We consider imperfect two-mode bosonic quantum transducers that cannot
completely transfer an initial source-system quantum state due to insufficient
coupling strength or other non-idealities. We show that such transducers can
generically be made perfect by using interference and phase-sensitive
amplification. Our approach is based on the realization that a particular kind
of imperfect transducer (one which implements a swapped quantum non-demolition
(QND) gate) can be made into a perfect one-way transducer using feed-forward

An application of quantum communications is the transmission of qubits to
create shared symmetric encryption keys in a process called Quantum Key
Distribution (QKD). Contrary to public-private key encryption, symmetric
encryption is safe from (quantum) computing attacks, i.e. it provides forward
security and is thus attractive for secure communications. In this paper we
argue that for free-space quantum communications, especially with satellites,
if one assumes that man-in-the-middle attacks can be detected by classical

The real stabilizer fragment of quantum mechanics was shown to have a
complete axiomatization in terms of the angle-free fragment of the ZX-calculus.
This fragment of the ZXcalculus--although abstractly elegant--is stated in
terms of identities, such as spider fusion which generally do not have
interpretations as circuit transformations. We complete the category CNOT
generated by the controlled not gate and the computational ancillary bits,
presented by circuit relations, to the real stabilizer fragment of quantum

The variational quantum eigensolver is one of the most promising approaches
for performing chemistry simulations using noisy intermediate-scale quantum
(NISQ) processors. The efficiency of this algorithm depends crucially on the
ability to prepare multi-qubit trial states on the quantum processor that
either include, or at least closely approximate, the actual energy eigenstates
of the problem being simulated while avoiding states that have little overlap

Monte Carlo simulations are performed in classical phase space for a
one-dimensional quantum harmonic crystal. Symmetrization effects for spinless
bosons and fermions are quantified. The algorithm is tested for a range of
parameters against exact results that use 20,000 energy levels. It is shown
that the singlet mean field approximation is very accurate at high
temperatures, and that the pair mean field approximation gives a systematic
improvement in the intermediate and low temperature regime. The latter is

Optical clocks based on atoms and ions achieve exceptional precision and
accuracy, with applications to relativistic geodesy, tests of relativity, and
searches for dark matter. Achieving such performance requires balancing
competing desirable features, including a high particle number, isolation of
atoms from collisions, insensitivity to motional effects, and high duty-cycle
operation. Here we demonstrate a new platform based on arrays of ultracold
strontium atoms confined within optical tweezers that realizes a novel

We describe how an ensemble of four-level atoms in the diamond-type
configuration can be applied to create a fully controllable effective coupling
between two cavity modes. The diamond-type configuration allows one to use a
bimodal cavity that supports modes of different frequencies or different
circular polarisations, because each mode is coupled only to its own
transition. This system can be used for mapping a quantum state of one cavity
mode onto the other mode on demand. Additionally, it can serve as a fast

In the past decades, quantum plasmonics has become an active area due to its
potential applications in on-chip plasmonic devices for quantum information
processing. However, the fundamental physical process, i.e., how a quantum
state of light evolves in the photon-plasmon conversion process, has not been
clearly understood. Here, we report a complete characterization of the
plasmon-assisted extraordinary optical transmission process through quantum
process tomography. By inputting various coherent states to interact with the

A device-independent dimension test for a Bell experiment aims to estimate
the underlying Hilbert space dimension that is required to produce given
measurement statistical data without any other assumptions concerning the
quantum apparatus. Previous work mostly deals with the two-party version of
this problem. In this paper, we propose a very general and robust approach to
test the dimension of any subsystem in a multiparty Bell experiment. Our
dimension test stems from the study of a new multiparty scenario which we call

Hybrid quantum systems have the potential of mitigating current challenges in
developing a scalable quantum computer. Of particular interest is the
hybridization between atomic and superconducting qubits. We demonstrate a novel
experimental setup for transferring and trapping ultracold atoms inside a
millikelvin cryogenic environment, where interactions between atomic and
superconducting qubits can be established, paving the way for hybrid quantum
systems. $^{87}\text{Rb}$ atoms are prepared in a conventional magneto-optical