Paul Skrzypczyk :
Research  Quantum nonlocality
My research interests cover a
few areas, namely
Quantum nonlocality: Trying to understand the nonlocal
correlations which are exhibited by measurement outcomes made on
entangled quantum states.
Thermodynamics of small systems: Understand what concepts from
thermodynamics mean, and how they look, in the quantum regime.
Quantum behaviour: Using the ideas of Modular variables, pre and
postselection and weak values to better understand systems exhibiting
truly quantum behaviour.
My work to date can be found on arXiv.org.
My supervisor is Sandu
Popescu.

Bound
nonlocality and activation
Nicolas
Brunner,
Daniel Cavalcanti, Alejo Salles, Paul Skrzypczyk
arXiv:1009.4207
We investigate nonlocality distillation using measures of nonlocality
based on the ElitzurPopescuRohrlich decomposition. For a certain
number of copies of a given nonlocal correlation, we define two
quantities of interest: (i) the nonlocal cost, and (ii) the
distillable nonlocality. We find that there exist correlations whose
distillable nonlocality is strictly smaller than their nonlocal cost.
Thus nonlocality displays a form of irreversibility which we term
bound nonlocality. Finally we show that nonlocal distillability can
be activated.


On
the efficiency of very small refrigerators
Paul
Skrzypczyk, Nicolas Brunner, Noah Linden, Sandu Popescu
arXiv:1009.0865
We investigate whether size imposes a fundamental constraint on the
efficiency of small thermal machines. We analyse in detail a model of a
small selfcontained refrigerator consisting of three qubits. We show
that this system can reach the Carnot efficiency, and thus demonstrate
that there exists no complementarity between size and
efficiency.


Physics
within a quantum reference frame
Renato M.
Angelo, Nicolas Brunner, Sandu Popescu, Antony J. Short, Paul Skrzypczyk
arXiv:1007.2292
We investigate the physics of quantum reference frames. Specifically,
we study several simple scenarios involving a small number of quantum
particles, whereby we promote one of these particles to the role of a
quantum observer and ask what is the description of the rest of the
system, as seen by this observer? We highlight the interesting aspects
of such questions by presenting a number of apparent paradoxes. By
unraveling these paradoxes we get a better understanding of the physics
of quantum reference frames.


How
small can thermal machines be? Towards the smallest possible
refrigerator
Noah Linden,
Sandu Popescu and Paul Skrzypczyk
Phys. Rev. Lett 105,
130401 (2010)
We investigate the fundamental dimensional limits to thermodynamic
machines. In particular, we show that it is possible to construct
selfcontained refrigerators (i.e., not requiring external sources of
work) consisting of only a small number of qubits and/or qutrits. We
present three different models, consisting of two qubits, a qubit and a
qutrit with nearestneighbor interactions, and a single qutrit,
respectively. We then investigate the fundamental limits to their
performance; in particular, we show that it is possible to cool towards
absolute zero.


Closed
sets of nonlocal correlations
Jonathan
Allcock, Nicolas Brunner, Noah Linden, Sandu Popescu, Paul Skrzypczyk
and Tamas Vertesi
Phys. Rev. A 80,
062107 (2009)
In this work we introduce a fundamental concept  closed sets of
correlations  for studying nonlocal correlations. We argue that sets
of correlations corresponding to informationtheoretic principles, or
more generally to consistent physical theories, must be closed under a
natural set of operations. Hence, studying the closure of sets of
correlations gives insight into which informationtheoretic principles
are genuinely different, and which are ultimately equivalent. This
concept also has implications for understanding why quantum
nonlocality is limited, and for finding constraints on physical
theories beyond quantum mechanics.


Nonlocality
distillation and postquantum theories with trivial
communication complexity
Nicolas
Brunner and Paul Skrzypczyk
Phys. Rev. Lett. 102,
160403 (2009)
Here we first present a protocol for deterministically distilling
nonlocality, building upon a recent result of Forster et al. [Phys.
Rev. Lett. 102,
120401 (2009)]. The protocol
works efficiently for a specific class of postquantum nonlocal boxes,
which we term correlated nonlocal boxes. In the asymptotic limit, all
correlated nonlocal boxes are distilled to the maximally nonlocal box
of Popescu and Rohrlich. Then, taking advantage of a result of
Brassard et al.
[Phys.
Rev. Lett. 96,
250401 (2006)] we show that all
correlated nonlocal boxes make communication complexity trivial, and
therefore appear very unlikely to exist in nature. Some of these
nonlocal boxes are arbitrarily close to the set of classical
correlations.


Couplers
for NonLocality Swapping
Paul
Skrzypczyk and Nicolas Brunner
New J. Phys. 11,
073014 (2009)
Studying generalized nonlocal theories brings insight to the
foundations of quantum mechanics. In this work we focus on nonlocality
swapping, the analogue of quantum entanglement swapping. In order to
implement such a protocol, one needs a coupler that performs the
equivalent of quantum joint measurements on generalized `boxlike'
states. By establishing a connection to Bell inequalities, we define
consistent couplers for theories containing an arbitrary amount of
nonlocality, which leads us to introduce the concepts of perfect and
minimal couplers. Surprisingly, Tsirelson's bound for quantum
nonlocality naturally appears in this study.


Emergence
of Quantum Correlations from NonLocality Swapping
Paul
Skrzypczyk, Nicolas Brunner and Sandu Popescu
Phys. Rev. Lett. 102,
110402 (2009)
By studying generalized nonsignalling theories, the hope is to find
out what makes quantum mechanics so special. In this work we revisit
the paradigmatic model of nonsignalling boxes and introduce the
concept of a genuine box. This allows us to present the first
generalized nonsignalling model featuring quantumlike dynamics. In
particular, we present the coupler, a device enabling nonlocality
swapping, the analogue of quantum entanglement swapping, as well as
teleportation. Interestingly, part of the boundary between quantum and
postquantum correlations emerges from this study.
