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.

Efficient decoupling of a non-linear qubit mode from its environment

  1. Frederik Pfeiffer,
  2. Max Werninghaus,
  3. Christian Schweizer,
  4. Niklas Bruckmoser,
  5. Leon Koch,
  6. Niklas J. Glaser,
  7. Gerhard Huber,
  8. David Bunch,
  9. Franz X. Haslbeck,
  10. M. Knudsen,
  11. Gleb Krylov,
  12. Klaus Liegener,
  13. Achim Marx,
  14. Lea Richard,
  15. João H. Romeiro,
  16. Federico Roy,
  17. Johannes Schirk,
  18. Christian Schneider,
  19. Malay Singh,
  20. Lasse Södergren,
  21. Ivan Tsitsilin,
  22. Florian Wallner,
  23. Carlos A. Riofrío,
  24. and Stefan Filipp
To control and measure the state of a quantum system it must necessarily be coupled to external degrees of freedom. This inevitably leads to spontaneous emission via the Purcell effect,
photon-induced dephasing from measurement back-action, and errors caused by unwanted interactions with nearby quantum systems. To tackle this fundamental challenge, we make use of the design flexibility of superconducting quantum circuits to form a multi-mode element — an artificial molecule — with symmetry-protected modes. The proposed circuit consists of three superconducting islands coupled to a central island via Josephson junctions. It exhibits two essential non-linear modes, one of which is flux-insensitive and used as the protected qubit mode. The second mode is flux-tunable and serves via a cross-Kerr type coupling as a mediator to control the dispersive coupling of the qubit mode to the readout resonator. We demonstrate the Purcell protection of the qubit mode by measuring relaxation times that are independent of the mediated dispersive coupling. We show that the coherence of the qubit is not limited by photon-induced dephasing when detuning the mediator mode from the readout resonator and thereby reducing the dispersive coupling. The resulting highly protected qubit with tunable interactions may serve as a basic building block of a scalable quantum processor architecture, in which qubit decoherence is strongly suppressed.