435 research outputs found
Decoherence-free manipulation of photonic memories for quantum computation
We present a protocol to construct an arbitrary quantum circuit. The quantum
bits (qubits) are encoded in polarisation states of single photons. They are
stored in spatially separated dense media deposed in an optical cavity.
Specific sequences of pulses address individually the storage media to encode
the qubits and to implement a universal set of gates. The proposed protocol is
decoherence-free in the sense that spontaneous emission and cavity damping are
avoided. We discuss a coupling scheme for experimental implementation in Neon
atoms.Comment: 5 pages, 4 figures, submitted to Phys. Re
Linking Measures for Macroscopic Quantum States via Photon-Spin Mapping
We review and compare several measures that identify quantum states that are
"macroscopically quantum". These measures were initially formulated either for
photonic systems or spin ensembles. Here, we compare them through a simple
model which maps photonic states to spin ensembles. On the one hand, we reveal
problems for some spin measures to handle correctly photonic states that
typically are considered to be macroscopically quantum. On the other hand, we
find significant similarities between other measures even though they were
differently motivated.Comment: 12 pages, 1 figure; published in a special issue of Optics
Communications: "Macroscopic quantumness: theory and applications in optical
sciences"; v2: minor change
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
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
A Novel Piezoelectric Microtransformer for Autonmous Sensors Applications
This work relates to a novel piezoelectric transformer to be used in an
autonomous sensor unit, possibly in conjunction with a RF-MEMS retro-modulator.Comment: Submitted on behalf of EDA Publishing Association
(http://irevues.inist.fr/handle/2042/16838
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
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
Noise-resistant device-independent certification of Bell state measurements
Device-independent certification refers to the characterization of an
apparatus without reference to the internal description of other devices. It is
a trustworthy certification method, free of assumption on the underlying
Hilbert space dimension and on calibration methods. We show how it can be used
to quantify the quality of a Bell state measurement, whether deterministic,
partial or probabilistic. Our certification is noise resistant and opens the
way towards the device-independent self-testing of Bell state measurements in
existing experiments.Comment: 5+5 pages, 3+3 figures. See also related work by Marc Olivier Renou
et a
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