Generation and storage of optical entanglement in a solid state spin-wave quantum memory
Date
2016
Authors
Ferguson, Katherine Rose
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Abstract
This thesis investigates an entangled light source with an
in-built quantum memory based on the protocol of rephased
amplified spontaneous emission (RASE). RASE has promising
applications as a building-block of a quantum repeater: a device
essential for extending the range of current quantum
communication links. To be useful RASE must be able to produce
high fidelity non-classical light with high efficiency, and be
able to store multimode entanglement for long times. This thesis
characterises the RASE source and determines to what degree these
requirements can be met.
The experimental RASE demonstration was conducted in a rare-earth
ion doped crystal. Rare-earth ions provide a particularly
promising platform for developing quantum technologies as they
possess long coherence times on both the optical and hyperfine
transitions.
In the RASE protocol an inverted ensemble of two-level atoms
amplifies the vacuum fluctuations resulting in amplified
spontaneous emission (ASE). This results in entanglement between
the output optical field and the collective modes of the
amplifying ensemble. The collective atomic state dephases due to
the inhomogeneous broadening of the ensemble but this can be
rephased using photon echo techniques. When the ensemble
rephases, a second optical field, the rephased amplified
spontaneous emission (RASE), is emitted and is entangled with the
ASE. In this thesis, a modified four-level rephasing scheme is
used that allows the single photon signals to be spectrally
resolved from any coherent background emission associated with
the bright driving fields. In addition, four-level RASE
incorporates storage on the long-lived hyperfine ground states.
Two experiments are described in this thesis. First, a free-space
four-level RASE demonstration using continuous-variable
detection. In this experiment the different sources of noise were
characterised and low noise operation was shown to be possible.
Entanglement of the ASE and RASE was confirmed by violating the
inseparability criterion with 98.6% confidence. In addition,
entanglement was demonstrated after storage of the collective
atomic state on the spin states and RASE was shown to be
temporally multimode, with almost perfect distinguishability
between two temporal modes demonstrated. The degree of
entanglement between the ASE and RASE was limited by the
rephasing efficiency, which saturated at 3%. It was determined
that distortion of the rephasing pulses as they propagate through
the optically thick ensemble was the probable cause of the low
efficiency.
The second experiment was a preliminary cavity-enhanced RASE
demonstration. Theoretically perfect rephasing efficiency can be
obtained by placing the crystal in an impedance-matched optical
cavity. The initial cavity design showed encouraging evidence of
an enhancement in the rephasing efficiency, with a 4-fold
improvement over the free-space experiment. Improvements to the
cavity design were proposed to allow a further increase in the
rephasing efficiency of RASE.
In summary, this thesis provides an extensive characterisation of
an entangled light source with an in-built quantum memory based
on rephasing spontaneous emission from an ensemble of ions.
Importantly, the RASE scheme allows generation and storage of
entanglement in a single protocol, which holds great promise for
the development of integrated quantum networks.
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Quantum information, Single photon sources, Quantum memories, Rare-earth ion crystals
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Thesis (PhD)
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