305 research outputs found
Retraction of "Near-Deterministic Discrimination of All Bell States with Linear Optics," Phys. Rev. Lett. 107, 080403 (2011) and Erratum Phys. Rev. Lett. 107, 219901 (2011)
The original versions (1 and 2) of this paper paper contain a fatal error.
All my attempts to patch the error have failed. As a service to the community I
explain the error in some detail.Comment: The original paper (v. 1 and 2) was retracted from Phys. Rev. Lett.
107, 080403 (2011) and its Erratum Phys. Rev. Lett. 107. 219901 (2011
Complete and Deterministic discrimination of polarization Bell state assisted by momentum entanglement
A complete and deterministic Bell state measurement was realized by a simple
linear optics experimental scheme which adopts 2-photon polarization-momentum
hyperentanglement. The scheme, which is based on the discrimination among the
single photon Bell states of the hyperentangled state, requires the adoption of
standard single photon detectors. The four polarization Bell states have been
measured with average fidelity by using the linear momentum
degree of freedom as the ancilla. The feasibility of the scheme has been
characterized as a function of the purity of momentum entanglement.Comment: 4 pages, v2, comments adde
All non-classical correlations can be activated into distillable entanglement
We devise a protocol in which general non-classical multipartite correlations
produce a physically relevant effect, leading to the creation of bipartite
entanglement. In particular, we show that the relative entropy of quantumness,
which measures all non-classical correlations among subsystems of a quantum
system, is equivalent to and can be operationally interpreted as the minimum
distillable entanglement generated between the system and local ancillae in our
protocol. We emphasize the key role of state mixedness in maximizing
non-classicality: Mixed entangled states can be arbitrarily more non-classical
than separable and pure entangled states.Comment: 4+4 pages, 1 figure. Published versio
Experimental reversion of the optimal quantum cloning and flipping processes
The quantum cloner machine maps an unknown arbitrary input qubit into two
optimal clones and one optimal flipped qubit. By combining linear and
non-linear optical methods we experimentally implement a scheme that, after the
cloning transformation, restores the original input qubit in one of the output
channels, by using local measurements, classical communication and feedforward.
This significant teleportation-like method demonstrates how the information is
preserved during the cloning process. The realization of the reversion process
is expected to find useful applications in the field of modern multi-partite
quantum cryptography.Comment: 10 pages, 3 figure
Characterizing quantumness via entanglement creation
In [M. Piani et al., arXiv:1103.4032 (2011)] an activation protocol was
introduced which maps the general non-classical (multipartite) correlations
between given systems into bipartite entanglement between the systems and local
ancillae by means of a potentially highly entangling interaction. Here, we
study how this activation protocol can be used to entangle the starting systems
themselves via entanglement swapping through a measurement on the ancillae.
Furthermore, we bound the relative entropy of quantumness (a naturally arising
measure of non-classicality in the scheme of Piani et al. above) for a special
class of separable states, the so-called classical-quantum states. In
particular, we fully characterize the classical-quantum two-qubit states that
are maximally non-classical.Comment: 13 pages, 1 figure, submitted to special issue of IJQ
Phase-Covariant Quantum Benchmarks
We give a quantum benchmark for teleportation and quantum storage experiments
suited for pure and mixed test states. The benchmark is based on the average
fidelity over a family of phase-covariant states and certifies that an
experiment can not be emulated by a classical setup, i.e., by a
measure-and-prepare scheme. We give an analytical solution for qubits, which
shows important differences with standard state estimation approach, and
compute the value of the benchmark for coherent and squeezed states, both pure
and mixed.Comment: 4 pages, 2 figure
Real-time calibration of coherent-state receivers: learning by trial and error
The optimal discrimination of coherent states of light with current
technology is a key problem in classical and quantum communication, whose
solution would enable the realization of efficient receivers for long-distance
communications in free-space and optical fiber channels. In this article, we
show that reinforcement learning (RL) protocols allow an agent to learn
near-optimal coherent-state receivers made of passive linear optics,
photodetectors and classical adaptive control. Each agent is trained and tested
in real time over several runs of independent discrimination experiments and
has no knowledge about the energy of the states nor the receiver setup nor the
quantum-mechanical laws governing the experiments. Based exclusively on the
observed photodetector outcomes, the agent adaptively chooses among a set of ~3
10^3 possible receiver setups, and obtains a reward at the end of each
experiment if its guess is correct. At variance with previous applications of
RL in quantum physics, the information gathered in each run is intrinsically
stochastic and thus insufficient to evaluate exactly the performance of the
chosen receiver. Nevertheless, we present families of agents that: (i) discover
a receiver beating the best Gaussian receiver after ~3 10^2 experiments; (ii)
surpass the cumulative reward of the best Gaussian receiver after ~10^3
experiments; (iii) simultaneously discover a near-optimal receiver and attain
its cumulative reward after ~10^5 experiments. Our results show that RL
techniques are suitable for on-line control of quantum receivers and can be
employed for long-distance communications over potentially unknown channels.Comment: 14+3 pages, 11 figure
Growth of graph states in quantum networks
We propose a scheme to distribute graph states over quantum networks in the
presence of noise in the channels and in the operations. The protocol can be
implemented efficiently for large graph sates of arbitrary (complex) topology.
We benchmark our scheme with two protocols where each connected component is
prepared in a node belonging to the component and subsequently distributed via
quantum repeaters to the remaining connected nodes. We show that the fidelity
of the generated graphs can be written as the partition function of a classical
Ising-type Hamiltonian. We give exact expressions of the fidelity of the linear
cluster and results for its decay rate in random graphs with arbitrary
(uncorrelated) degree distributions.Comment: 16 pages, 7 figure
Conditional beam splitting attack on quantum key distribution
We present a novel attack on quantum key distribution based on the idea of
adaptive absorption [calsam01]. The conditional beam splitting attack is shown
to be much more efficient than the conventional beam spitting attack, achieving
a performance similar to the, powerful but currently unfeasible, photon number
splitting attack. The implementation of the conditional beam splitting attack,
based solely on linear optical elements, is well within reach of current
technology.Comment: Submitted to Phys. Rev.
- …