8,285 research outputs found
Superior memory efficiency of quantum devices for the simulation of continuous-time stochastic processes
Continuous-time stochastic processes pervade everyday experience, and the
simulation of models of these processes is of great utility. Classical models
of systems operating in continuous-time must typically track an unbounded
amount of information about past behaviour, even for relatively simple models,
enforcing limits on precision due to the finite memory of the machine. However,
quantum machines can require less information about the past than even their
optimal classical counterparts to simulate the future of discrete-time
processes, and we demonstrate that this advantage extends to the
continuous-time regime. Moreover, we show that this reduction in the memory
requirement can be unboundedly large, allowing for arbitrary precision even
with a finite quantum memory. We provide a systematic method for finding
superior quantum constructions, and a protocol for analogue simulation of
continuous-time renewal processes with a quantum machine.Comment: 13 pages, 8 figures, title changed from original versio
Continuous variable qumodes as non-destructive probes of quantum systems
With the rise of quantum technologies, it is necessary to have practical and
preferably non-destructive methods to measure and read-out from such devices. A
current line of research towards this has focussed on the use of ancilla
systems which couple to the system under investigation, and through their
interaction, enable properties of the primary system to be imprinted onto and
inferred from the ancillae. We propose the use of continuous variable qumodes
as ancillary probes, and show that the interaction Hamiltonian can be fully
characterised and directly sampled from measurements of the qumode alone. We
suggest how such probes may also be used to determine thermodynamical
properties, including reconstruction of the partition function. We show that
the method is robust to realistic experimental imperfections such as
finite-sized measurement bins and squeezing, and discuss how such probes are
already feasible with current experimental setups.Comment: 8 pages, 3 figure
Multipartite Entangled Spatial Modes of Ultracold Atoms Generated and Controlled by Quantum Measurement
We show that the effect of measurement back-action results in the generation
of multiple many-body spatial modes of ultracold atoms trapped in an optical
lattice, when scattered light is detected. The multipartite mode entanglement
properties and their nontrivial spatial overlap can be varied by tuning the
optical geometry in a single setup. This can be used to engineer quantum states
and dynamics of matter fields. We provide examples of multimode generalizations
of parametric down-conversion, Dicke, and other states, investigate the
entanglement properties of such states, and show how they can be transformed
into a class of generalized squeezed states. Further, we propose how these
modes can be used to detect and measure entanglement in quantum gases.Comment: 6 Pages, 3 Figures, Supplemental Material include
Brain Imaging Correlates of Cognitive Impairment in Depression
This review briefly summarises recent research on the neural basis of cognition in depression. Two broad areas are covered: emotional and non-emotional processing. We consider how research findings support models of depression based on disrupted cortico-limbic circuitry, and how modern connectivity analysis techniques can be used to test such models explicitly. Finally we discuss clinical implications of cognitive imaging in depression, and specifically the possible role for these techniques in diagnosis and treatment planning
Optimal stochastic modelling with unitary quantum dynamics
Identifying and extracting the past information relevant to the future
behaviour of stochastic processes is a central task in the quantitative
sciences. Quantum models offer a promising approach to this, allowing for
accurate simulation of future trajectories whilst using less past information
than any classical counterpart. Here we introduce a class of phase-enhanced
quantum models, representing the most general means of causal simulation with a
unitary quantum circuit. We show that the resulting constructions can display
advantages over previous state-of-art methods - both in the amount of
information they need to store about the past, and in the minimal memory
dimension they require to store this information. Moreover, we find that these
two features are generally competing factors in optimisation - leading to an
ambiguity in what constitutes the optimal model - a phenomenon that does not
manifest classically. Our results thus simultaneously offer new quantum
advantages for stochastic simulation, and illustrate further qualitative
differences in behaviour between classical and quantum notions of complexity.Comment: 9 pages, 5 figure
Universal Quantum Viscosity in a Unitary Fermi Gas
A Fermi gas of atoms with resonant interactions is predicted to obey
universal hydrodynamics, where the shear viscosity and other transport
coefficients are universal functions of the density and temperature. At low
temperatures, the viscosity has a universal quantum scale where
is the density, while at high temperatures the natural scale is
where is the thermal momentum. We employ breathing mode damping to
measure the shear viscosity at low temperature. At high temperature , we
employ anisotropic expansion of the cloud to find the viscosity, which exhibits
precise scaling. In both experiments, universal hydrodynamic
equations including friction and heating are used to extract the viscosity. We
estimate the ratio of the shear viscosity to the entropy density and compare to
that of a perfect fluid.Comment: 13 pages, 3 figure
Surveying structural complexity in quantum many-body systems
Quantum many-body systems exhibit a rich and diverse range of exotic
behaviours, owing to their underlying non-classical structure. These systems
present a deep structure beyond those that can be captured by measures of
correlation and entanglement alone. Using tools from complexity science, we
characterise such structure. We investigate the structural complexities that
can be found within the patterns that manifest from the observational data of
these systems. In particular, using two prototypical quantum many-body systems
as test cases - the one-dimensional quantum Ising and Bose-Hubbard models - we
explore how different information-theoretic measures of complexity are able to
identify different features of such patterns. This work furthers the
understanding of fully-quantum notions of structure and complexity in quantum
systems and dynamics.Comment: 9 pages, 5 figure
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