557 research outputs found
Measurement-Assisted Quantum Communication in Spin Channels with Dephasing
We propose a protocol for countering the effects of dephasing in quantum
state transfer over a noisy spin channel weakly coupled to the sender and
receiver qubits. Our protocol, based on performing regular global measurements
on the channel, significantly suppresses the nocuous environmental effects and
offers much higher fidelities than the traditional no-measurement approach. Our
proposal can also operate as a robust two-qubit entangling gate over distant
spins. Our scheme counters any source of dephasing, including those for which
the well established dynamical decoupling approach fails. Our protocol is
probabilistic, given the intrinsic randomness in quantum measurements, but its
success probability can be maximized by adequately tuning the rate of the
measurements.Comment: 8 pages, 9 figure
Topologically protected quantization of work
The transport of a particle in the presence of a potential that changes
periodically in space and in time can be characterized by the amount of work
needed to shift a particle by a single spatial period of the potential. In
general, this amount of work, when averaged over a single temporal period of
the potential, can take any value in a continuous fashion. Here we present a
topological effect inducing the quantization of the average work. We find that
this work is equal to the first Chern number calculated in a unit cell of a
space-time lattice. Hence, this quantization of the average work is
topologically protected. We illustrate this phenomenon with the example of an
atom whose center of mass motion is coupled to its internal degrees of freedom
by electromagnetic waves.Comment: 10 pages (including Supplemental Material), 1 figure, 1 table; closer
to published versio
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
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
- …