Efficiently entangling pairs of qubits is essential to fully harness the

power of quantum computing. Here, we devise an exact protocol that

simultaneously entangles arbitrary pairs of qubits on a trapped-ion quantum

computer. The protocol requires classical computational resources polynomial in

the system size, and very little overhead in the quantum control compared to a

single-pair case. We demonstrate an exponential improvement in both classical

and quantum resources over the current state of the art. We implement the

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We consider a weakly-interacting Bose-Einstein condensate that rotates in

either a harmonic, or a weakly-anharmonic trapping potential. Performing

numerical calculations, we investigate the behaviour of the gas in these two

cases as the angular momentum, or equivalently as the rotational frequency of

the trap increases. While in the case of a purely-harmonic potential the gas

makes a transition from the mean-field regime to the correlated, "Laughlin",

regime, in the case of anharmonic confinement the mean-field approximation

Quantum computers are on the brink of surpassing the capabilities of even the

most powerful classical computers. This naturally raises the question of how

one can trust the results of a quantum computer when they cannot be compared to

classical simulation. Here we present a scalable verification technique that

exploits the principles of measurement-based quantum computing to link quantum

circuits of different input size, depth, and structure. Our approach enables

We have proposed in several recent papers a critical view of some parts of

quantum mechanics (QM) that is methodologically unusual because it rests on

analysing the language of QM by using some elementary but fundamental tools of

mathematical logic. Our approach proves that some widespread beliefs about QM

can be questioned and establishes new links with a classical view, which is

significant in the debate on the interpretations of QM. We propose here a brief

We develop an efficient variational approach to studying dynamics of a

localized quantum spin coupled to a bath of mobile spinful bosons. We use

parity symmetry to decouple the impurity spin from the environment via a

canonical transformation and reduce the problem to a model of the interacting

bosonic bath. We describe coherent time evolution of the latter using bosonic

Gaussian states as a variational ansatz. We provide full analytical expressions

for equations describing variational time evolution that can be applied to

Three-body recombination in quantum gases is traditionally associated with

heating, but it was recently found that it can also cool the gas. We show that

in a partially condensed three-dimensional homogeneous Bose gas three-body loss

could even purify the sample, that is, reduce the entropy per particle and

increase the condensed fraction $\eta$. We predict that the evolution of $\eta$

under continuous three-body loss can, depending on small changes in the initial

We propose a novel quantum diffraction imaging technique whereby one photon

of an entangled pair is diffracted off a sample and detected in coincidence

with its twin. The image is obtained by scanning the photon that did not

interact with matter. We show that when a dynamical quantum system interacts

with an external field, the phase information is imprinted in the state of the

field in a detectable way. The contribution to the signal from photons that

A remarkable aspect of quantum theory is that certain measurement outcomes

are entirely unpredictable to all possible observers. Such quantum events can

be harnessed to generate numbers whose randomness is asserted based upon the

underlying physical processes. We formally introduce and experimentally

demonstrate an ultrafast optical quantum randomness generator that uses a

totally untrusted photonic source. While considering completely general quantum

attacks, we certify randomness at a rate of $1.1\,\mathrm{Gbps}$ with a

Because quantum measurements have probabilistic outcomes they can seem to

violate conservation laws in individual experiments. Despite these appearances,

strict conservation of momentum, energy, and angular momentum can be shown to

be consistent with the assumption that the entangling interactions that

constitute measurements induce a real collapse of the wave function. The

essential idea is that measured systems always have some pre-existing

entanglement relations with (usually larger) systems, and that apparent changes

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.