40 years of quantum computing

In 2022 we celebrate four decades of quantum computing by looking back at the milestones of the field and forward to the challenges and opportunities that lie ahead.

The first quantum error-correcting code was devised by Peter Shor 25 years ago. Ever since there have been numerous advances on both the theoretical and experimental fronts, and quantum error correction turned out to have unexpected applications.

In 2000, David DiVincenzo gave a set of basic criteria for building a quantum computer, which have guided research for the past 20 years. Today, despite not all the criteria having been cleared, additional complex requirements have emerged.

Twenty-five years ago, a paper by Ignacio Cirac and Peter Zoller turned quantum computing from a bold theoretical idea to an experimental race to build an actual device. Today, engineering challenges remain, but first-generation practical quantum computers seem tangible.

Quantum computing technologies are advancing, and the class of addressable problems is expanding. What market strategies are quantum computing companies and start-ups adopting?

As superconducting quantum computing research progresses in industry and academic laboratories, researchers are increasingly aware of the importance of the big picture — a full quantum stack where each layer presents specific and related technical challenges.

More than 40 years ago the first Bell tests translated a purely philosophical conundrum to a physical experiment. In doing so, they changed our understanding of quantum mechanics and contributed to the development of quantum technologies.

The advent of commercial quantum devices has ushered in the era of near-term quantum computing. Variational quantum algorithms are promising candidates to make use of these devices for achieving a practical quantum advantage over classical computers.

Quantum annealing is a widely used heuristic algorithm for optimization and sampling, implemented in commercial processors. This Review provides a critical assessment of the field and points to new opportunities for a quantum advantage via recently developed alternative quantum annealing protocols.

A variety of quantum programming languages have been developed over the past few years, enabling newcomers and seasoned practitioners alike. This Review gives a brief introduction to quantum programming, overviewing some of the existing languages and the ecosystem around them.

Quantum technologies require an extremely precise functioning of their components which is ensured by sophisticated tools for device characterization. This Technical Review surveys and assesses the currently available tools according to their overall complexity, information gain, and underlying assumptions.

Photonics is one of the key platforms for emerging quantum technologies, but its full potential can only be harnessed by exploiting miniaturization via on-chip integration. This Roadmap charts new directions and discusses the challenges associated with the hybrid integration of a variety of materials, devices and components.

Magnetic molecules have been widely proposed for different quantum technologies due to their bewildering quantum properties. This Review describes techniques of paramount importance for the characterization, understanding and, ultimately, manipulation of the electronic properties of these systems.

Semiconductor qubits are expected to have diverse future quantum applications. This Review discusses semiconductor qubit implementations from the perspective of an ecosystem of applications, such as quantum simulation, sensing, computation and communication.

Quantum simulation with ultracold atomic gases is an established platform for investigating complex quantum processes. Focusing on optical lattice experiments, this Technical Review overviews the available tools and their applications to the simulation of solid-state physics problems.

Ion traps enable the precise control and manipulation of the quantum state of a trapped ion. This Technical Review discusses the way in which ion-trap microchips can be fabricated and integrated with advanced on-chip features for implementing practical quantum technologies.

The integration of gate-defined quantum dots with superconducting resonators results in a hybrid architecture that holds promise for quantum information processing. This Review discusses recent experimental results in the field, including the achievement of strong coupling between single microwave photons and the charge and spin degrees of freedom, and examines the underlying physics.

The study of higher-dimensional quantum states has seen numerous conceptual and technological developments. This review discusses various techniques for the generation and processing of qudits, which are stored in the momentum, path, time-/frequency-bins, or the orbital angular momentum of photons.

The dynamics of quantum information is opening new perspectives on the behaviour of complex many-body systems. This Perspective covers progress made with atomic gases and trapped ions for accessing the dynamics of quantum correlations, entanglement and information scrambling in a broad parameter regime.

Entanglement is often considered the defining feature separating classical physics from quantum physics and provides the basis for many quantum technologies. This Review discusses recent progress in the challenging task of conclusively proving that a physical system features entanglement, surveying detection and certification methods.