James O'Sullivan

Deterministic generation of a 20-qubit two-dimensional photonic cluster state

  1. James O'Sullivan,
  2. Kevin Reuer,
  3. Aleksandr Grigorev,
  4. Xi Dai,
  5. Alonso Hernández-Antón,
  6. Manuel H. Muñoz-Arias,
  7. Christoph Hellings,
  8. Alexander Flasby,
  9. Dante Colao Zanuz,
  10. Jean-Claude Besse,
  11. Alexandre Blais,
  12. Daniel Malz,
  13. Christopher Eichler,
  14. and Andreas Wallraff
Multidimensional cluster states are a key resource for robust quantum communication, measurement-based quantum computing and quantum metrology. Here, we present a device capable of
emitting large-scale entangled microwave photonic states in a two dimensional ladder structure. The device consists of a pair of coupled superconducting transmon qubits which are each tuneably coupled to a common output waveguide. This architecture permits entanglement between each transmon and a deterministically emitted photonic qubit. By interleaving two-qubit gates with controlled photon emission, we generate 2 x n grids of time- and frequency-multiplexed cluster states of itinerant microwave photons. We measure a signature of localizable entanglement across up to 20 photonic qubits. We expect the device architecture to be capable of generating a wide range of other tensor network states such as tree graph states, repeater states or the ground state of the toric code, and to be readily scalable to generate larger and higher dimensional states.
arxiv pdf

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