This thesis investigates the coherence properties of the
hyperfine transitions of the 151Eu3+ ions in Eu3+:Y2SiO5 and
evaluates the potential of developing quantum memories using
these transitions.
Quantum memories for light with long storage times are required
for quantum commu- nication applications. For these memories to
be useful they need to have storage times long compared to the
transmission times across the communication network. For a global
optical communication network this requires storage time longer
than 100 ms. Rare-earth doped crystals have been identified as a
suitable storage material. The storage time of these systems is
limited by the coherence time of the hyperfine transitions of the
optically active rare-earth ions. In previous work it had been
demonstrated that coherence times as long as 1.4 seconds could be
achieved for hyperfine transitions in Pr3+:Y2SiO5 by ap- plying a
particular magnetic field such that the first order Zeeman shift
of the transition nulled. This technique is known as zero
first-order Zeeman (ZEFOZ). Due to the relatively large second
order Zeeman efficient of the transitions in Pr3+:Y2SiO5, an
extension of the coherence time, significantly beyond the 1.4
second mark using ZEFOZ, is not expected. However, it has been
predicted that coherence times more than two orders of magnitude
longer could be achieved in Eu3+:Y2SiO5 due to the smaller second
order Zeeman shifts associated with the relevant hyperfine
transitions.
The dominant decoherence mechanism for the hyperfine transitions
in diluted Eu3+:Y2SiO5 is the magnetic field perturbations caused
by the random spin reconfigu- ration of the Y3+ ions in the host.
By applying the ZEFOZ technique, previously used in Pr3+:Y2SiO5,
the sensitivity of the transition’s frequency to environmental
magnetic field perturbations was significantly reduced. Further,
this strong ZEFOZ magnetic field was also shown to induce a
frozen core around the Eu3+ ion, which resulted in a signifi-
cant suppression of the reconfiguration of the nearby Y3+ spins.
The combined effect of the reduced sensitivity and frozen core
effect allowed a decoherence rate of 8 × 10−5 s−1 over 100
milliseconds to be demonstrated. The observed decoherence rate is
at least an order of magnitude lower than that of any other
system suitable for an optical quantum memory. Furthermore, by
employing dynamic decoherence control, a coherence time of 370 ±
60 minutes was achieved. This 6 hour coherence time observed here
opens up the possibility of distributing quantum entanglement via
the physical transport of memories as an alternative to optical
communications.
It was found that even at the critical point alignment the
observed coherence times showed that the Y3+ spin flips remain
the dominant decoherence mechanism. To aid in the development of
future strategies to further extend the coherence time beyond 6
hours, a study of the Y3+ spin dynamics in the frozen core was
conducted. Four of the Y3+ sites were resolved and a complete
mapping of all frozen-core Y3+ sites was limited by the
inhomogeneity of the applied magnetic field. The Rabi frequency,
the coherence time and lifetime as well as the interaction
strength with the Eu3+ ion of one of these Y3+ ions were
measured. The observed lifetime of the Y3+ ion is 27 s, which is
four orders of magnitude longer than the low field value. With
the technique developed, a detailed understanding of the
frozen-core dynamics is possible, which would allow an extension
of the hyperfine coherence time of the Eu3+ ion towards the
lifetime limit.
In summary, this thesis provides a detailed characterisation of
the decoherence mecha- nisms of the hyperfine transitions in
Eu3+:Y2SiO5. The potential of using rare-earth doped crystals for
the long-term storage of quantum information with applications to
long-range quantum communications is identified. The demonstrated
long coherence time of the quantum transitions for information
storage allows a new way of entanglement distribu- tion:
entanglement is transported by physically transporting the memory
crystal rather than the light. This approach opens a new regime
for both quantum communication and fundamental tests of quantum
mechanics
