122 research outputs found
Quantum steering: a review with focus on semidefinite programming
Quantum steering refers to the non-classical correlations that can be
observed between the outcomes of measurements applied on half of an entangled
state and the resulting post-measured states that are left with the other
party. From an operational point of view, a steering test can be seen as an
entanglement test where one of the parties performs uncharacterised
measurements. Thus, quantum steering is a form of quantum inseparability that
lies in between the well-known notions of Bell nonlocality and entanglement.
Moreover, quantum steering is also related to several asymmetric quantum
information protocols where some of the parties are considered untrusted.
Because of these facts, quantum steering has received a lot of attention both
theoretically and experimentally. The main goal of this review is to give an
overview of how to characterise quantum steering through semidefinite
programming. This characterisation provides efficient numerical methods to
address a number of problems, including steering detection, quantification, and
applications. We also give a brief overview of some important results that are
not directly related to semidefinite programming. Finally, we make available a
collection of semidefinite programming codes that can be used to study the
topics discussed in this articleComment: v2: 31 pages, 2 figures. Published version. New material added.
Matlab codes to accompany this review can be found at https://git.io/vax9
The smallest refrigerators can reach maximal efficiency
We investigate whether size imposes a fundamental constraint on the
efficiency of small thermal machines. We analyse in detail a model of a small
self-contained refrigerator consisting of three qubits. We show analytically
that this system can reach the Carnot efficiency, thus demonstrating that there
exists no complementarity between size and efficiency.Comment: 9 pages, 1 figure. v2: published versio
Quantum refrigerator driven by current noise
We proposed a scheme to implement a self-contained quantum refrigerator
system composed of three rf-SQUID qubits, or rather, flux-biased phase qubits.
The three qubits play the roles of the target, the refrigerator and the heat
engine respectively. We provide different effective temperatures for the three
qubits, by imposing external current noises of different strengths. The
differences of effective temperatures give rise to the flow of free energy and
that drives the refrigerator system to cool down the target. We also show that
the efficiency of the system approaches the Carnot efficiency.Comment: 5 pages, 1 figur
Multi-port beamsplitters based on multi-core optical fibers for high-dimensional quantum information
Platelet aggregation and risk of stent thrombosis or bleeding in interventionally treated diabetic patients with acute coronary syndrome
Strong nonlocality: A trade-off between states and measurements
Measurements on entangled quantum states can produce outcomes that are
nonlocally correlated. But according to Tsirelson's theorem, there is a
quantitative limit on quantum nonlocality. It is interesting to explore what
would happen if Tsirelson's bound were violated. To this end, we consider a
model that allows arbitrary nonlocal correlations, colloquially referred to as
"box world". We show that while box world allows more highly entangled states
than quantum theory, measurements in box world are rather limited. As a
consequence there is no entanglement swapping, teleportation or dense coding.Comment: 11 pages, 2 figures, very minor change
Experimental multipartite entanglement and randomness certification of the W state in the quantum steering scenario
Recently [Cavalcanti \textit{et al.} Nat Commun \textbf{6}, 7941 (2015)]
proposed a method to certify the presence of entanglement in asymmetric
networks, where some users do not have control over the measurements they are
performing. Such asymmetry naturally emerges in realistic situtations, such as
in cryptographic protocols over quantum networks. Here we implement such
"semi-device independent" techniques to experimentally witness all types of
entanglement on a three-qubit photonic W state. Furthermore we analise the
amount of genuine randomness that can be certified in this scenario from any
bipartition of the three-qubit W state.Comment: v2: text improved, steering witness redefined so that their violation
provides the steering robustness, experimental data included in an appendi
On defining the Hamiltonian beyond quantum theory
Energy is a crucial concept within classical and quantum physics. An
essential tool to quantify energy is the Hamiltonian. Here, we consider how to
define a Hamiltonian in general probabilistic theories, a framework in which
quantum theory is a special case. We list desiderata which the definition
should meet. For 3-dimensional systems, we provide a fully-defined recipe which
satisfies these desiderata. We discuss the higher dimensional case where some
freedom of choice is left remaining. We apply the definition to example toy
theories, and discuss how the quantum notion of time evolution as a phase
between energy eigenstates generalises to other theories.Comment: Authors' accepted manuscript for inclusion in the Foundations of
Physics topical collection on Foundational Aspects of Quantum Informatio
Thermodynamic principles and implementations of quantum machines
The efficiency of cyclic heat engines is limited by the Carnot bound. This
bound follows from the second law of thermodynamics and is attained by engines
that operate between two thermal baths under the reversibility condition
whereby the total entropy does not increase. By contrast, the efficiency of
engines powered by quantum non-thermal baths has been claimed to surpass the
thermodynamic Carnot bound. The key to understanding the performance of such
engines is a proper division of the energy supplied by the bath to the system
into heat and work, depending on the associated change in the system entropy
and ergotropy. Due to their hybrid character, the efficiency bound for quantum
engines powered by a non-thermal bath does not solely follow from the laws of
thermodynamics. Hence, the thermodynamic Carnot bound is inapplicable to such
hybrid engines. Yet, they do not violate the principles of thermodynamics.
An alternative means of boosting machine performance is the concept of
heat-to-work conversion catalysis by quantum non-linear (squeezed) pumping of
the piston mode. This enhancement is due to the increased ability of the
squeezed piston to store ergotropy. Since the catalyzed machine is fueled by
thermal baths, it adheres to the Carnot bound.
We conclude by arguing that it is not quantumness per se that improves the
machine performance, but rather the properties of the baths, the working fluid
and the piston that boost the ergotropy and minimize the wasted heat in both
the input and the output.Comment: As a chapter of: F. Binder, L. A. Correa, C. Gogolin, J. Anders, and
G. Adesso (eds.), "Thermodynamics in the quantum regime - Recent Progress and
Outlook", (Springer International Publishing
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