PRX Quantum (Dec 2021)

Fault-Tolerant Qubit from a Constant Number of Components

  • Kianna Wan,
  • Soonwon Choi,
  • Isaac H. Kim,
  • Noah Shutty,
  • Patrick Hayden

DOI
https://doi.org/10.1103/PRXQuantum.2.040345
Journal volume & issue
Vol. 2, no. 4
p. 040345

Abstract

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With gate error rates in multiple technologies now below the threshold required for fault-tolerant quantum computation, the major remaining obstacle to useful quantum computation is scaling, a challenge greatly amplified by the huge overhead imposed by quantum error correction itself. We propose a fault-tolerant quantum computing scheme that can nonetheless be assembled from a small number of experimental components, potentially dramatically reducing the engineering challenges associated with building a large-scale fault-tolerant quantum computer. Our scheme has a threshold of 0.39% for depolarizing noise, assuming that memory errors are negligible. In the presence of memory errors, the logical error rate decays exponentially with sqrt[T/τ], where T is the memory coherence time and τ is the timescale for elementary gates. Our approach is based on a novel procedure for fault-tolerantly preparing three-dimensional cluster states using a single actively controlled qubit and a pair of delay lines. Although a circuit-level error may propagate to a high-weight error, the effect of this error on the prepared state is always equivalent to that of a constant-weight error. We describe how the requisite gates can be implemented using existing technologies in quantum photonic and phononic systems. With continued improvements in only a few components, we expect these systems to be promising candidates for demonstrating fault-tolerant quantum computation with a comparatively modest experimental effort.