196 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
A practical, unitary simulator for non-Markovian complex processes
Stochastic processes are as ubiquitous throughout the quantitative sciences
as they are notorious for being difficult to simulate and predict. In this
letter we propose a unitary quantum simulator for discrete-time stochastic
processes which requires less internal memory than any classical analogue
throughout the simulation. The simulator's internal memory requirements equal
those of the best previous quantum models. However, in contrast to previous
models it only requires a (small) finite-dimensional Hilbert space. Moreover,
since the simulator operates unitarily throughout, it avoids any unnecessary
information loss. We provide a stepwise construction for simulators for a large
class of stochastic processes hence directly opening the possibility for
experimental implementations with current platforms for quantum computation.
The results are illustrated for an example process.Comment: 12 pages, 5 figure
Unified framework for quantumness -- coherence, discord, and entanglement
From an operational perspective, quantumness characterizes the exotic
behavior in a physical process which cannot be explained with Newtonian
physics. There are several widely used measures of quantumness, including
coherence, discord, and entanglement, each proven to be essential resources in
particular situations. There exists evidence of fundamental connections amongst
the three measures. However, those quantumnesses are still regarded differently
and such connections are yet to be elucidated. Here, we introduce a general
framework of defining a unified quantumness with an operational motivation
founded on the capability of interferometry. The quantumness appears
differently as coherence, discord, and entanglement in different scenarios with
local measurement, weak reference frame free measurement, and strong reference
frame free measurement, respectively. Our results also elaborate how these
three measures are related and how they can be transformed from each other.
This framework can be further extended to other scenarios and serves as a
universal quantumness measure.Comment: 9 pages, 4 figure
Unconditional preparation of entanglement between atoms in cascaded optical cavities
We propose a scheme to unconditionally entangle the internal states of atoms
trapped in separate high finesse optical cavities. The scheme uses the
technique of quantum reservoir engineering in a cascaded cavity QED setting,
and for ideal (lossless) coupling between the cavities generates an entangled
pure state. Highly entangled states are also shown to be possible for
realizable cavity QED parameters and with nonideal coupling.Comment: 4 pages, 5 figures, submitted to Physical Revie
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
Optimal classical simulation of state-independent quantum contextuality
Simulating quantum contextuality with classical systems requires memory. A
fundamental yet open question is what is the minimum memory needed and,
therefore, the precise sense in which quantum systems outperform classical
ones. Here, we make rigorous the notion of classically simulating quantum
state-independent contextuality (QSIC) in the case of a single quantum system
submitted to an infinite sequence of measurements randomly chosen from a finite
QSIC set. We obtain the minimum memory needed to simulate arbitrary QSIC sets
via classical systems under the assumption that the simulation should not
contain any oracular information. In particular, we show that, while
classically simulating two qubits tested with the Peres-Mermin set requires
bits, simulating a single qutrit tested with the
Yu-Oh set requires, at least, bits.Comment: 7 pages, 4 figure
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