Atom-light interfaces for quantum information processing
Date
2018
Authors
Everett, Jesse Llewellyn
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Abstract
The emergence of quantum physics from the page to the lab and the
world at large is an exciting development of recent years. The
prospects of absolutely secure communication and efficient
simulation of physical systems have spurred great human effort
into understanding these possibilities and turning them into
realities.
Photons are the most easily manipulated quantum particles and are
a promising candidate for implementing these technologies.
Limitations of photons include the difficulty of keeping objects
that move at the speed of light, and producing strong
interactions between particles that do not normally interact. The
work presented in this thesis is motivated by the possibility of
overcoming these limitations.
The ability to faithfully store and reproduce a quantum state is
essential for many quantum information technologies. Quantum
memories for light have been developed over the last two decades
to provide this ability. The group at the Australian National
University developed the gradient echo memory (GEM): A quantum
state of light can be controllably stored and released from an
atomic ensemble by the use of additional optical fields and
magnetic field gradients. This scheme was previously shown to
preserve the quantum characteristics of the light.
We used the GEM scheme with a cold rubidium ensemble to create
the first optical memory that simultaneously beat the no-cloning
limit, a benchmark for many of the technologies relying on
quantum memories, and the loss rate for a delay line composed of
optical fibre.
We also created an analogue to a pulsed optical resonator using
GEM with a warm rubidium vapour. This was done by replacing the
circulating optical field of a resonator with light stored in the
memory, and replacing the coupling of light into and out of that
circulating mode with storage and recall from the memory. The
bandwidth and repetition rate of this resonator were rapidly
tunable as they were controlled by external optical and magnetic
fields.
We worked on implementing GEM with strings of thousands of atoms
strongly coupled to the evanescent field of an optical nanofibre.
This raised new possibilities for creating a true random access
memory that would allow a more flexible use of the multi-mode
capacity of GEM.
We developed the theory for a novel type of stationary light in
the gradient echo memory. Our stationary light scheme relies on
the destructive interference of counter-propagating optical
fields throughout the memory. The optical intensity scales with
optical depth, as with other forms of stationary light. However,
as the destructive interference could be set up over a much
greater distance, more of the optical depth is available for
generating stationary light.
Finally, we studied how a control-phase gate for single-photon
optical states could be implemented using a nonlinear interaction
with stationary light. The stationary light generated by one
state modulates the phase of another state stored in the memory.
The second state modifies the stationary light, also producing a
back-action on the first state and generating the required
cross-phase shift.
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optical quantum memory, atomic ensemble, stationary light, cross-phase modulation, gradient echo memory, electromagnetically induced transparency, optical quantum computing
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