462 research outputs found
Heating without heat: thermodynamics of passive energy filters between finite systems
Passive filters allowing the exchange of particles in a narrow band of energy
are currently used in micro-refrigerators and energy transducers. In this
letter, we analyze their thermal properties using linear irreversible
thermodynamics and kinetic theory, and discuss a striking phenomenon: the
possibility of increasing or decreasing simultaneously the temperatures of two
systems without any supply of energy. This occurs when the filter induces a
flow of particles whose energy is between the average energies of the two
systems. Here we show that this selective transfer of particles does not need
the action of any sort of Maxwell demon and can be carried out by passive
filters without compromising the second law of thermodynamics. The phenomenon
allows us to design cycles between two reservoirs at temperatures
that are able to reach temperatures below or above .Comment: 5 pages, 3 figure
Measuring the purity of a qubit state: entanglement estimation with fully separable measurements
Given a finite number of copies of a qubit state we compute the maximum
fidelity that can be attained using joint-measurement protocols for estimating
its purity. We prove that in the asymptotic limit,
separable-measurement protocols can be as efficient as the optimal
joint-measurement one if classical communication is used. This in turn shows
that the optimal estimation of the entanglement of a two-qubit state can also
be achieved asymptotically with fully separable measurements. The relationship
between our global Bayesian approach and the quantum Cramer-Rao bound is also
discussed.Comment: 5 pages, 1 figure, RevTeX, improved versio
Quantum reverse-engineering and reference frame alignment without non-local correlations
Estimation of unknown qubit elementary gates and alignment of reference
frames are formally the same problem. Using quantum states made out of
qubits, we show that the theoretical precision limit for both problems, which
behaves as , can be asymptotically attained with a covariant protocol
that exploits the quantum correlation of internal degrees of freedom instead of
the more fragile entanglement between distant parties. This cuts by half the
number of qubits needed to achieve the precision of the dense covariant coding
protocol
Phase-Covariant Quantum Benchmarks
We give a quantum benchmark for teleportation and quantum storage experiments
suited for pure and mixed test states. The benchmark is based on the average
fidelity over a family of phase-covariant states and certifies that an
experiment can not be emulated by a classical setup, i.e., by a
measure-and-prepare scheme. We give an analytical solution for qubits, which
shows important differences with standard state estimation approach, and
compute the value of the benchmark for coherent and squeezed states, both pure
and mixed.Comment: 4 pages, 2 figure
Recycling of quantum information: Multiple observations of quantum systems
Given a finite number of copies of an unknown qubit state that have already
been measured optimally, can one still extract any information about the
original unknown state? We give a positive answer to this question and quantify
the information obtainable by a given observer as a function of the number of
copies in the ensemble, and of the number of independent observers that, one
after the other, have independently measured the same ensemble of qubits before
him. The optimality of the protocol is proven and extensions to other states
and encodings are also studied. According to the general lore, the state after
a measurement has no information about the state before the measurement. Our
results manifestly show that this statement has to be taken with a grain of
salt, specially in situations where the quantum states encode confidential
information.Comment: 4 page
Multi-copy programmable discrimination of general qubit states
Quantum state discrimination is a fundamental primitive in quantum statistics
where one has to correctly identify the state of a system that is in one of two
possible known states. A programmable discrimination machine performs this task
when the pair of possible states is not a priori known, but instead the two
possible states are provided through two respective program ports. We study
optimal programmable discrimination machines for general qubit states when
several copies of states are available in the data or program ports. Two
scenarios are considered: one in which the purity of the possible states is a
priori known, and the fully universal one where the machine operates over
generic mixed states of unknown purity. We find analytical results for both,
the unambiguous and minimum error, discrimination strategies. This allows us to
calculate the asymptotic performance of programmable discrimination machines
when a large number of copies is provided, and to recover the standard state
discrimination and state comparison values as different limiting cases.Comment: Based on version published in Physical Review A, some errors in
appendix A corrected. 13 pages, 4 figure
Separable Measurement Estimation of Density Matrices and its Fidelity Gap with Collective Protocols
We show that there exists a gap between the performance of separable and
collective measurements in qubit mixed-state estimation that persists in the
large sample limit. We characterize such gap in terms of the corresponding
bounds on the mean fidelity. We present an adaptive protocol that attains the
separable-measurement bound. This (optimal separable) protocol uses von Neumann
measurements and can be easily implemented with current technology.Comment: version published in PR
- âŠ