Universal quantum computation with symmetric qubit clusters coupled to an environment

TL;DR

This paper proposes a scalable quantum computing scheme using symmetric qubit clusters, called quansistors, which are protected from decoherence and errors through symmetry, enabling robust universal quantum operations.

Contribution

It introduces a novel approach employing omega-rotation invariance in qubit clusters to enhance error resilience and scalability in quantum computation.

Findings

Symmetry makes logical operations resilient to anisotropic qubit rotations.

Dynamical decoupling from leads enables controllable quantum memory units.

The scheme is adaptable to higher-level systems and other symmetries.

Abstract

One of the most challenging problems for the realization of a scalable quantum computer is to design a physical device that keeps the error rate for each quantum processing operation low. These errors can originate from the accuracy of quantum manipulation, such as the sweeping of a gate voltage in solid state qubits or the duration of a laser pulse in optical schemes. Errors also result from decoherence, which is often regarded as more crucial in the sense that it is inherent to the quantum system, being fundamentally a consequence of the coupling to the external environment. Grouping small collections of qubits into clusters with symmetries can protect parts of the calculation from decoherence. We use 4-level cores with a straightforward generalization of discrete rotational symmetry, omega-rotation invariance, to encode pairs of coupled qubits and universal 2-qubit logical gates. We include quantum errors as a main source of decoherence, and show that symmetry makes logical operations particularly resilient to untimely anisotropic qubit rotations. We propose a scalable scheme for universal quantum computation where cores play the role of quantum-computational transistors, quansistors. Initialization and readout are achieved by coupling to leads. The external leads are explicitly considered and are assumed to be the other main source of decoherence. We show that quansistors can be dynamically decoupled from the leads by tuning their internal parameters, giving them the versatility required to act as controllable quantum memory units. With this dynamical decoupling, logical operations within quansistors are also symmetry-protected from unbiased noise in their parameters. We identify technologies that could implement omega-rotation invariance. Many of our results can be generalized to higher-level omega-rotation-invariant systems, or adapted to clusters with other symmetries.