1,332 research outputs found
Certified randomness in quantum physics
The concept of randomness plays an important role in many disciplines. On one
hand, the question of whether random processes exist is fundamental for our
understanding of nature. On the other hand, randomness is a resource for
cryptography, algorithms and simulations. Standard methods for generating
randomness rely on assumptions on the devices that are difficult to meet in
practice. However, quantum technologies allow for new methods for generating
certified randomness. These methods are known as device-independent because do
not rely on any modeling of the devices. Here we review the efforts and
challenges to design device-independent randomness generators.Comment: 18 pages, 3 figure
On the structure of a reversible entanglement generating set for tripartite states
We show that Einstein–Podolsky–Rosen–Bohm (EPR) and Greenberger–Horne–Zeilinger–Mermin
(GHZ) states can not generate, through local manipulation and in the asymptotic limit, all forms of
three–partite pure–state entanglement in a reversible way. The techniques that we use suggest that
there may be a connection between this result and the irreversibility that occurs in the asymptotic
preparation and distillation of bipartite mixed states
Entanglement preserving local thermalization
We investigate whether entanglement can survive the thermalization of
subsystems. We present two equivalent formulations of this problem: (1) Can two
isolated agents, accessing only pre-shared randomness, locally thermalize
arbitrary input states while maintaining some entanglement? (2) Can
thermalization with local heat baths, which may be classically correlated but
do not exchange information, locally thermalize arbitrary input states while
maintaining some entanglement? We answer these questions in the positive at
every nonzero temperature and provide bounds on the amount of preserved
entanglement. We provide explicit protocols and discuss their thermodynamic
interpretation: we suggest that the underlying mechanism is a speed-up of the
subsystem thermalization process. We also present extensions to multipartite
systems. Our findings show that entanglement can survive locally performed
thermalization processes accessing only classical correlations as a resource.
They also suggest a broader study of the channel's ability to preserve
resources and of the compatibility between global and local dynamics.Comment: 6+7 pages, 1 figure, closed to the published versio
Optimal randomness certification in the quantum steering and prepare-and-measure scenarios
Quantum mechanics predicts the existence of intrinsically random processes.
Contrary to classical randomness, this lack of predictability can not be
attributed to ignorance or lack of control. Here we find the optimal method to
quantify the amount of local or global randomness that can be extracted in two
scenarios: (i) the quantum steering scenario, where two parties measure a
bipartite system in an unknown state but one of them does not trust his
measurement apparatus, and (ii) the prepare-and-measure scenario, where
additionally the quantum state is known. We use our methods to compute the
maximal amount of local and global randomness that can be certified by
measuring systems subject to noise and losses and show that local randomness
can be certified from a single measurement if and only if the detectors used in
the test have detection efficiency higher than 50%.Comment: 11 pages, 6 figures. v2: Published versio
Non-secret correlations can be used to distribute secrecy
A counter-intuitive result in entanglement theory was shown in [PRL 91 037902
(2003)], namely that entanglement can be distributed by sending a separable
state through a quantum channel. In this work, following an analogy between the
entanglement and secret key distillation scenarios, we derive its classical
analog: secrecy can be distributed by sending non-secret correlations through a
private channel. This strengthens the close relation between entanglement and
secrecy.Comment: 4 page
Self-testing multipartite entangled states through projections onto two systems
Finding ways to test the behaviour of quantum devices is a timely enterprise,
especially in the light of the rapid development of quantum technologies.
Device-independent self-testing is one desirable approach, as it makes minimal
assumptions on the devices being tested. In this work, we address the question
of which states can be self-tested. This has been answered recently in the
bipartite case [Nat. Comm. 8, 15485 (2017)], while it is largely unexplored in
the multipartite case, with only a few scattered results, using a variety of
different methods: maximal violation of a Bell inequality, numerical SWAP
method, stabilizer self-testing etc. In this work, we investigate a simple, and
potentially unifying, approach: combining projections onto two-qubit spaces
(projecting parties or degrees of freedom) and then using maximal violation of
the tilted CHSH inequalities. This allows to obtain self-testing of Dicke
states and partially entangled GHZ states with two measurements per party, and
also to recover self-testing of graph states (previously known only through
stabilizer methods). Finally, we give the first self-test of a class
multipartite qudit states: we generalize the self-testing of partially
entangled GHZ states by adapting techniques from [Nat. Comm. 8, 15485 (2017)],
and show that all multipartite states which admit a Schmidt decomposition can
be self-tested with few measurements.Comment: The title is changed and the presentation is slightly restructure
Optimal randomness certification from one entangled bit
By performing local projective measurements on a two-qubit entangled state
one can certify in a device-independent way up to one bit of randomness. We
show here that general measurements, defined by positive-operator-valued
measures, can certify up to two bits of randomness, which is the optimal amount
of randomness that can be certified from an entangled bit. General measurements
thus provide an advantage over projective ones for device-independent
randomness certification.Comment: 7 pages, 1 figure, computational details at
http://nbviewer.ipython.org/github/peterwittek/ipython-notebooks/blob/master/Optimal%20randomness%20generation%20from%20entangled%20quantum%20states.ipyn
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