Author(s): Yafei Wen, Pai Zhou, Zhongxiao Xu, Liang Yuan, Haoyi Zhang, Shengzhi Wang, Long Tian, Shujing Li, and Hai Wang

A source that can generate atom-photon quantum correlations or entanglement based on a quantum memory is a basic building block of quantum repeaters (QRs). To achieve high entanglement generation rates in ensemble-based QRs, spatial-, temporal-, and spectral-multimode memories are needed. Previous t...

[Phys. Rev. A 100, 012342] Published Thu Jul 25, 2019

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Author(s): Jia-Qi Zhou, Ling Cai, Qi-Ping Su, and Chui-Ping Yang

Implementation of a discrete-time quantum walk (DTQW) with superconducting qubits is difficult since on-chip superconducting qubits cannot hop between lattice sites. We propose an efficient protocol for the implementation of DTQW in circuit quantum electrodynamics (QED), in which only $N+1$ qutrits,...

[Phys. Rev. A 100, 012343] Published Thu Jul 25, 2019

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We review theoretical and experimental highlights in transport in

two-dimensional materials focussing on key developments over the last five

years. Topological insulators are finding applications in magnetic devices,

while Hall transport in doped samples and the general issue of topological

protection remain controversial. In transition metal dichalcogenides

valley-dependent electrical and optical phenomena continue to stimulate

state-of-the-art experiments. In Weyl semimetals the properties of Fermi arcs

Quantum metrology makes use of coherent superpositions to detect weak

signals. While in principle the sensitivity can be improved by increasing the

density of sensing particles, in practice this improvement is severely hindered

by interactions between them. Using a dense ensemble of interacting electronic

spins in diamond, we demonstrate a novel approach to quantum metrology. It is

based on a new method of robust quantum control, which allows us to

simultaneously eliminate the undesired effects associated with spin-spin

Iterative phase estimation has long been used in quantum computing to

estimate Hamiltonian eigenvalues. This is done by applying many repetitions of

the same fundamental simulation circuit to an initial state, and using

statistical inference to glean estimates of the eigenvalues from the resulting

data. Here, we show a generalization of this framework where each of the steps

in the simulation uses a different Hamiltonian. This allows the precision of

the Hamiltonian to be changed as the phase estimation precision increases.

We introduce a method for the conditional generation of nonclassical states

of light in a cavity. We consider two-level atoms traveling along the

transverse direction to the cavity axis and show that by conditioning on one of

the output measurements nonclassical field states are generated. The two-level

atoms are prepared in the ground state and we conditioned on the events in

which they are also detected in the ground state. Nonclassical properties of

the cavity mode are identified and characterized. This includes: quadrature

In laboratory and numerical experiments, physical quantities are known with a

finite precision and described by rational numbers. Based on this, we deduce

that quantum control problems both for open and closed systems are in general

not algorithmically solvable. To prove this statement, we develop a technique

based on establishing the equivalence between quantum control problems and

Diophantine equations, which are polynomial equations with integer coefficients

In this article we reconstruct the Frauchiger and Renner's argument [1]

taking into account that the assertions of the argument are made at different

times. In order to do that, we use a formalism that allows dealing with quantum

properties at different times: the Theory of Consistent Histories. We show that

the supposedly contradictory conclusion of the argument requires computing

probabilities in a family of histories that does not satisfy the consistency

condition, a non legitimatemove in this theory.

We show that the set of not-completely-positive (NCP) maps is unbounded,

unless further assumptions are made. This is done by first proposing a

reasonable definition of a valid NCP map, which is nontrivial because NCP maps

may lack a full positivity domain. The definition is motivated by specific

examples. We prove that for valid NCP maps, the eigenvalue spectrum of the

corresponding dynamical matrix is not bounded. Based on this, we argue that in

general the volume measure of qubit maps, including NCP maps, is not well

In device-independent quantum information processing Bell inequalities are

not only used as detectors of nonlocality, but also as certificates of relevant

quantum properties. In order for these certificates to work, one very often

needs Bell inequalities that are maximally violated by specific quantum states.

Recently, in [A. Salavrakos et al., Phys. Rev. Lett. 119, 040402 (2017)] a

general class of Bell inequalities, with arbitrary numbers of measurements and