3,147 research outputs found
Complete experimental toolbox for alignment-free quantum communication
Quantum communication employs the counter-intuitive features of quantum
physics to perform tasks that are im- possible in the classical world. It is
crucial for testing the foundations of quantum theory and promises to rev-
olutionize our information and communication technolo- gies. However, for two
or more parties to execute even the simplest quantum transmission, they must
establish, and maintain, a shared reference frame. This introduces a
considerable overhead in communication resources, par- ticularly if the parties
are in motion or rotating relative to each other. We experimentally demonstrate
how to circumvent this problem with the efficient transmission of quantum
information encoded in rotationally invariant states of single photons. By
developing a complete toolbox for the efficient encoding and decoding of
quantum infor- mation in such photonic qubits, we demonstrate the fea- sibility
of alignment-free quantum key-distribution, and perform a proof-of-principle
alignment-free entanglement distribution and violation of a Bell inequality.
Our scheme should find applications in fundamental tests of quantum mechanics
and satellite-based quantum communication.Comment: Main manuscript: 7 pages, 3 figures; Supplementary Information: 7
pages, 3 figure
A resource theory of quantum memories and their faithful verification with minimal assumptions
We provide a complete set of game-theoretic conditions equivalent to the
existence of a transformation from one quantum channel into another one, by
means of classically correlated pre/post processing maps only. Such conditions
naturally induce tests to certify that a quantum memory is capable of storing
quantum information, as opposed to memories that can be simulated by
measurement and state preparation (corresponding to entanglement-breaking
channels). These results are formulated as a resource theory of genuine quantum
memories (correlated in time), mirroring the resource theory of entanglement in
quantum states (correlated spatially). As the set of conditions is complete,
the corresponding tests are faithful, in the sense that any non
entanglement-breaking channel can be certified. Moreover, they only require the
assumption of trusted inputs, known to be unavoidable for quantum channel
verification. As such, the tests we propose are intrinsically different from
the usual process tomography, for which the probes of both the input and the
output of the channel must be trusted. An explicit construction is provided and
shown to be experimentally realizable, even in the presence of arbitrarily
strong losses in the memory or detectors.Comment: Addition of a quantitative study of memories as resources, and
reformulated part of the results in that ligh
Measuring multipartite entanglement via dynamic susceptibilities
Entanglement plays a central role in our understanding of quantum many body
physics, and is fundamental in characterising quantum phases and quantum phase
transitions. Developing protocols to detect and quantify entanglement of
many-particle quantum states is thus a key challenge for present experiments.
Here, we show that the quantum Fisher information, representing a witness for
genuinely multipartite entanglement, becomes measurable for thermal ensembles
via the dynamic susceptibility, i.e., with resources readily available in
present cold atomic gas and condensed-matter experiments. This moreover
establishes a fundamental connection between multipartite entanglement and
many-body correlations contained in response functions, with profound
implications close to quantum phase transitions. There, the quantum Fisher
information becomes universal, allowing us to identify strongly entangled phase
transitions with a divergent multipartiteness of entanglement. We illustrate
our framework using paradigmatic quantum Ising models, and point out potential
signatures in optical-lattice experiments.Comment: 5+5 pages, 3+2 figure
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