What is a quantum computer, and how do we build one?

TL;DR

This paper defines general operational criteria for quantum computers beyond the circuit model, emphasizing scalability and fault-tolerance, and provides a decision framework for choosing implementation paradigms.

Contribution

It introduces a formal definition of quantum computers with four core criteria applicable to various paradigms, expanding beyond the traditional DiVincenzo criteria.

Findings

Quantum computers require quantum memory, controlled evolution, cooling, and readout.

Scalability and fault-tolerance are essential for quantum computer implementation.

A decision tree helps identify suitable physical paradigms for building quantum computers.

Abstract

The DiVincenzo criteria for implementing a quantum computer have been seminal in focussing both experimental and theoretical research in quantum information processing. These criteria were formulated specifically for the circuit model of quantum computing. However, several new models for quantum computing (paradigms) have been proposed that do not seem to fit the criteria well. The question is therefore what are the general criteria for implementing quantum computers. To this end, a formal operational definition of a quantum computer is introduced. It is then shown that according to this definition a device is a quantum computer if it obeys the following four criteria: Any quantum computer must (1) have a quantum memory; (2) facilitate a controlled quantum evolution of the quantum memory; (3) include a method for cooling the quantum memory; and (4) provide a readout mechanism for subsets of the quantum memory. The criteria are met when the device is scalable and operates fault-tolerantly. We discuss various existing quantum computing paradigms, and how they fit within this framework. Finally, we lay out a roadmap for selecting an avenue towards building a quantum computer. This is summarized in a decision tree intended to help experimentalists determine the most natural paradigm given a particular physical implementation.