41 research outputs found
Quantum Measurements, Energy Conservation and Quantum Clocks
We consider a spin chain extending from Alice to Bob with next neighbors
interactions, initially in its ground state. Assuming that Bob measures the
last spin of the chain, the energy of the spin chain has to increase, at least
on average, due to the measurement disturbance. Presumably, the energy is
provided by Bob's measurement apparatus. Assuming now that, simultaneously to
Bob's measurement, Alice measures the first spin, we show that either energy is
not conserved, - implausible - or the projection postulate doesn't apply, and
that there is signalling. An explicit measurement model shows that energy is
conserved (as expected), but that the spin chain energy increase is not
provided by the measurement apparatus(es), that the projection postulate is not
always valid - illustrating the Wigner-Araki-Yanase (WAY) theorem - and that
there is signalling, indeed. The signalling is due to the non-local interaction
Hamiltonian. This raises the question of a suitable quantum information
inspired model of such non-local Hamiltonians.Comment: 7 pages + appendices, 6 figure
Tight Bell inequalities from polytope slices
We derive new tight bipartite Bell inequalities for various scenarios. A
bipartite Bell scenario is defined by the numbers of settings and
outcomes per party, , and , for Alice and Bob, respectively. We
derive the complete set of facets of the local polytopes of ,
, , and . We provide extensive lists of facets
for , and . For each inequality we compute
the maximum quantum violation, the resistance to noise, and the minimal
symmetric detection efficiency required to close the detection loophole, for
qubits, qutrits and ququarts. Based on these results, we identify scenarios
which perform better in terms of visibility, resistance to noise, or both, when
compared to CHSH. Such scenarios could find important applications in quantum
communication
High precision measurement of the Dzyaloshinsky-Moriya interaction between two rare-earth ions in a solid
We report on a direct measurement of the pair-wise anti-symmetric exchange
interaction, known as the Dzyaloshinsky-Moriya interaction (DMI), in a
Nd3+-doped YVO4 crystal. To this end we introduce a broadband electron spin
resonance technique coupled with an optical detection scheme which selectively
detects only one Nd3+-Nd3+ pair. Using this technique we can fully determine
the spin-spin coupling tensor, allowing us to experimentally determine both the
strength and direction of the DMI vector. We believe that this ability to fully
determine the interaction Hamiltonian is of interest for studying the numerous
magnetic phenomena where the DMI interaction is of fundamental importance,
including multiferroics. We also detect a singlet-triplet transition within the
pair, with a highly suppressed magnetic-field dependence, which suggests that
such systems could form singlet-triplet qubits with long coherence times for
quantum information applications
Semi-device-independent quantum key distribution based on a coherence equality
We introduce the first example of a semi-device-independent quantum key
distribution (SDI-QKD) protocol with a classical Alice and Bob. The protocol is
based on the Coherence Equality (CE) game recently introduced by del Santo and
Daki\'c, which verifies a coherent quantum superposition of communication
trajectories in a de-localized way. We show the protocol to be
semi-device-independent since the only trusted operations occur in the users'
labs, and establish security against an adversary with bounded quantum memory.
Finally, we recast the setup of the protocol as a counterfactual test of
nonlocality, and provide additional insight into the CE game.Comment: 18 pages, 6 figure
Characterising correlations under informational restrictions
The strength of correlations observed between two separated events hinges on
the amount of information transmitted between them. We characterise the
correlations that can be created in classical and quantum experiments which
feature a given amount of communicated information. For classical models, we
present a complete characterisation of informationally restricted correlations
in terms of linear programming. For quantum models, we develop a hierarchy of
increasingly precise semidefinite relaxations to bound the set of
informationally restricted quantum correlations. We leverage these techniques
to i) derive device-independent witnesses of the information content of quantum
communication, ii) the derivation of strict inequalities for different quantum
information resources and iii) a new avenue for semi-device-independent random
number generation based on the information assumption.Comment: First versio
Third law of thermodynamics and the scaling of quantum computers
The third law of thermodynamics, also known as the Nernst unattainability
principle, puts a fundamental bound on how close a system, whether classical or
quantum, can be cooled to a temperature near to absolute zero. On the other
hand, a fundamental assumption of quantum computing is to start each
computation from a register of qubits initialized in a pure state, i.e., at
zero temperature. These conflicting aspects, at the interface between quantum
computing and thermodynamics, are often overlooked or, at best, addressed only
at a single-qubit level. In this work, we argue how the existence of a small,
but finite, effective temperature, which makes the initial state a mixed state,
poses a real challenge to the fidelity constraints required for the scaling of
quantum computers. Our theoretical results, carried out for a generic quantum
circuit with -qubit input states, are validated by test runs performed on a
real quantum processor.Comment: 9 pages, 5 figure
Quantum-inspired classification based on quantum state discrimination
We present quantum-inspired algorithms for classification tasks inspired by
the problem of quantum state discrimination. By construction, these algorithms
can perform multiclass classification, prevent overfitting, and generate
probability outputs. While they could be implemented on a quantum computer, we
focus here on classical implementations of such algorithms. The training of
these classifiers involves Semi-Definite Programming. We also present a
relaxation of these classifiers that utilizes Linear Programming (but that can
no longer be interpreted as a quantum measurement). Additionally, we consider a
classifier based on the Pretty Good Measurement (PGM) and show how to implement
it using an analogue of the so-called Kernel Trick, which allows us to study
its performance on any number of copies of the input state. We evaluate these
classifiers on the MNIST and MNIST-1D datasets and find that the PGM generally
outperforms the other quantum-inspired classifiers and performs comparably to
standard classifiers.Comment: 19 pages, 4 figure
Efficient optical pumping using hyperfine levels in Nd:YSiO and its application to optical storage
Efficient optical pumping is an important tool for state initialization in
quantum technologies, such as optical quantum memories. In crystals doped with
Kramers rare-earth ions, such as erbium and neodymium, efficient optical
pumping is challenging due to the relatively short population lifetimes of the
electronic Zeeman levels, of the order of 100 ms at around 4 K. In this article
we show that optical pumping of the hyperfine levels in isotopically enriched
Nd:YSiO crystals is more efficient, owing to the longer
population relaxation times of hyperfine levels. By optically cycling the
population many times through the excited state a nuclear-spin flip can be
forced in the ground-state hyperfine manifold, in which case the population is
trapped for several seconds before relaxing back to the pumped hyperfine level.
To demonstrate the effectiveness of this approach in applications we perform an
atomic frequency comb memory experiment with 33% storage efficiency in
Nd:YSiO, which is on a par with results obtained in
non-Kramers ions, e.g. europium and praseodymium, where optical pumping is
generally efficient due to the quenched electronic spin. Efficient optical
pumping in neodymium-doped crystals is also of interest for spectral filtering
in biomedical imaging, as neodymium has an absorption wavelength compatible
with tissue imaging. In addition to these applications, our study is of
interest for understanding spin dynamics in Kramers ions with nuclear spin.Comment: 8 pages, 6 figure