128 research outputs found
Minimising the heat dissipation of quantum information erasure
Quantum state engineering and quantum computation rely on information erasure
procedures that, up to some fidelity, prepare a quantum object in a pure state.
Such processes occur within Landauer's framework if they rely on an interaction
between the object and a thermal reservoir. Landauer's principle dictates that
this must dissipate a minimum quantity of heat, proportional to the entropy
reduction that is incurred by the object, to the thermal reservoir. However,
this lower bound is only reachable for some specific physical situations, and
it is not necessarily achievable for any given reservoir. The main task of our
work can be stated as the minimisation of heat dissipation given probabilistic
information erasure, i.e., minimising the amount of energy transferred to the
thermal reservoir as heat if we require that the probability of preparing the
object in a specific pure state be no smaller than
. Here is the maximum
probability of information erasure that is permissible by the physical context,
and the error. To determine the achievable minimal heat
dissipation of quantum information erasure within a given physical context, we
explicitly optimise over all possible unitary operators that act on the
composite system of object and reservoir. Specifically, we characterise the
equivalence class of such optimal unitary operators, using tools from
majorisation theory, when we are restricted to finite-dimensional Hilbert
spaces. Furthermore, we discuss how pure state preparation processes could be
achieved with a smaller heat cost than Landauer's limit, by operating outside
of Landauer's framework
Symmetry-enhanced supertransfer of delocalized quantum states
Coherent hopping of excitation rely on quantum coherence over physically
extended states. In this work, we consider simple models to examine the effect
of symmetries of delocalized multi-excitation states on the dynamical
timescales, including hopping rates, radiative decay, and environmental
interactions. While the decoherence (pure dephasing) rate of an extended state
over N sites is comparable to that of a non-extended state, superradiance leads
to a factor of N enhancement in decay and absorption rates. In addition to
superradiance, we illustrate how the multi-excitonic states exhibit
`supertransfer' in the far-field regime: hopping from a symmetrized state over
N sites to a symmetrized state over M sites at a rate proportional to MN. We
argue that such symmetries could play an operational role in physical systems
based on the competition between symmetry-enhanced interactions and localized
inhomogeneities and environmental interactions that destroy symmetry. As an
example, we propose that supertransfer and coherent hopping play a role in
recent observations of anomolously long diffusion lengths in nano-engineered
assembly of light-harvesting complexes.Comment: 6 page
Environment-assisted analog quantum search
Two main obstacles for observing quantum advantage in noisy
intermediate-scale quantum computers (NISQ) are the finite precision effects
due to control errors, or disorders, and decoherence effects due to thermal
fluctuations. It has been shown that dissipative quantum computation is
possible in presence of an idealized fully-engineered bath. However, it is not
clear, in general, what performance can be achieved by NISQ when internal bath
degrees of freedom are not controllable. In this work, we consider the task of
quantum search of a marked node on a complete graph of nodes in the
presence of both static disorder and non-zero coupling to an environment. We
show that, given fixed and finite levels of disorder and thermal fluctuations,
there is an optimal range of bath temperatures that can significantly improve
the success probability of the algorithm. Remarkably for a fixed disorder
strength , the system relaxation time decreases for higher temperatures
within a robust range of parameters. In particular, we demonstrate that for
strong disorder, the presence of a thermal bath increases the success
probability from to at least . While the asymptotic
running time is approximately maintained, the need to repeat the algorithm many
times and issues associated with unitary over-rotations can be avoided as the
system relaxes to an absorbing steady state. Furthermore, we discuss for what
regimes of disorder and bath parameters quantum speedup is possible and mention
conditions for which similar phenomena can be observed in more general families
of graphs. Our work highlights that in the presence of static disorder, even
non-engineered environmental interactions can be beneficial for a quantum
algorithm
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