Parity-dependent state transfer for direct entanglement generation

  1. Federico A. Roy,
  2. João H. Romeiro,
  3. Leon Koch,
  4. Ivan Tsitsilin,
  5. Johannes Schirk,
  6. Niklas J. Glaser,
  7. Niklas Bruckmoser,
  8. Malay Singh,
  9. Franz X. Haslbeck,
  10. Gerhard B. P. Huber,
  11. Gleb Krylov,
  12. Achim Marx,
  13. Frederik Pfeiffer,
  14. Christian M. F. Schneider,
  15. Christian Schweizer,
  16. Florian Wallner,
  17. David Bunch,
  18. Lea Richard,
  19. Lasse Södergren,
  20. Klaus Liegener,
  21. Max Werninghaus,
  22. and Stefan Filipp
As quantum information technologies advance they face challenges in scaling and connectivity. In particular, two necessities remain independent of the technological implementation:
the need for connectivity between distant qubits and the need for efficient generation of entanglement. Perfect State Transfer is a technique which realises the time optimal transfer of a quantum state between distant nodes of qubit lattices with only nearest-neighbour couplings, hence providing an important tool to improve device connectivity. Crucially, the transfer protocol results in effective parity-dependent non-local interactions, extending its utility to the efficient generation of entangled states. Here, we experimentally demonstrate Perfect State Transfer and the generation of multi-qubit entanglement on a chain of superconducting qubits. The system consists of six fixed-frequency transmon qubits connected by tunable couplers, where the couplings are controlled via parametric drives. By simultaneously activating all couplings and engineering their individual amplitudes and frequencies, we implement Perfect State Transfer on up to six qubits and observe the respective single-excitation dynamics for different initial states. We then apply the protocol in the presence of multiple excitations and verify its parity-dependent property, where the number of excitations within the chain controls the phase of the transferred state. Finally, we utilise this property to prepare a multi-qubit Greenberger-Horne-Zeilinger state using only a single transfer operation, demonstrating its application for efficient entanglement generation.

Single Shot i-Toffoli Gate in Dispersively Coupled Superconducting Qubits

  1. Aneirin J. Baker,
  2. Gerhard B. P. Huber,
  3. Niklas J. Glaser,
  4. Federico Roy,
  5. Ivan Tsitsilin,
  6. Stefan Filipp,
  7. and Michael J. Hartmann
Quantum algorithms often benefit from the ability to execute multi-qubit (>2) gates. To date such multi-qubit gates are typically decomposed into single- and two-qubit gates, particularly
in superconducting qubit architectures. The ability to perform multi-qubit operations in a single step could vastly improve the fidelity and execution time of many algorithms. Here, we propose a single shot method for executing an i-Toffoli gate, a three-qubit gate gate with two control and one target qubit, using currently existing superconducting hardware. We show numerical evidence for a process fidelity over 98% and a gate time of 500 ns for superconducting qubits interacting via tunable couplers. Our method can straight forwardly be extended to implement gates with more than two control qubits at similar fidelities.