129 research outputs found
The size of quantum superpositions as measured with "classical" detectors
We propose a criterion which defines whether a superposition of two photonic
components is macroscopic. It is based on the ability to discriminate these
components with a particular class of "classical" detectors, namely a photon
number measurement with a resolution coarse-grained by noise. We show how our
criterion can be extended to a measure of the size of macroscopic
superpositions by quantifying the amount of noise that can be tolerated and
taking the distinctness of two Fock states differing by N photons as a
reference. After applying our measure to several well-known examples, we
demonstrate that the superpositions which meet our criterion are very sensitive
to phase fluctuations. This suggests that quantifying the macroscopicity of a
superposition state through the distinguishability of its components with
"classical" detectors is not only a natural measure but also explains why it is
difficult to observe superpositions at the macroscopic scale.Comment: 5 pages, 3 figures, updated versio
Proposal for Implementing Device-Independent Quantum Key Distribution based on a Heralded Qubit Amplification
In device-independent quantum key distribution (DIQKD), the violation of a
Bell inequality is exploited to establish a shared key that is secure
independently of the internal workings of the QKD devices. An experimental
implementation of DIQKD, however, is still awaited, since hitherto all optical
Bell tests are subject to the detection loophole, making the protocol
unsecured. In particular, photon losses in the quantum channel represent a
fundamental limitation for DIQKD. Here, we introduce a heralded qubit amplifier
based on single-photon sources and linear optics that provides a realistic
solution to overcome the problem of channel losses in Bell tests.Comment: 5 pages, 4 figures, 6 page appendi
How difficult it is to prove the quantumness of macroscropic states?
General wisdom tells us that if two quantum states are ``macroscopically
distinguishable'' then their superposition should be hard to observe. We make
this intuition precise and general by quantifying the difficulty to observe the
quantum nature of a superposition of two states that can be distinguished
without microscopic accuracy. First, we quantify the distinguishability of any
given pair of quantum states with measurement devices lacking microscopic
accuracy, i.e. measurements suffering from limited resolution or limited
sensitivity. Next, we quantify the required stability that have to be fulfilled
by any measurement setup able to distinguish their superposition from a mere
mixture. Finally, by establishing a relationship between the stability
requirement and the ``macroscopic distinguishability'' of the two superposed
states, we demonstrate that indeed, the more distinguishable the states are,
the more demanding are the stability requirements.Comment: 6 pages, 2 figure
Macroscopic optomechanics from displaced single-photon entanglement
Displaced single-photon entanglement is a simple form of optical
entanglement, obtained by sending a photon on a beamsplitter and subsequently
applying a displacement operation. We show that it can generate, through a
momentum transfer in the pulsed regime, an optomechanical entangled state
involving macroscopically distinct mechanical components, even if the
optomechanical system operates in the single-photon weak coupling regime. We
discuss the experimental feasibility of this approach and show that it might
open up a way for testing unconventional decoherence models.Comment: 10 pages, 4 figures, submission coordinated with Gohbadi et al. who
reported on similar result
Purification of single-photon entanglement with linear optics
We show that single-photon entangled states of the form |0>|1>+|1>|0> can be
purified with a simple linear-optics based protocol, which is eminently
feasible with current technology. Besides its conceptual interest, this result
is relevant for attractive quantum repeater protocols.Comment: 4 pages, 3 figure
Analysis of a quantum memory for photons based on controlled reversible inhomogeneous broadening
We present a detailed analysis of a quantum memory for photons based on
controlled and reversible inhomogeneous broadening (CRIB). The explicit
solution of the equations of motion is obtained in the weak excitation regime,
making it possible to gain insight into the dependence of the memory efficiency
on the optical depth, and on the width and shape of the atomic spectral
distributions. We also study a simplified memory protocol which does not
require any optical control fields.Comment: 9 pages, 4 figures (Accepted for publication in Phys. Rev. A
Quantum Repeaters based on Single Trapped Ions
We analyze the performance of a quantum repeater protocol based on single
trapped ions. At each node, single trapped ions embedded into high finesse
cavities emit single photons whose polarization is entangled with the ion
state. A specific detection of two photons at a central station located
half-way between two nodes heralds the entanglement of two remote ions.
Entanglement can be extended to long distances by applying successive
entanglement swapping operations based on two-ion gate operations that have
already been demonstrated experimentally with high precision. Our calculation
shows that the distribution rate of entanglement achievable with such an
ion-based quantum repeater protocol is higher by orders of magnitude than the
rates that are achievable with the best known schemes based on atomic ensemble
memories and linear optics. The main reason is that for trapped ions the
entanglement swapping operations are performed deterministically, in contrast
to success probabilities below 50 percent per swapping with linear optics. The
scheme requires efficient collection of the emitted photons, which can be
achieved with cavities, and efficient conversion of their wavelength, which can
be done via stimulated parametric down-conversion. We also suggest how to
realize temporal multiplexing, which offers additional significant speed-ups in
entanglement distribution, with trapped ions
Heralded single phonon preparation, storage and readout in cavity optomechanics
We analyze theoretically how to use the radiation pressure coupling between a
mechanical oscillator and an optical cavity field to generate in a heralded way
a single quantum of mechanical motion (a Fock state), and release on-demand the
stored excitation as a single photon. Starting with the oscillator close to its
ground state, a laser pumping the upper motional sideband leads to dynamical
backaction amplification and to the creation of correlated photon-phonon pairs.
The detection of one Stokes photon thus projects the macroscopic oscillator
into a single-phonon Fock state. The non-classical nature of this mechanical
state can be demonstrated by applying a readout laser on the lower sideband
(i.e. optical cooling) to map the phononic state to a photonic mode, and by
performing an autocorrelation measurement on the anti-Stokes photons. We
discuss the relevance of our proposal for the future of cavity optomechanics as
an enabling quantum technology.Comment: Accepted for publication in Physical Review Letters. Added References
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