We experimentally demonstrate that a single-photon detector ID210
commercially available from ID Quantique is vulnerable to blinding and can be
fully controlled by bright illumination. In quantum key distribution, this
vulnerability can be exploited by an eavesdropper to perform a faked-state
attack giving her full knowledge of the key without being noticed. We consider
the attack on standard BB84 protocol and a subcarrier-wave scheme, and outline
a possible countermeasure.

We present an explicit treatment of the two-particle-irreducible (2PI)
effective action for a zero-dimensional field theory. The advantage of this
simple playground is that we are required to deal only with functions rather
than functionals, making complete analytic approximations accessible and full
numerical evaluation of the exact result possible. Moreover, it permits us to
plot intuitive graphical representations of the behaviour of the effective
action, as well as the objects out of which it is built. We illustrate the

Recently, there are more and more organizations offering quantum-cloud
services, where any client can access a quantum computer remotely through the
internet. In the near future, these cloud servers may claim to offer quantum
computing power out of reach of classical devices. An important task is to make
sure that there is a real quantum computer running, instead of a simulation by
a classical device. Here we explore the applicability of a cryptographic
verification scheme that avoids the need of implementing a full quantum

Self-testing is a method to infer the underlying physics of a quantum
experiment in a black box scenario. As such it represents the strongest form of
certification for quantum systems. In recent years a considerable self-testing
literature has been developed, leading to progress in related
device-independent quantum information protocols and deepening our
understanding of quantum correlations. In this work we give a thorough and
self-contained introduction and review of self-testing and its application to

Certain solutions of Einstein's equations in anti-de Sitter spacetime can be
engineered, using extended gravitational thermodynamics, to yield `holographic
heat engines', devices that turn heat into useful mechanical work. On the other
hand, there are constructions (both experimental and theoretical) where a
series of operations is performed on a small quantum system, defining what are
known as `quantum heat engines'. We propose that certain holographic heat

We study the notion of causal orders for the cases of (classical and quantum)
circuits and spacetime events. We show that every circuit can be immersed into
a classical spacetime, preserving the compatibility between the two causal
structures. Within the process matrix formalism, we analyse the realisations of
the quantum switch using 4 and 3 gates in classical spacetimes with fixed
causal orders, and the realisation of a gravitational switch with only 2 gates

Author(s): Philippe Faist, Mario Berta, and Fernando Brandão
Thermodynamics imposes restrictions on what state transformations are possible. In the macroscopic limit of asymptotically many independent copies of a state—as for instance in the case of an ideal gas—the possible transformations become reversible and are fully characterized by the free energy. In ...
[Phys. Rev. Lett. 122, 200601] Published Fri May 24, 2019

Author(s): Xiao-Ming Zhang, Jianan Li, Xin Wang, and Man-Hong Yung
In quantum control theory, a question of fundamental and practical interest is how an arbitrary unitary transformation can be decomposed into a minimum number of elementary rotations for implementation, subject to various physical constraints. Examples include the singlet-triplet (ST) and exchange-o...
[Phys. Rev. A 99, 052339] Published Fri May 24, 2019

Author(s): Yongliang Zhang, Lina Shi, Ruo-Yang Zhang, Jinglai Duan, Jack Ng, C. T. Chan, and Kin Hung Fung
We develop a metric-torsion theory for chiral structures by using a generalized framework of transformation optics. We show that the chirality is uniquely determined by a metric with the local rotational degree of freedom. In analogy to the dislocation continuum, the chirality can be alternatively i...
[Phys. Rev. Lett. 122, 200201] Published Fri May 24, 2019

Author(s): Natalia Giovenale, Federico M. Pont, Pablo Serra, and Omar Osenda
The entanglement content of superpositions of pairs of degenerate eigenstates of a bipartite system is considered in the case that both eigenstates are also eigenstates of the $z$ component of the total angular momentum. It is shown that the von Neumann entropy of the state that is obtained tracing ...
[Phys. Rev. A 99, 052340] Published Fri May 24, 2019