215 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
Fast SWAP gate by adiabatic passage
We present a process for the construction of a SWAP gate which does not
require a composition of elementary gates from a universal set. We propose to
employ direct techniques adapted to the preparation of this specific gate. The
mechanism, based on adiabatic passage, constitutes a decoherence-free method in
the sense that spontaneous emission and cavity damping are avoided.Comment: 5 pages, 4 figures, submitted to Phys. Re
Arbitrary state controlled-unitary gate by adiabatic passage
We propose a robust scheme involving atoms fixed in an optical cavity to
directly implement the universal controlled-unitary gate. The present technique
based on adiabatic passage uses novel dark states well suited for the
controlled-rotation operation. We show that these dark states allow the robust
implementation of a gate that is a generalisation of the controlled-unitary
gate to the case where the control qubit can be selected to be an arbitrary
state. This gate has potential applications to the rapid implementation of
quantum algorithms such as of the projective measurement algorithm. This
process is decoherence-free since excited atomic states and cavity modes are
not populated during the dynamics.Comment: 6 pages, 6 figure, submitted to Phys. Rev.
CNOT gate by adiabatic passage with an optical cavity
We propose a scheme for the construction of a CNOT gate by adiabatic passage
in an optical cavity. In opposition to a previously proposed method, the
technique is not based on fractional adiabatic passage, which requires the
control of the ratio of two pulse amplitudes. Moreover, the technique
constitutes a decoherence-free method in the sense that spontaneous emission
and cavity damping are avoided since the dynamics follows dark states.Comment: 6 pages, 4 figures, submitted to EPJ
Preparation of nondegenerate coherent superpositions in a three-state ladder system assisted by Stark Shifts
We propose a technique to prepare coherent superpositions of two
nondegenerate quantum states in a three-state ladder system, driven by two
simultaneous fields near resonance with an intermediate state. The technique,
of potential application to enhancement of nonlinear processes, uses adiabatic
passage assisted by dynamic Stark shifts induced by a third laser field. The
method offers significant advantages over alternative techniques: (\i) it does
not require laser pulses of specific shape and duration and (\ii) it requires
less intense fields than schemes based on two-photon excitation with
non-resonant intermediate states. We discuss possible experimental
implementation for enhancement of frequency conversion in mercury atoms.Comment: 22 pages, 8 figures, 1 table, submitted to PHys. Rev.
Heralded photon amplification for quantum communication
Heralded noiseless amplification based on single-photon sources and linear
optics is ideally suited for long-distance quantum communication tasks based on
discrete variables. We experimentally demonstrate such an amplifier, operating
at telecommunication wavelengths. Coherent amplification is performed with a
gain of G=1.98+/-0.2, for a state with a maximum expected gain G=2. We also
demonstrate that there is no need for a stable phase reference between the
initial signal state and the local auxiliary photons used by the amplifier.
These results highlight the potential of heralded quantum amplifiers for
long-distance quantum communication, and bring device-independent quantum key
distribution one step closer.Comment: 5 pages, 4 figure
Noisy pre-processing facilitating a photonic realisation of device-independent quantum key distribution
Device-independent quantum key distribution provides security even when the
equipment used to communicate over the quantum channel is largely
uncharacterized. An experimental demonstration of device-independent quantum
key distribution is however challenging. A central obstacle in photonic
implementations is that the global detection efficiency, i.e., the probability
that the signals sent over the quantum channel are successfully received, must
be above a certain threshold. We here propose a method to significantly relax
this threshold, while maintaining provable device-independent security. This is
achieved with a protocol that adds artificial noise, which cannot be known or
controlled by an adversary, to the initial measurement data (the raw key).
Focusing on a realistic photonic setup using a source based on spontaneous
parametric down conversion, we give explicit bounds on the minimal required
global detection efficiency.Comment: 5+16 pages, 4 figure
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