63 research outputs found
Small quantum absorption refrigerator in the transient regime: time scales, enhanced cooling and entanglement
A small quantum absorption refrigerator, consisting of three qubits, is
discussed in the transient regime. We discuss time scales for coherent
dynamics, damping, and approach to the steady state, and we study cooling and
entanglement. We observe that cooling can be enhanced in the transient regime,
in the sense that lower temperatures can be achieved compared to the
steady-state regime. This is a consequence of coherent dynamics, but can occur
even when this dynamics is strongly damped by the dissipative thermal
environment, and we note that precise control over couplings or timing is not
needed to achieve enhanced cooling. We also show that the amount of
entanglement present in the refrigerator can be much larger in the transient
regime compared to the steady-state. These results are of relevance to future
implementations of quantum thermal machines.Comment: 8 pages, 5 figure
Bell scenarios with communication
Classical and quantum physics provide fundamentally different predictions
about experiments with separate observers that do not communicate, a phenomenon
known as quantum nonlocality. This insight is a key element of our present
understanding of quantum physics, and also enables a number of information
processing protocols with security beyond what is classically attainable.
Relaxing the pivotal assumption of no communication leads to new insights into
the nature quantum correlations, and may enable new applications where security
can be established under less strict assumptions. Here, we study such
relaxations where different forms of communication are allowed. We consider
communication of inputs, outputs, and of a message between the parties. Using
several measures, we study how much communication is required for classical
models to reproduce quantum or general no-signalling correlations, as well as
how quantum models can be augmented with classical communication to reproduce
no-signalling correlations.Comment: 12 pages, 3 figures. Includes a more detailed explanation of results
appearing in the appendix of arXiv:1411.4648 [quant-ph
Device-Independent Tests of Entropy
We show that the entropy of a message can be tested in a device-independent
way. Specifically, we consider a prepare-and-measure scenario with classical or
quantum communication, and develop two different methods for placing lower
bounds on the communication entropy, given observable data. The first method is
based on the framework of causal inference networks. The second technique,
based on convex optimization, shows that quantum communication provides an
advantage over classical, in the sense of requiring a lower entropy to
reproduce given data. These ideas may serve as a basis for novel applications
in device-independent quantum information processing
Testing nonlocality of a single photon without a shared reference frame
The question of testing the nonlocality of a single photon has raised much
debate over the last years. The controversy is intimately related to the issue
of providing a common reference frame for the observers to perform their local
measurements. Here we address this point by presenting a simple scheme for
demonstrating the nonlocality of a single photon which does not require a
shared reference frame. Specifically, Bell inequality violation can be obtained
with certainty with unaligned devices, even if the relative frame fluctuates
between each experimental run of the Bell test. Our scheme appears feasible
with current technology, and may simplify the realization of quantum
communication protocols based on single-photon entanglement.Comment: 5 pages, 3 figure
Fundamental limits on low-temperature quantum thermometry with finite resolution
While the ability to measure low temperatures accurately in quantum systems
is important in a wide range of experiments, the possibilities and the
fundamental limits of quantum thermometry are not yet fully understood
theoretically. Here we develop a general approach to low-temperature quantum
thermometry, taking into account restrictions arising not only from the sample
but also from the measurement process. We derive a fundamental bound on the
minimal uncertainty for any temperature measurement that has a finite
resolution. A similar bound can be obtained from the third law of
thermodynamics. Moreover, we identify a mechanism enabling sub-exponential
scaling, even in the regime of finite resolution. We illustrate this effect in
the case of thermometry on a fermionic tight-binding chain with access to only
two lattice sites, where we find a quadratic divergence of the uncertainty. We
also give illustrative examples of ideal quantum gases and a square-lattice
Ising model, highlighting the role of phase transitions.Comment: Published version. Main text: 12 pages, 5 figures; see also related
work by K. Hovhannisyan and L. A. Correa at arXiv:1712.0308
Bell tests for continuous variable systems using hybrid measurements and heralded amplifiers
We present Bell tests for optical continuous variable systems, combining both
hybrid measurements (i.e. measuring both particle and wave aspects of light)
and heralded amplifiers. We discuss two types of schemes, in which the
amplifier is located either at the source, or at the parties' laboratories. The
inclusion of amplifiers helps to reduce the detrimental effect of losses in the
setup. In particular, we show that the requirements in terms of detection
efficiency and transmission losses are significantly reduced, approaching the
experimentally accessible regime.Comment: 6 pages, 5 figure
Autonomous quantum thermal machine for generating steady-state entanglement
We discuss a simple quantum thermal machine for the generation of
steady-state entanglement between two interacting qubits. The machine is
autonomous in the sense that it uses only incoherent interactions with thermal
baths, but no source of coherence or external control. By weakly coupling the
qubits to thermal baths at different temperatures, inducing a heat current
through the system, steady-state entanglement is generated far from thermal
equilibrium. Finally, we discuss two possible implementations, using
superconducting flux qubits or a semiconductor double quantum dot. Experimental
prospects for steady-state entanglement are promising in both systems.Comment: 14 pages, 4 figure
A contextuality witness inspired by optimal state discrimination
Many protocols and tasks in quantum information science rely inherently on
the fundamental notion of contextuality to provide advantages over their
classical counterparts, and contextuality represents one of the main
differences between quantum and classical physics. In this work we present a
witness for preparation contextuality inspired by optimal two-state
discrimination. The main idea is based on finding the accessible averaged
success and error probabilities in both classical and quantum models. We can
then construct a noncontextuality inequality and associated witness which we
find to be robust against depolarising noise and loss in the form of
inconclusive events.Comment: 5 pages main text, 3 figures, 3 pages supplemental materia
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