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

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

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