5 research outputs found
Development of Persistent Quantum Memories
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
Quantum information processing using frozen core Y 3+ spins in Eu 3+ :Y 2 SiO 5
In this paper, we present a method to investigate and control the dynamics of the nearby host nuclear spins (the 'frozen core') about a rare-earth ion doped in a crystal. Optically detected, double quantum magnetic resonance measurements were conducted on Eu3+ Y2SiO5. The distinct magnetic resonant frequencies of nearby Y3+ spins were measured along with the lifetime and coherence time of an individual Y3+ spin. We demonstrate an entangling gate between the Eu3+ spins and a Y3+ spin associated with a particular position. Further, we propose a method to initialize the Y3+ spin states, enabling the Y3+ spins to be used as a quantum resource for quantum information applications.This work was supported by the Australian Research Council Centre of Excellence for Quantum Computation and Communication Technology (Grant No. CE110001027). MZ is supported by the Science, Technology and Innovation Commission of Shenzhen Municipality (No. ZDSYS20170303165926217, No.
JCYJ20170412152620376)) and Guangdong Innovative and Entrepreneurial Research Team Program (Grant No. 2016ZT06D348). RLA is a recipient of an Australian Research Council Discovery Early Career Researcher Award (project No. DE170100099)
Minimizing Zeeman sensitivity on optical and hyperfine transitions in EuCl 3 . 6H 2 O to extend coherence times
In this paper we characterize the spin Hamiltonian and magnetic dependence of the ground and excited states of the F07→5D0 transition of EuCl3·6H2O. From this information, magnetic field values at which the optical and hyperfine transitions undergo tu
Optically addressable nuclear spins in a solid with a six-hour coherence time
Space-like separation of entangled quantum states is a central concept in fundamental investigations of quantum mechanics and in quantum communication applications. Optical approaches are ubiquitous in the distribution of entanglement because entangled photons are easy to generate and transmit. However, extending this direct distribution beyond a range of a few hundred kilometres to a worldwide network is prohibited by losses associated with scattering, diffraction and absorption during transmission. A proposal to overcome this range limitation is the quantum repeater protocol, which involves the distribution of entangled pairs of optical modes among many quantum memories stationed along the transmission channel. To be effective, the memories must store the quantum information encoded on the optical modes for times that are long compared to the direct optical transmission time of the channel. Here we measure a decoherence rate of 8 × 10(-5) per second over 100 milliseconds, which is the time required for light transmission on a global scale. The measurements were performed on a ground-state hyperfine transition of europium ion dopants in yttrium orthosilicate ((151)Eu(3+):Y2SiO5) using optically detected nuclear magnetic resonance techniques. 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 decoupling, a coherence time of 370 ± 60 minutes was achieved at 2 kelvin. It has been almost universally assumed that light is the best long-distance carrier for quantum information. However, the coherence time observed here is long enough that nuclear spins travelling at 9 kilometres per hour in a crystal would have a lower decoherence with distance than light in an optical fibre. This enables some very early approaches to entanglement distribution to be revisited, in particular those in which the spins are transported rather than the light.This work was supported by the Australian Research Council Centre of Excellence for Quantum Computation and Communication Technology (CE110001027), and M.J.S. was supported by an Australian Research Council Future
Fellowship (FT110100919). J.J.L. was supported by the Marsden Fund of the Royal Society of New Zealand (contract UOO1221)