In a recent paper [Phys. Rev. A {\bf 96}, 053822 (2017)], we proposed a strategy to generate bipartite and quadripartite continuous-variable entanglement of bright quantum states based on degenerate down-conversion in a pair of evanescently coupled nonlinear $\chi^{(2)}$ waveguides. Here, we show that the resources needed for obtaining these features can be optimized by exploiting the regime of second harmonic generation: the combination of depletion and coupling among pump beams indeed supplies all necessary wavelengths and appropriate phase mismatch along propagation. Our device thus entangles the two fundamental classical input fields without the participation of any harmonic ancilla. Depending on the propagation distance, the generated harmonics are entangled in bright or vacuum modes. We also evidence two-color bipartite and quadripartite entanglement over the interacting modes. The proposed device represents a boost in continuous-variable integrated quantum optics since it enables a broad range of quantum effects in a very simple scheme, which optimizes the resources and can be easily realized with current technology.

Author(s): Luca Pezzè, Augusto Smerzi, Markus K. Oberthaler, Roman Schmied, and Philipp Treutlein

Entanglement is the basis of quantum technologies aimed at revolutionizing measurements, computing, and communications. This article reviews methods to improve measurement precision and sensitivity by harnessing entangled states of many atomic probe particles. The achievements of different experimental entanglement schemes are presented with theoretical analyses of their fundamental and practical limits, discussing prospects for applications in clocks, frequency standards, and measurements of forces and fields.

[Rev. Mod. Phys. 90, 035005] Published Wed Sep 05, 2018

Author(s): Daniel Braun, Gerardo Adesso, Fabio Benatti, Roberto Floreanini, Ugo Marzolino, Morgan W. Mitchell, and Stefano Pirandola

Entangled quantum states can enhance measurement precision. But quantum mechanics harbors other possibilities for enhancing precision, including some that have nothing to do with entanglement. This review surveys various strategies with unentangled probes by which measurements have been improved. Among the approaches considered are those that rely on particle statistics and correlations in highly mixed states. Often nonentangled states are more robust, and these approaches are feasible in current experiments: here the current states of research are shown in cold atoms, nonlinear optics, and nanomechanical oscillators.

[Rev. Mod. Phys. 90, 035006] Published Wed Sep 05, 2018

Author(s): J. Perczel, P. Kómár, and M. D. Lukin

The imaging properties of a lens that may perfectly focus electromagnetic waves is investigated at the quantum level. The lens is shown to be diffraction limited, and its role in entangling operations based on dipole-dipole interactions is discussed.

[Phys. Rev. A 98, 033803] Published Wed Sep 05, 2018

An optical design called Maxwell’s fish eye lens could produce quantum entanglement between atoms separated by an arbitrary distance, new calculations show.

[Physics] Published Wed Sep 05, 2018

Categories: Physics

Author(s): Shiroman Prakash, Akalank Jain, Bhakti Kapur, and Shubangi Seth

We study single-qutrit gates composed of Clifford and T gates, using the qutrit version of the T gate proposed by Howard and Vala [M. Howard and J. Vala, Phys. Rev. A **86**, 022316 (2012)]. We propose a normal form for single-qutrit gates analogous to the Matsumoto-Amano normal form for qubits. We prov...

[Phys. Rev. A 98, 032304] Published Wed Sep 05, 2018

Author(s): Manuel Schrauth, Jefferson S. E. Portela, and Florian Goth

In 1974, Harris proposed his celebrated criterion: Continuous phase transitions in d-dimensional systems are stable against quenched spatial randomness whenever dν>2, where ν is the clean critical exponent of the correlation length. Forty years later, motivated by violations of the Harris criteri...

[Phys. Rev. Lett. 121, 100601] Published Tue Sep 04, 2018

Author(s): Qing Sun, Jie Hu, Lin Wen, Han Pu, and An-Chun Ji

We consider two spin-1/2 fermions inside an optical cavity which supports a single-mode quantized light field. We demonstrate that the atom-light coupling (ALC) gives rise to the two-atom polariton states, where the two atoms are highly entangled with cavity photons. We focus on the case where the c...

[Phys. Rev. A 98, 033801] Published Tue Sep 04, 2018

Author(s): Sheng-Jun Yang, Jun Rui, Han-Ning Dai, Xian-Min Jin, Shuai Chen, and Jian-Wei Pan

Quantum interface of coherent optical field and atomic excitations plays an important role in quantum metrology and quantum information science. The electromagnetically-induced-transparency (EIT) technique has shown versatile and powerful capability in many applications during the last decades. By u...

[Phys. Rev. A 98, 033802] Published Tue Sep 04, 2018

Temperatures on Earth’s surface exhibit “pink noise”—a finding that could explain the global warming hiatus in the first decade of this century.

[Physics] Published Tue Sep 04, 2018

Categories: Physics

Author(s): Amandeep Singh, Harpreet Singh, Kavita Dorai, and Arvind

We undertake experimental detection of the entanglement present in arbitrary three-qubit pure quantum states on an NMR quantum information processor. Measurements of only four observables suffice to experimentally differentiate between the six classes of states which are inequivalent under stochasti...

[Phys. Rev. A 98, 032301] Published Tue Sep 04, 2018

Author(s): Eyal Cornfeld, Moshe Goldstein, and Eran Sela

In the presence of symmetry, entanglement measures of quantum many-body states can be decomposed into contributions from distinct symmetry sectors. Here we investigate the decomposability of negativity, a measure of entanglement between two parts of a generally open system in a mixed state. While th...

[Phys. Rev. A 98, 032302] Published Tue Sep 04, 2018

Author(s): Xiao-Long Hu, Zong-Wen Yu, and Xiang-Bin Wang

We present an efficient decoy-state protocol measurement-device-independent quantum key distribution without vacuum source, with the heralded single-photon source (HSPS) and global optimization. This protocol is able to give a high key rate and avoid the security problem caused by the vacuum source ...

[Phys. Rev. A 98, 032303] Published Tue Sep 04, 2018

We experimentally realize a simple scheme to minimize errors in multi-qubit entangling operations related to residual excitation of mediating bosonic modes. The technique employs discrete phase shifts in the oscillating field driving the gate to ensure all modes are de-excited at arbitrary user-defined times. We demonstrate its use across a range of parameters with a pair of $^{171}\text{Yb}^{+}$ ions and observe a significant reduction in gate error under non-ideal conditions. The technique provides a unified framework to achieve robustness against both static and time-varying error sources.

By modeling quantum chaotic dynamics with ensembles of random operators, we explore how a deep learning architecture known as a convolutional neural network (CNN) can be used to detect pseudorandom behavior in qubit systems. We analyze samples consisting of pieces of correlation functions and find that a CNN is capable of determining the degree of pseudorandomness which a system is subject to. This is done without computing any correlators explicitly. Interestingly, even samples drawn from two-point functions are found to be sufficient to solve this classification problem. This presents the possibility of using deep learning algorithms to explore late time behavior in chaotic quantum systems which have been inaccessible to simulation.

The Schmidt number is an entanglement measure whose logarithm quantifies the zero-error entanglement cost of generating a given quantum state using local operations and classical communication (LOCC). %However, the Schmidt number is a notoriously difficult quantity to compute, and its relationship to other entanglement measures is largely unknown. In this paper we show that the Schmidt number is highly non-multiplicative in the sense that for any integer $n$, there exists states whose Schmidt number remains constant when taking $n$ copies of the given state. These states also provide a rare instance in which the regularized zero-error entanglement cost can be computed exactly. We then explore the question of increasing the Schmidt number by quantum operations. We describe a class of bipartite quantum operations that preserve the Schmidt number for pure state transformations, and yet they can increase the Schmidt number by an arbitrarily large amount when generating mixed states. Our results are obtained by making connections to the resource theory of quantum coherence and generalizing the class of dephasing-covariant incoherent operations (DIO) to the bipartite setting.

We study a cavity-QED setup consisting of a two-level system coupled to a single cavity mode with two-photon relaxation. The system dynamics is modeled via a Lindblad master equation consisting of the Rabi Hamiltonian and a two-photon dissipator. We show that an even-photon relaxation preserves the $\mathbb{Z}_2$-symmetry of the Rabi model and provide a framework to study the corresponding non-Hermitian dynamics in the number-parity representation of the closed Rabi model. We discuss the role of different terms in the two-photon dissipator and show how one can extend existing results for the closed Rabi spectrum to the open case. Furthermore, we characterize the role of the $\mathbb{Z}_2$-symmetry in the excitation-relaxation dynamics of the system as a function of light-matter coupling. We observe that initial states with even/odd parity manifest completely distinct transient and steady state behaviors.

We describe the construction of a fast field cycling device capable of sweeping a 4-order-of-magnitude range of magnetic fields, from ~1mT to 7T, in under 700ms. Central to this system is a high-speed sample shuttling mechanism between a superconducting magnet and a magnetic shield, with the capability to access arbitrary fields in between with high resolution. Our instrument serves as a versatile platform to harness the inherent dichotomy of spin dynamics on offer at low and high fields - in particular, the low anisotropy, fast spin manipulation, and rapid entanglement growth at low field as well as the long spin lifetimes, spin specific control, and efficient inductive measurement possible at high fields. Exploiting these complementary capabilities in a single device open up applications in a host of problems in quantum control, sensing, and information storage, besides in nuclear hypepolarization, relaxometry and imaging. In particular, in this paper, we focus on the ability of the device to enable low-field hyperpolarization of 13C nuclei in diamond via optically pumped electronic spins associated with Nitrogen Vacancy (NV) defect centers.

In this work, we investigate the possibility of using artificial neural network to build ansatz quantum many-body states. The progresses on representing quantum many body states by stochastic recurrent neural network, restricted or unrestricted Boltzmann machine, are reviewed. Besides, we discuss the possibility of representing quantum states using feed-forward neural network which is relatively less studied in literatures. At last, entanglement features of the quantum neural network states are discussed for comparison with the tensor network states.

Quantum computers with Kerr-nonlinear parametric oscillators (KPOs) have recently been proposed by the author and others. Quantum computation using KPOs is based on quantum adiabatic bifurcations of the KPOs, which lead to quantum superpositions of coherent states, such as Schrodinger cat states. Therefore, these quantum computers are referred to as "quantum bifurcation machines (QbMs)." QbMs can be used for qauntum adiabatic optimization and universal quantum computation. Superconducting circuits with Josephson junctions, Josephson parametric oscillators (JPOs) in particular, are promising for physical implementation of KPOs. Thus, KPOs and QbMs offer not only a new path toward the realization of quantum bits (qubits) and quantum computers, but also a new application of JPOs. Here we theoretically explain the physics of KPOs and QbMs, comparing them with their dissipative counterparts. Their physical implementations with superconducting circuits are also presented.