17 research outputs found
Perturbative operator approach to high-precision light-pulse atom interferometry
Light-pulse atom interferometers are powerful quantum sensors, however, their
accuracy for example in tests of the weak equivalence principle is limited by
various spurious influences like magnetic stray fields or blackbody radiation.
Pushing the accuracy therefore requires a detailed assessment of the size of
such deleterious effects. Here, we present a systematic operator expansion to
obtain phase shifts and contrast analytically in powers of the perturbation.
The result can either be employed for robust straightforward order-of-magnitude
estimates or for rigorous calculations. Together with general conditions for
the validity of the approach, we provide a particularly useful formula for the
phase including wave-packet effects
Universality-of-Clock-Rates Test using Atom Interferometry with Scaling
We propose a competitive quantum test of the universality of clock rates that
depends on the proper time of a freely-falling particle, scaling cubic with the
laboratory time. In contrast to current tests with fountain clocks, our
proposed atom-interferometric scheme can be made robust against initial
conditions and recoil effects, making optical frequencies accessible even for
long interferometer durations. We study the influence of parasitic effects and
discuss implementations with strontium isotopes that may even outperform
current tests with fountain clocks.Comment: 9 pages, 3 figures, 1 tabl
Cutting multi-control quantum gates with ZX calculus
Circuit cutting, the decomposition of a quantum circuit into independent
partitions, has become a promising avenue towards experiments with larger
quantum circuits in the noisy-intermediate scale quantum (NISQ) era. While
previous work focused on cutting qubit wires or two-qubit gates, in this work
we introduce a method for cutting multi-controlled Z gates. We construct a
decomposition and prove the upper bound on the associated
sampling overhead, where is the number of cuts in the circuit. This bound
is independent of the number of control qubits but can be further reduced to
for the special case of CCZ gates. Furthermore, we
evaluate our proposal on IBM hardware and experimentally show noise resilience
due to the strong reduction of CNOT gates in the cut circuits
A Survey on Quantum Reinforcement Learning
Quantum reinforcement learning is an emerging field at the intersection of
quantum computing and machine learning. While we intend to provide a broad
overview of the literature on quantum reinforcement learning (our
interpretation of this term will be clarified below), we put particular
emphasis on recent developments. With a focus on already available noisy
intermediate-scale quantum devices, these include variational quantum circuits
acting as function approximators in an otherwise classical reinforcement
learning setting. In addition, we survey quantum reinforcement learning
algorithms based on future fault-tolerant hardware, some of which come with a
provable quantum advantage. We provide both a birds-eye-view of the field, as
well as summaries and reviews for selected parts of the literature.Comment: 62 pages, 16 figure
Interference of Clocks: A Quantum Twin Paradox
The phase of matter waves depends on proper time and is therefore susceptible
to special-relativistic (kinematic) and gravitational time dilation (redshift).
Hence, it is conceivable that atom interferometers measure general-relativistic
time-dilation effects. In contrast to this intuition, we show that light-pulse
interferometers without internal transitions are not sensitive to gravitational
time dilation, whereas they can constitute a quantum version of the
special-relativistic twin paradox. We propose an interferometer geometry
isolating the effect that can be used for quantum-clock interferometry.Comment: 9 Pages, 2 Figure
Uncovering Instabilities in Variational-Quantum Deep Q-Networks
Deep Reinforcement Learning (RL) has considerably advanced over the past
decade. At the same time, state-of-the-art RL algorithms require a large
computational budget in terms of training time to converge. Recent work has
started to approach this problem through the lens of quantum computing, which
promises theoretical speed-ups for several traditionally hard tasks. In this
work, we examine a class of hybrid quantum-classical RL algorithms that we
collectively refer to as variational quantum deep Q-networks (VQ-DQN). We show
that VQ-DQN approaches are subject to instabilities that cause the learned
policy to diverge, study the extent to which this afflicts reproduciblity of
established results based on classical simulation, and perform systematic
experiments to identify potential explanations for the observed instabilities.
Additionally, and in contrast to most existing work on quantum reinforcement
learning, we execute RL algorithms on an actual quantum processing unit (an IBM
Quantum Device) and investigate differences in behaviour between simulated and
physical quantum systems that suffer from implementation deficiencies. Our
experiments show that, contrary to opposite claims in the literature, it cannot
be conclusively decided if known quantum approaches, even if simulated without
physical imperfections, can provide an advantage as compared to classical
approaches. Finally, we provide a robust, universal and well-tested
implementation of VQ-DQN as a reproducible testbed for future experiments.Comment: Authors Maja Franz, Lucas Wolf, Maniraman Periyasamy contributed
equally (name order randomised). To be published in the Journal of The
Franklin Institut
Atom-interferometric test of the universality of gravitational redshift and free fall
Light-pulse atom interferometers constitute powerful quantum sensors for
inertial forces. They are based on delocalised spatial superpositions and the
combination with internal transitions directly links them to atomic clocks.
Since classical tests of the gravitational redshift are based on a comparison
of two clocks localised at different positions under gravity, it is promising
to explore whether the aforementioned interferometers constitute a competitive
alternative for tests of general relativity. Here we present a specific
geometry which together with state transitions leads to a scheme that is
concurrently sensitive to both violations of the universality of free fall and
gravitational redshift, two premises of general relativity. The proposed
interferometer does not rely on a superposition of internal states, but merely
on transitions between them, and therefore generalises the concept of physical
atomic clocks and quantum-clock interferometry. An experimental realisation
seems feasible with already demonstrated techniques in state-of-the-art
facilities.Comment: 8 pages, 4 figure
Theoretical approach to high-precision atom interferometry
The wave properties of matter in quantum mechanics first postulated by de Broglie in
1923 as well as Einstein’s theory of general relativity have radically changed our
perception of the world at the beginning of the twentieth century. While each theory
is extremely successful and well tested within its range of validity, a unification of both
theories has so far resisted any attempt.
However, the advances in precision of modern matter-wave interferometers have paved
the way to designing experiments at the interface of gravity and quantum mechanics.
Indeed, quantum mechanical devices are on the brink of becoming sensitive enough to
challenge predictions of general relativity such as the weak equivalence principle or set
bounds on alternative gravitational theories.
Reaching sensitivities required for these experiments necessitates a careful assessment
of deleterious effects some of which might be atom-atom interactions or the influence
of the gravitational potential of the laboratory setup itself. Estimation of the size of
such effects calls for refined theoretical tools for the description of light-pulse atom
interferometry which is the subject of the present thesis
Reply to “Comment on ‘Perturbative operator approach to high-precision light-pulse atom interferometry’ ”
Recently, we introduced [C. Ufrecht and E. Giese, Phys. Rev. A 101, 053615 (2020)] a technique to calculate the phase of light-pulse atom interferometers caused by the presence of perturbation potentials and underlined its power by an illustrative example. In the preceding Comment [B. Dubetsky, Phys. Rev. A 102, 027301 (2020)], it was pointed out that other, less idealized situations could have been calculated as well. Our Reply emphasizes that our method is correct, the results from our example can be trivially generalized to other perturbations, and intricate effects of local environments can be even more prominent but also treated by our technique