Characterization and tomography of a hidden qubit

TL;DR

This paper explores the challenges of controlling and measuring hidden qubits in quantum computing architectures, demonstrating full control and characterization in a superconducting two-qubit system with limited direct access.

Contribution

It introduces a method to control and characterize hidden qubits via connected control qubits and two-qubit gates, with experimental validation in a superconducting device.

Findings

Full control and measurement of hidden qubits achieved

Iterative process developed for quantum process tomography

High gate fidelities demonstrated in superconducting system

Abstract

In circuit-based quantum computing, the available gate set typically consists of single-qubit gates acting on each individual qubit and at least one entangling gate between pairs of qubits. In certain physical architectures, however, some qubits may be 'hidden' and lacking direct addressability through dedicated control and readout lines, for instance because of limited on-chip routing capabilities, or because the number of control lines becomes a limiting factor for many-qubit systems. In this case, no single-qubit operations can be applied to the hidden qubits and their state cannot be measured directly. Instead, they may be controlled and read out only via single-qubit operations on connected 'control' qubits and a suitable set of two-qubit gates. We first discuss the impact of such restricted control capabilities on the quantum volume of specific qubit coupling networks. We then experimentally demonstrate full control and measurement capabilities in a superconducting two-qubit device with local single-qubit control and iSWAP and controlled-phase two-qubit interactions enabled by a tunable coupler. We further introduce an iterative tune-up process required to completely characterize the gate set used for quantum process tomography and evaluate the resulting gate fidelities.