Thomas B. Smith

Protected phase gate for the 0-π qubit using its internal modes

  1. Xanda C Kolesnikow,
  2. Thomas B. Smith,
  3. Felix Thomsen,
  4. Abhijeet A Alase,
  5. and Andrew C. Doherty
Protected superconducting qubits such as the 0-π qubit promise to substantially reduce physical error rates through a multi-mode encoding. This protection comes at the cost of controllability,
as standard techniques for quantum gates are ineffective. We propose a protected phase gate for the 0-π qubit that utilises an internal mode of the circuit as an ancilla. The gate is achieved by varying the qubit-ancilla coupling via a tunable Josephson element. Our scheme is a modified version of a protected gate proposed by Brooks, Kitaev and Preskill that uses an external oscillator as an ancilla. We find that our scheme is compatible with the protected regime of the 0-π qubit, and does not suffer from spurious coupling to additional modes of the 0-π circuit. Through numerical simulations, we study how the gate error scales with the circuit parameters of the 0-π qubit and the tunable Josephson element that enacts the gate.
arxiv pdf

Universal flux-based control of a π-SQUID

  1. J. Wilson Staples,
  2. Thomas B. Smith,
  3. and Andrew C. Doherty
We describe a protocol for the universal control of non-ideal π-periodic superconducting qubits. Our proposal relies on a π-SQUID: a superconducting loop formed by two π-periodic
circuit elements, with an external magnetic flux threading the circuit. The system exhibits an extensive sweet spot around half-flux where residual 2π-periodic Cooper pair tunneling is highly suppressed. We demonstrate that universal single-qubit operations can be realised by tuning the flux adiabatically and diabatically within this broad sweet spot. We also assess how residual 2π-periodicity in π-SQUIDs impacts holonomic phase gates.
arxiv pdf