John J.L. Morton

Random-access quantum memory using chirped pulse phase encoding

  1. James O'Sullivan,
  2. Oscar W. Kennedy,
  3. Kamanasish Debnath,
  4. Joseph Alexander,
  5. Christoph W. Zollitsch,
  6. Mantas Šimėnas,
  7. Akel Hashim,
  8. Christopher N Thomas,
  9. Stafford Withington,
  10. Irfan Siddiqi,
  11. Klaus Mølmer,
  12. and John J.L. Morton
and quantum information"]processors [arXiv:1109.3743]. As in conventional computing, key attributes of such memories are high storage density and, crucially, random access, or the ability to read from or write to an arbitrarily chosen register. However, achieving such random access with quantum memories [arXiv:1904.09643] in a dense, hardware-efficient manner remains a challenge, for example requiring dedicated cavities per qubit [arXiv:1109.3743] or pulsed field gradients [arXiv:0908.0101]. Here we introduce a protocol using chirped pulses to encode qubits within an ensemble of quantum two-level systems, offering both random access and naturally supporting dynamical decoupling to enhance the memory lifetime. We demonstrate the protocol in the microwave regime using donor spins in silicon coupled to a superconducting cavity, storing up to four multi-photon microwave pulses and retrieving them on-demand up to 2~ms later. A further advantage is the natural suppression of superradiant echo emission, which we show is critical when approaching unit cooperativity. This approach offers the potential for microwave random access quantum memories with lifetimes exceeding seconds [arXiv:1301.6567, arXiv:2005.09275], while the chirped pulse phase encoding could also be applied in the optical regime to enhance quantum repeaters and networks.
arxiv pdf

Coherent storage of microwave excitations in rare-earth nuclear spins

  1. Gary Wolfowicz,
  2. Hannes Maier-Flaig,
  3. Robert Marino,
  4. Alban Ferrier,
  5. Hervé Vezin,
  6. John J.L. Morton,
  7. and Philippe Goldner
Interfacing between various elements of a computer – from memory to processors to long range communication – will be as critical for quantum computers as it is for classical
computers today. Paramagnetic rare earth doped crystals, such as Nd3+:Y2SiO5 (YSO), are excellent candidates for such a quantum interface: they are known to exhibit long optical coherence lifetimes (for communication via optical photons), possess a nuclear spin (memory) and have in addition an electron spin that can offer hybrid coupling with superconducting qubits (processing). Here we study two of these three elements, demonstrating coherent storage and retrieval between electron and 145Nd nuclear spin states in Nd3+:YSO. We find nuclear spin coherence times can reach 9 ms at ≈5 K, about two orders of magnitude longer than the electron spin coherence, while quantum state and process tomography of the storage/retrieval operation reveal an average state fidelity of 0.86. The times and fidelities are expected to further improve at lower temperatures and with more homogeneous radio-frequency excitation.
arxiv pdf