1,030 research outputs found
Extended linear regime of cavity-QED enhanced optical circular birefringence induced by a charged quantum dot
Giant optical Faraday rotation (GFR) and giant optical circular birefringence
(GCB) induced by a single quantum-dot spin in an optical microcavity can be
regarded as linear effects in the weak-excitation approximation if the input
field lies in the low-power limit [Hu et al, Phys.Rev. B {\bf 78}, 085307(2008)
and ibid {\bf 80}, 205326(2009)]. In this work, we investigate the transition
from the weak-excitation approximation moving into the saturation regime
comparing a semiclassical approximation with the numerical results from a
quantum optics toolbox [S.M. Tan, J. Opt. B {\bf 1}, 424 (1999)]. We find that
the GFR and GCB around the cavity resonance in the strong coupling regime are
input-field independent at intermediate powers and can be well described by the
semiclassical approximation. Those associated with the dressed state resonances
in the strong coupling regime or merging with the cavity resonance in the
Purcell regime are sensitive to input field at intermediate powers, and cannot
be well described by the semiclassical approximation due to the quantum dot
saturation. As the GFR and GCB around the cavity resonance are relatively
immune to the saturation effects, the rapid read out of single electron spins
can be carried out with coherent state and other statistically fluctuating
light fields. This also shows that high speed quantum entangling gates, robust
against input power variations, can be built exploiting these linear effects.Comment: Section IV has been added to show the linear GFR/GCB is not affected
by high-order dressed state resonances in reflection/transmission spectra. 11
pages, 9 figure
Hong-Ou-Mandel interferometry on a biphoton beat note
Hong-Ou-Mandel interference, the fact that identical photons that arrive
simultaneously on different input ports of a beam splitter bunch into a common
output port, can be used to measure optical delays between different paths. It
is generally assumed that great precision in the measurement requires that
photons contain many frequencies, i.e., a large bandwidth. Here we challenge
this well-known assumption and show that the use of two well-separated
frequencies embedded in a quantum entangled state (discrete color entanglement)
suffices to achieve great precision. We determine optimum working points using
a Fisher Information analysis and demonstrate the experimental feasibility of
this approach by detecting thermally-induced delays in an optical fiber. These
results may significantly facilitate the use of quantum interference for
quantum sensing, by avoiding some stringent conditions such as the requirement
for large bandwidth signals
A quantum key distribution protocol for rapid denial of service detection
We introduce a quantum key distribution protocol designed to expose fake
users that connect to Alice or Bob for the purpose of monopolising the link and
denying service. It inherently resists attempts to exhaust Alice and Bob's
initial shared secret, and is 100% efficient, regardless of the number of
qubits exchanged above the finite key limit. Additionally, secure key can be
generated from two-photon pulses, without having to make any extra
modifications. This is made possible by relaxing the security of BB84 to that
of the quantum-safe block cipher used for day-to-day encryption, meaning the
overall security remains unaffected for useful real-world cryptosystems such as
AES-GCM being keyed with quantum devices.Comment: 13 pages, 3 figures. v2: Shifted focus of paper towards DoS and added
protocol 4. v1: Accepted to QCrypt 201
Do the laws of physics prohibit counterfactual communication?
It has been conjectured that counterfactual communication is impossible, even
for post-selected quantum particles. We strongly challenge this by proposing
exactly such a counterfactual scheme where---unambiguously---none of Alice's
photons that make it has been to Bob. We demonstrate counterfactuality
theoretically and experimentally by means of weak measurements, as well as
conceptually using consistent histories. Importantly, the accuracy of Alice
learning Bob's bit can be made arbitrarily close to unity with no trace left by
Bob on Alice's photon.Comment: Experiment conducted in the lab, showing no weak trace from Bob at
either D0 or D1. 5 pages, 5 figure
On the effects of self- and cross-phase modulation on photon purity for four-wave mixing photon-pair sources
We consider the effect of self-phase modulation and cross-phase modulation on
the joint spectral amplitude of photon pairs generated by spontaneous four-wave
mixing. In particular, the purity of a heralded photon from a pair is
considered, in the context of schemes that aim to maximise the purity and
minimise correlation in the joint spectral amplitude using birefringent
phase-matching and short pump pulses. We find that non-linear phase modulation
effects will be detrimental, and will limit the quantum interference visibility
that can be achieved at a given generation rate. An approximate expression for
the joint spectral amplitude with phase modulation is found by considering the
group velocity walk-off between each photon and the pump, but neglecting the
group-velocity dispersion at each wavelength. The group-velocity dispersion can
also be included with a numerical calculation, and it is shown that it only has
a small effect on the purity for the realistic parameters considered.Comment: 11 pages, 10 figure
Generating entanglement with low Q-factor microcavities
We propose a method of generating entanglement using single photons and
electron spins in the regime of resonance scattering. The technique involves
matching the spontaneous emission rate of the spin dipole transition in bulk
dielectric to the modified rate of spontaneous emission of the dipole coupled
to the fundamental mode of an optical microcavity. We call this regime
resonance scattering where interference between the input photons and those
scattered by the resonantly coupled dipole transition result in a reflectivity
of zero. The contrast between this and the unit reflectivity when the cavity is
empty allow us to perform a non demolition measurement of the spin and to non
deterministically generate entanglement between photons and spins. The chief
advantage of working in the regime of resonance scattering is that the required
cavity quality factors are orders of magnitude lower than is required for
strong coupling, or Purcell enhancement. This makes engineering a suitable
cavity much easier particularly in materials such as diamond where etching high
quality factor cavities remains a significant challenge
Experimental demonstration of a measurement-based realisation of a quantum channel
We introduce and experimentally demonstrate a method for realising a quantum
channel using the measurement-based model. Using a photonic setup and modifying
the bases of single-qubit measurements on a four-qubit entangled cluster state,
representative channels are realised for the case of a single qubit in the form
of amplitude and phase damping channels. The experimental results match the
theoretical model well, demonstrating the successful performance of the
channels. We also show how other types of quantum channels can be realised
using our approach. This work highlights the potential of the measurement-based
model for realising quantum channels which may serve as building blocks for
simulations of realistic open quantum systems.Comment: 8 pages, 4 figure
Heralding Single Photons from Pulsed Parametric Down-Conversion
We describe an experiment in which photon pairs from a pulsed parametric
down-conversion source were coupled into single-mode fibers. Detecting one of
the photons heralded the presence of the other photon in its fiber with a
probability of 83%. The heralded photons were then used in a simple
multi-photon interference experiment to illustrate their potential for quantum
information applications.Comment: 4 pages, 7 figures. Version 2 has minor revision
Does entanglement depend on the timing of the impacts at the beam-splitters?
A new nonlocality experiment with moving beam-splitters is proposed. The
experiment is analysed according to conventional quantum mechanics, and to an
alternative nonlocal description in which superposition depends not only on
indistinguishability but also on the timing of the impacts at the
beam-splitters.Comment: 5 pages of Latex and 2 eps figures. Submitted to Phys. Lett.
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