513 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
Loss-resistant state teleportation and entanglement swapping using a quantum-dot spin in an optical microcavity
We present a scheme for efficient state teleportation and entanglement
swapping using a single quantum-dot spin in an optical microcavity based on
giant circular birefringence. State teleportation or entanglement swapping is
heralded by the sequential detection of two photons, and is finished after the
spin measurement. The spin-cavity unit works as a complete Bell-state analyzer
with a built-in spin memory allowing loss-resistant repeater operation. This
device can work in both the weak coupling and the strong coupling regime, but
high efficiencies and high fidelities are only achievable when the side leakage
and cavity loss is low. We assess the feasibility of this device, and show it
can be implemented with current technology. We also propose a spin manipulation
method using single photons, which could be used to preserve the spin coherence
via spin echo techniques.Comment: The manuscript is extended, including BSA fidelity, efficiency, and a
compatible scheme for spin manipulations and spin echoes to prolong the spin
coherenc
The entanglement beam splitter: a quantum-dot spin in a double-sided optical microcavity
We propose an entanglement beam splitter (EBS) using a quantum-dot spin in a
double-sided optical microcavity. In contrast to the conventional optical beam
splitter, the EBS can directly split a photon-spin product state into two
constituent entangled states via transmission and reflection with high fidelity
and high efficiency (up to 100 percent). This device is based on giant optical
circular birefringence induced by a single spin as a result of cavity quantum
electrodynamics and the spin selection rule of trion transition (Pauli
blocking). The EBS is robust and it is immune to the fine structure splitting
in a realistic quantum dot. This quantum device can be used for
deterministically creating photon-spin, photon-photon and spin-spin
entanglement as well as a single-shot quantum non-demolition measurement of a
single spin. Therefore, the EBS can find wide applications in quantum
information science and technology.Comment: 7 pages, 5 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
Entangling photons using a charged quantum dot in a microcavity
We present two novel schemes to generate photon polarization entanglement via
single electron spins confined in charged quantum dots inside microcavities.
One scheme is via entangled remote electron spins followed by
negatively-charged exciton emissions, and another scheme is via a single
electron spin followed by the spin state measurement. Both schemes are based on
giant circular birefringence and giant Faraday rotation induced by a single
electron spin in a microcavity. Our schemes are deterministic and can generate
an arbitrary amount of multi-photon entanglement. Following similar procedures,
a scheme for a photon-spin quantum interface is proposed.Comment: 4 pages, 4 figure
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
Investigation of defect cavities formed in three-dimensional woodpile photonic crystals
We report the optimisation of optical properties of single defects in
three-dimensional (3D) face-centred-cubic (FCC) woodpile photonic crystal (PC)
cavities by using plane-wave expansion (PWE) and finite-difference time-domain
(FDTD) methods. By optimising the dimensions of a 3D woodpile PC, wide photonic
band gaps (PBG) are created. Optical cavities with resonances in the bandgap
arise when point defects are introduced in the crystal. Three types of single
defects are investigated in high refractive index contrast (Gallium
Phosphide-Air) woodpile structures and Q-factors and mode volumes ()
of the resonant cavity modes are calculated. We show that, by introducing an
air buffer around a single defect, smaller mode volumes can be obtained. We
demonstrate high Q-factors up to 700000 and cavity volumes down to
. The estimates of and are then used to
quantify the enhancement of spontaneous emission and the possibility of
achieving strong coupling with nitrogen-vacancy (NV) colour centres in diamond.Comment: 12 pages, 11 figure
Experimental Realization of a One-way Quantum Computer Algorithm Solving Simon's Problem
We report an experimental demonstration of a one-way implementation of a
quantum algorithm solving Simon's Problem - a black box period-finding problem
which has an exponential gap between the classical and quantum runtime. Using
an all-optical setup and modifying the bases of single-qubit measurements on a
five-qubit cluster state, key representative functions of the logical two-qubit
version's black box can be queried and solved. To the best of our knowledge,
this work represents the first experimental realization of the quantum
algorithm solving Simon's Problem. The experimental results are in excellent
agreement with the theoretical model, demonstrating the successful performance
of the algorithm. With a view to scaling up to larger numbers of qubits, we
analyze the resource requirements for an n-qubit version. This work helps
highlight how one-way quantum computing provides a practical route to
experimentally investigating the quantum-classical gap in the query complexity
model.Comment: 9 pages, 5 figure
Counterfactual Ghost Imaging
We give a protocol for ghost imaging in a way that is always counterfactual—while imaging an object, no light interacts with that object. This extends the idea of counterfactuality beyond communication, showing how this interesting phenomenon can be leveraged for metrology. Given, in the infinite limit, no photons ever go to the imaged object, it presents a method of imaging even the most light-sensitive of objects without damaging them. Even when not in the infinite limit, it still provides a many-fold improvement in visibility and signal-to-noise ratio over previous protocols, with over an order of magnitude reduction in absorbed intensity
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