2,648 research outputs found
Unitarity and fuzzball complementarity: "Alice fuzzes but may not even know it!"
We investigate the recent black hole firewall argument. For a black hole in a
typical state we argue that unitarity requires every quantum of radiation
leaving the black hole to carry information about the initial state. An
information-free horizon is thus inconsistent with unitary at every step of the
evaporation process (in particular both before and after Page time). The
required horizon-scale structure is manifest in the fuzzball proposal which
provides a mechanism for holding up the structure. In this context we want to
address the experience of an infalling observer and discuss the recent fuzzball
complementarity proposal. Unlike black hole complementarity and observer
complementarity which postulate asymptotic observers experience a hot membrane
while infalling ones pass freely through the horizon, fuzzball complementarity
postulates that fine-grained operators experience the details of the fuzzball
microstate and coarse-grained operators experience the black hole. In
particular, this implies that an infalling detector tuned to energy E ~ T,
where T is the asymptotic Hawking temperature, does not experience free infall
while one tuned to E >> T does.Comment: v3: 33 pages + citations, 8 figures, version accepted for publicatio
Non-equilibrium readiness and accuracy of Gaussian Quantum Thermometers
The dimensionality of a thermometer is key in the design of quantum
thermometry schemes. In general, the phenomenology that is typical of
finite-dimensional quantum thermometry does not apply to infinite dimensional
ones. We analyse the dynamical and metrological features of non-equilibrium
Gaussian Quantum Thermometers: on one hand, we highlight how quantum
entanglement can enhance the readiness of composite Gaussian thermometers; on
the other hand, we show that non-equilibrium conditions do not guarantee the
best sensitivities in temperature estimation, thus suggesting the reassessment
of the working principles of quantum thermometry
Toward quantum simulations of biological information flow
Recent advances in the spectroscopy of biomolecules have highlighted the
possibility of quantum coherence playing an active role in biological energy
transport. The revelation that quantum coherence can survive in the hot and wet
environment of biology has generated a lively debate across both the physics
and biology communities. In particular, it remains unclear to what extent
non-trivial quantum effects are utilised in biology and what advantage, if any,
they afford. We propose an analogue quantum simulator, based on currently
available techniques in ultra-cold atom physics, to study a model of energy and
electron transport based on the Holstein Hamiltonian By simulating the salient
aspects of a biological system in a tunable laboratory setup, we hope to gain
insight into the validity of several theoretical models of biological quantum
transport in a variety of relevant parameter regimes.Comment: 8 Pages, 2 Figures, Non-technical contributing article for the
Interface Focus Theme Issue `Computability and the Turning centenary'.
Interface Focus
http://rsfs.royalsocietypublishing.org/content/early/2012/03/22/rsfs.2011.0109.shor
Numerical evaluation of the fidelity error threshold for the surface code
We study how the resilience of the surface code is affected by the coupling
to a non-Markovian environment at zero temperature. The qubits in the surface
code experience an effective dynamics due to the coupling to the environment
that induces correlations among them. The range of the effective induced
qubit-qubit interaction depends on parameters related to the environment and
the duration of the quantum error correction cycle. We show numerically that
different interaction ranges set different intrinsic bounds on the fidelity of
the code. These bounds are unrelated to the error thresholds based on
stochastic error models. We introduce a definition of stabilizers based on
logical operators that allows us to efficiently implement a Metropolis
algorithm to determine upper bounds to the fidelity error threshold
Probing nonlinear adiabatic paths with a universal integrator
We apply a flexible numerical integrator to the simulation of adiabatic
quantum computation with nonlinear paths. We find that a nonlinear path may
significantly improve the performance of adiabatic algorithms versus the
conventional straight-line interpolations. The employed integrator is suitable
for solving the time-dependent Schr\"odinger equation for any qubit
Hamiltonian. Its flexible storage format significantly reduces cost for storage
and matrix-vector multiplication in comparison to common sparse matrix schemes.Comment: 8 pages, 6 figure
Qubit thermometry for micromechanical resonators
We address estimation of temperature for a micromechanical oscillator lying
arbitrarily close to its quantum ground state. Motivated by recent experiments,
we assume that the oscillator is coupled to a probe qubit via Jaynes-Cummings
interaction and that the estimation of its effective temperature is achieved
via quantum limited measurements on the qubit. We first consider the ideal
unitary evolution in a noiseless environment and then take into account the
noise due to non dissipative decoherence. We exploit local quantum estimation
theory to assess and optimize the precision of estimation procedures based on
the measurement of qubit population, and to compare their performances with the
ultimate limit posed by quantum mechanics. In particular, we evaluate the
Fisher information (FI) for population measurement, maximize its value over the
possible qubit preparations and interaction times, and compare its behavior
with that of the quantum Fisher information (QFI). We found that the FI for
population measurement is equal to the QFI, i.e., population measurement is
optimal, for a suitable initial preparation of the qubit and a predictable
interaction time. The same configuration also corresponds to the maximum of the
QFI itself. Our results indicate that the achievement of the ultimate bound to
precision allowed by quantum mechanics is in the capabilities of the current
technology.Comment: 9 pages, 5 figures, revised version, to appear on PR
Achieving the Heisenberg limit in quantum metrology using quantum error correction
Quantum metrology has many important applications in science and technology,
ranging from frequency spectroscopy to gravitational wave detection. Quantum
mechanics imposes a fundamental limit on measurement precision, called the
Heisenberg limit, which can be achieved for noiseless quantum systems, but is
not achievable in general for systems subject to noise. Here we study how
measurement precision can be enhanced through quantum error correction, a
general method for protecting a quantum system from the damaging effects of
noise. We find a necessary and sufficient condition for achieving the
Heisenberg limit using quantum probes subject to Markovian noise, assuming that
noiseless ancilla systems are available, and that fast, accurate quantum
processing can be performed. When the sufficient condition is satisfied, a
quantum error-correcting code can be constructed which suppresses the noise
without obscuring the signal; the optimal code, achieving the best possible
precision, can be found by solving a semidefinite program.Comment: 16 pages, 2 figures, see also arXiv:1704.0628
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