386 research outputs found
A quantum interface between light and nuclear spins in quantum dots
The coherent coupling of flying photonic qubits to stationary matter-based
qubits is an essential building block for quantum communication networks. We
show how such a quantum interface can be realized between a traveling-wave
optical field and the polarized nuclear spins in a singly charged quantum dot
strongly coupled to a high-finesse optical cavity. By adiabatically eliminating
the electron a direct effective coupling is achieved. Depending on the laser
field applied, interactions that enable either write-in or read-out are
obtained.Comment: 10 pages, 5 figures, final versio
Distribution of continuous-variable entanglement by separable Gaussian states
Entangling two systems at distant locations using a {\it separable} mediating
ancilla is a counterintuitive phenomenon proposed for qubits by T. Cubitt {\it
et al}. [Phys. Rev. Lett. {\bf 91}, 037902 (2003)]. We show that such
entanglement distribution is possible with Gaussian states, using a certain
three-mode fully separable mixed Gaussian state and linear optics elements
readily available in experiments. Two modes of the state become entangled by
sequentially mixing them on two beam splitters, while the third one remains
separable in all stages of the protocol
Noise-Free Measurement of Harmonic Oscillators with Instantaneous Interactions
We present a method of measuring the quantum state of a harmonic oscillator
through instantaneous probe-system selective interactions of the
Jaynes-Cummings type. We prove that this scheme is robust to general
decoherence mechanisms, allowing the possibility of measuring fast-decaying
systems in the weak-coupling regime. This method could be applied to different
setups: motional states of trapped ions, microwave fields in cavity/circuit
QED, and even intra-cavity optical fields.Comment: 4 pages, no figure, published in Physical Review Letter
Entanglement generation via a completely mixed nuclear spin bath
We show that qubits coupled sequentially to a mesoscopic static completely
mixed spin bath via the Heisenberg interaction can become highly entangled.
Straightforward protocols for the generation of multipartite entangled
(Greenberger-Horne-Zeilinger-)states are presented. We show the feasibility of
an experimental realization in a quantum dot by the hyperfine interaction of an
electron with the nuclear spins.Comment: 4+pages, 3 figure
Standard super-activation for Gaussian channels requires squeezing
The quantum capacity of bosonic Gaussian quantum channels can be non-additive
in a particularly striking way: a pair of such optical-fiber type channels can
individually have zero quantum capacity but super-activate each other such that
the combined channel has strictly positive capacity. This has been shown in
[Nature Photonics 5, 624 (2011)] where it was conjectured that squeezing is a
necessary resource for this phenomenon. We provide a proof of this conjecture
by showing that for gauge covariant channels a Choi matrix with positive
partial transpose implies that the channel is entanglement-breaking. In
addition, we construct an example which shows that this implication fails to
hold for Gaussian channels which arise from passive interactions with a
squeezed environment.Comment: 5 pages, close to published versio
Unitarily localizable entanglement of Gaussian states
We consider generic -mode bipartitions of continuous variable
systems, and study the associated bisymmetric multimode Gaussian states. They
are defined as -mode Gaussian states invariant under local mode
permutations on the -mode and -mode subsystems. We prove that such states
are equivalent, under local unitary transformations, to the tensor product of a
two-mode state and of uncorrelated single-mode states. The entanglement
between the -mode and the -mode blocks can then be completely
concentrated on a single pair of modes by means of local unitary operations
alone. This result allows to prove that the PPT (positivity of the partial
transpose) condition is necessary and sufficient for the separability of -mode bisymmetric Gaussian states. We determine exactly their negativity and
identify a subset of bisymmetric states whose multimode entanglement of
formation can be computed analytically. We consider explicit examples of pure
and mixed bisymmetric states and study their entanglement scaling with the
number of modes.Comment: 10 pages, 2 figure
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