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.

Controlled-controlled-phase gates for superconducting qubits mediated by a shared tunable coupler

  1. Niklas J. Glaser,
  2. Federico Roy,
  3. and Stefan Filipp
Applications for noisy intermediate-scale quantum computing devices rely on the efficient entanglement of many qubits to reach a potential quantum advantage. Although entanglement is
typically generated using two-qubit gates, direct control of strong multi-qubit interactions can improve the efficiency of the process. Here, we investigate a system of three superconducting transmon-type qubits coupled via a single flux-tunable coupler. Tuning the frequency of the coupler by adiabatic flux pulses enables us to control the conditional energy shifts between the qubits and directly realize multi-qubit interactions. To accurately adjust the resulting controlled relative phases, we describe a gate protocol involving refocusing pulses and adjustable interaction times. This enables the implementation of the full family of pairwise controlled-phase (CPHASE) and controlled-controlled-phase (CCPHASE) gates. Numerical simulations result in fidelities around 99 % and gate times below 300 ns using currently achievable system parameters and decoherence rates.

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.