high-purity single photons, while the source brightness increases monotonically with increasing excitation pulse width. More importantly, our system can be operated resonantly and we show that in this case the oscillations in $g^{(2)}(0)$ is completely suppressed.

Quadrature bases that incorporate spatio-temporal degrees of freedom are derived as eigenstates of momentum dependent quadrature operators. The resulting bases are shown to be orthogonal for both the particle-number and spatio-temporal degrees of freedom. Using functional integration, we also demonstrate the completeness of these quadrature bases.

We investigate the entanglement dynamics of two interacting qubits in a common vacuum environment. The inevitable environment interaction leads to entanglement sudden death (ESD) in a two qubit entangled state system. The entanglement dynamics can be modified by the use of local unitary operations (quantum gates), applied on the system during its evolution. We show that these operations not only delays or avoids the ESD but also advances the entanglement revival with high concurrence value depending on the time of operation. We have analytically found out different time windows for switching with different quantum gates so that the ESD can be completely avoided in the subsequent evolution of the system. Our result offers practical applications in the field of quantum information processing where the entanglement is a necessary resource.