19 research outputs found
Multiple Observers Can Share the Nonlocality of Half of an Entangled Pair by Using Optimal Weak Measurements
We investigate the trade-off between information gain and disturbance for a
class of weak von Neumann measurements on spin- particles, and
derive the unusual measurement pointer state that saturates this trade-off. We
then consider the fundamental question of sharing the non-locality of a single
particle of an entangled pair among multiple observers, and demonstrate that by
exploiting the information gain disturbance trade-off, one can obtain an
arbitrarily long sequence of consecutive and independent violations of the
CHSH-Bell inequality.Comment: 16 pages, 6 figures, PRL versio
Adjusting inequalities for detection-loophole-free steering experiments
We study the problem of certifying quantum steering in a
detection-loophole-free manner in experimental situations that require
post-selection. We present a method to find the modified local-hidden-state
bound of steering inequalities in such a post-selected scenario. We then
present a construction of linear steering inequalities in arbitrary finite
dimension and show that they certify steering in a loophole-free manner as long
as the detection efficiencies are above the known bound below which steering
can never be demonstrated. We also show how our method extends to the scenarios
of multipartite steering and Bell nonlocality, in the general case where there
can be correlations between the losses of the different parties. In both cases
we present examples to demonstrate the techniques developed.Comment: 12 pages, 4 figure
Almost quantum correlations
There have been a number of attempts to derive the set of quantum non-local
correlations from reasonable physical principles. Here we introduce
, a set of multipartite supra-quantum correlations that has appeared
under different names in fields as diverse as graph theory, quantum gravity and
quantum information science. We argue that may correspond to the
set of correlations of a reasonable physical theory, in which case the research
program to reconstruct quantum theory from device-independent principles is met
with strong obstacles. In support of this conjecture, we prove that
is closed under classical operations and satisfies the physical principles of
Non-Trivial Communication Complexity, No Advantage for Nonlocal Computation,
Macroscopic Locality and Local Orthogonality. We also review numerical evidence
that almost quantum correlations satisfy Information Causality.Comment: 15+2 pages, 1 figur
Thermodynamics of quantum systems with multiple conserved quantities
We consider a generalisation of thermodynamics that deals with multiple
conserved quantities at the level of individual quantum systems. Each conserved
quantity, which, importantly, need not commute with the rest, can be extracted
and stored in its own battery. Unlike in standard thermodynamics, where the
second law places a constraint on how much of the conserved quantity (energy)
that can be extracted, here, on the contrary, there is no limit on how much of
any individual conserved quantity that can be extracted. However, other
conserved quantities must be supplied, and the second law constrains the
combination of extractable quantities and the trade-offs between them which are
allowed. We present explicit protocols which allow us to perform arbitrarily
good trade-offs and extract arbitrarily good combinations of conserved
quantities from individual quantum systems.Comment: 16 pages, 3 figure
Exploring the limits of no backward in time signalling
We present an operational and model-independent framework to investigate the
concept of no-backwards-in-time signaling. We define no-backwards-in-time
signaling conditions, closely related to the spatial no-signaling conditions.
These allow for theoretical possibilities in which the future affects the past,
nevertheless without signaling backwards in time. This is analogous to
non-local but no-signaling spatial correlations. Furthermore, our results shed
new light on situations with indefinite causal structure and their connection
to quantum theory.Comment: 10 pages, 3 figures, v2: reference adde
Pre- and post-selected quantum states: density matrices, tomography, and Kraus operators
We present a general formalism for charecterizing 2-time quantum states,
describing pre- and post-selected quantum systems. The most general 2-time
state is characterized by a `density vector' that is independent of
measurements performed between the preparation and post-selection. We provide a
method for performing tomography of an unknown 2-time density vector. This
procedure, which cannot be implemented by weak or projective measurements,
brings new insight to the fundamental role played by Kraus operators in quantum
measurements. Finally, after showing that general states and measurements are
isomorphic, we show that any measurement on a 2-time state can be mapped to a
measurement on a preselected bipartite state.Comment: 7 page
Quantum thermodynamics with multiple conserved quantities
In this chapter we address the topic of quantum thermodynamics in the
presence of additional observables beyond the energy of the system. In
particular we discuss the special role that the generalized Gibbs ensemble
plays in this theory, and derive this state from the perspectives of a
micro-canonical ensemble, dynamical typicality and a resource-theory
formulation. A notable obstacle occurs when some of the observables do not
commute, and so it is impossible for the observables to simultaneously take on
sharp microscopic values. We show how this can be circumvented, discuss
information-theoretic aspects of the setting, and explain how thermodynamic
costs can be traded between the different observables. Finally, we discuss open
problems and future directions for the topic.Comment: 18 pages, 3 figures; Chapter for the book "Thermodynamics in the
Quantum Regime - Recent Progress and Outlook", eds. F. Binder, L. A. Correa,
C. Gogolin, J. Anders and G. Adess