18,961 research outputs found
High efficiency coherent optical memory with warm rubidium vapour
By harnessing aspects of quantum mechanics, communication and information
processing could be radically transformed. Promising forms of quantum
information technology include optical quantum cryptographic systems and
computing using photons for quantum logic operations. As with current
information processing systems, some form of memory will be required. Quantum
repeaters, which are required for long distance quantum key distribution,
require optical memory as do deterministic logic gates for optical quantum
computing. In this paper we present results from a coherent optical memory
based on warm rubidium vapour and show 87% efficient recall of light pulses,
the highest efficiency measured to date for any coherent optical memory. We
also show storage recall of up to 20 pulses from our system. These results show
that simple warm atomic vapour systems have clear potential as a platform for
quantum memory
An AC Stark Gradient Echo Memory in Cold Atoms
The burgeoning fields of quantum computing and quantum key distribution have
created a demand for a quantum memory. The gradient echo memory scheme is a
quantum memory candidate for light storage that can boast efficiencies
approaching unity, as well as the flexibility to work with either two or three
level atoms. The key to this scheme is the frequency gradient that is placed
across the memory. Currently the three level implementation uses a Zeeman
gradient and warm atoms. In this paper we model a new gradient creation
mechanism - the ac Stark effect - to provide an improvement in the flexibility
of gradient creation and field switching times. We propose this scheme in
concert with a move to cold atoms (~1 mK). These temperatures would increase
the storage times possible, and the small ensemble volumes would enable large
ac Stark shifts with reasonable laser power. We find that memory bandwidths on
the order of MHz can be produced with experimentally achievable laser powers
and trapping volumes, with high precision in gradient creation and switching
times on the order of nanoseconds possible. By looking at the different
decoherence mechanisms present in this system we determine that coherence times
on the order of 10s of milliseconds are possible, as are delay-bandwidth
products of approximately 50 and efficiencies over 90%
Storage and Manipulation of Light Using a Raman Gradient Echo Process
The Gradient Echo Memory (GEM) scheme has potential to be a suitable protocol
for storage and retrieval of optical quantum information. In this paper, we
review the properties of the -GEM method that stores information in
the ground states of three-level atomic ensembles via Raman coupling. The
scheme is versatile in that it can store and re-sequence multiple pulses of
light. To date, this scheme has been implemented using warm rubidium gas cells.
There are different phenomena that can influence the performance of these
atomic systems. We investigate the impact of atomic motion and four-wave mixing
and present experiments that show how parasitic four-wave mixing can be
mitigated. We also use the memory to demonstrate preservation of pulse shape
and the backward retrieval of pulses.Comment: 26 pages, 13 figure
Quantum Cloning of Continuous Variable Entangled States
We consider the quantum cloning of continuous variable entangled states. This
is achieved by introducing two symmetric entanglement cloning machines (or
e-cloners): a local e-cloner and a global e-cloner; where we look at the
preservation of entanglement in the clones under the condition that the
fidelity of the clones is maximized. These cloning machines are implemented
using simple linear optical elements such as beam splitters and homodyne
detection along with squeeze gates. We show that the global e-cloner
out-performs the local e-cloner both in terms of the fidelity of the cloned
states as well as the strength of the entanglement of the clones. There is a
minimum strength of entanglement (3dB for the inseparability criterion and
5.7dB for the EPR paradox criterion) of the input state of the global e-cloner
that is required to preserve the entanglement in the clones.Comment: 11 pages, 6 figure
A Scalable, Self-Analyzing Digital Locking System for use on Quantum Optics Experiments
Digital control of optics experiments has many advantages over analog control
systems, specifically in terms of scalability, cost, flexibility, and the
integration of system information into one location. We present a digital
control system, freely available for download online, specifically designed for
quantum optics experiments that allows for automatic and sequential re-locking
of optical components. We show how the inbuilt locking analysis tools,
including a white-noise network analyzer, can be used to help optimize
individual locks, and verify the long term stability of the digital system.
Finally, we present an example of the benefits of digital locking for quantum
optics by applying the code to a specific experiment used to characterize
optical Schrodinger cat states.Comment: 7 pages, 5 figure
Generalized Background-Field Method
The graphical method discussed previously can be used to create new gauges
not reachable by the path-integral formalism. By this means a new gauge is
designed for more efficient two-loop QCD calculations. It is related to but
simpler than the ordinary background-field gauge, in that even the triple-gluon
vertices for internal lines contain only four terms, not the usual six. This
reduction simplifies the calculation inspite of the necessity to include other
vertices for compensation. Like the ordinary background-field gauge, this
generalized background-field gauge also preserves gauge invariance of the
external particles. As a check of the result and an illustration for the
reduction in labour, an explicit calculation of the two-loop QCD
-function is carried out in this new gauge. It results in a saving of
45% of computation compared to the ordinary background-field gauge.Comment: 17 pages, Latex, 18 figures in Postscrip
Configurable unitary transformations and linear logic gates using quantum memories
We show that a set of optical memories can act as a configurable linear
optical network operating on frequency-multiplexed optical states. Our protocol
is applicable to any quantum memories that employ off-resonant Raman
transitions to store optical information in atomic spins. In addition to the
configurability, the protocol also offers favourable scaling with an increasing
number of modes where N memories can be configured to implement an arbitrary
N-mode unitary operations during storage and readout. We demonstrate the
versatility of this protocol by showing an example where cascaded memories are
used to implement a conditional CZ gate.Comment: 5 pages, 2 figure
Array concepts for solid-state and vacuum microelectronics millimeter-wave generation
The authors have proposed that the increasing demand for contact watt-level coherent sources in the millimeter- and submillimeter-wave region can be satisfied by fabricating two-dimensional grids loaded with oscillators and multipliers for quasi-optical coherent spatial combining of the outputs of large numbers of low-power devices. This was first demonstrated through the successful fabrication of monolithic arrays with 2000 Schottky diodes. Watt-level power outputs were obtained in doubling to 66 GHz. In addition, a simple transmission-line model was verified with a quasi-optical reflectometer that measured the array impedance. This multiplier array work is being extended to novel tripler configurations using blocking barrier devices. The technique has also been extended to oscillator configurations where the grid structure is loaded with negative-resistance devices. This was first demonstrated using Gunn devices. More recently, a 25-element MESFET grid oscillating at 10 GHz exhibited power combining and self-locking. Currently, this approach is being extended to a 100-element monolithic array of Gunn diodes. This same approach should be applicable to planar vacuum electron devices such as the submillimeter-wave BWO (backward wave oscillator) and vacuum FET
Time- and frequency-domain polariton interference
We present experimental observations of interference between an atomic spin
coherence and an optical field in a {\Lambda}-type gradient echo memory. The
interference is mediated by a strong classical field that couples a weak probe
field to the atomic coherence through a resonant Raman transition. Interference
can be observed between a prepared spin coherence and another propagating
optical field, or between multiple {\Lambda} transitions driving a single spin
coherence. In principle, the interference in each scheme can yield a near unity
visibility.Comment: 11 pages, 5 figure
Biased EPR entanglement and its application to teleportation
We consider pure continuous variable entanglement with non-equal correlations
between orthogonal quadratures. We introduce a simple protocol which equates
these correlations and in the process transforms the entanglement onto a state
with the minimum allowed number of photons. As an example we show that our
protocol transforms, through unitary local operations, a single squeezed beam
split on a beam splitter into the same entanglement that is produced when two
squeezed beams are mixed orthogonally. We demonstrate that this technique can
in principle facilitate perfect teleportation utilising only one squeezed beam.Comment: 8 pages, 5 figure
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