Quantum computation with prethreshold superconducting qubits: Single-excitation subspace approach

TL;DR

This paper proposes a novel quantum computing approach using the single-excitation subspace of superconducting qubits, enabling high-dimensional operations in a single step suitable for near-term hardware.

Contribution

It introduces a SES-based quantum computation method that simplifies implementation on current hardware, bypassing complex gate decompositions.

Findings

Allows direct Hamiltonian generation in SES

Enables high-dimensional unitary operations in one step

Practical for early quantum computers with limited qubits

Abstract

We describe an alternative approach to quantum computation that is ideally suited for today's sub-threshold-fidelity qubits, and which can be applied to a family of hardware models that includes superconducting qubits with tunable coupling. In this approach, the computation on an n-qubit processor is carried out in the n-dimensional single-excitation subspace (SES) of the full 2^n-dimensional Hilbert space. Because any real Hamiltonian can be directly generated in the SES [E. J. Pritchett et al., arXiv:1008.0701], high-dimensional unitary operations can be carried out in a single step, bypassing the need to decompose into single- and two-qubit gates. Although technically nonscalable and unsuitable for applications (including Shor's) requiring enormous Hilbert spaces, this approach would make practical a first-generation quantum computer capable of achieving significant quantum speedup.