Qubit Devices Inch Toward Reality

  Some optimistic views but challenges still exist, good overview.

Qubit Devices Inch Toward Reality,  By Samuel Greengard

Communications of the ACM, November 2021, Vol. 64 No. 11, Pages 11-13   10.1145/3484988

The march toward functional quantum computing devices has taken a long and winding road. Although the concept has been around since the late 1970s, when physicists Paul Benioff, Richard Feynman, and others began to explore quantum information theory, only recently have actual devices begun to take shape. Several companies, including IBM, have developed prototype quantum computing systems, while many research organizations have experimental devices in early stages of development.

Yet unlike classical computing, which has evolved over more than 70 years and is now mature, quantum computing, which harnesses quantum physics to leverage the uncertainty of a quantum state versus the certitude of a classical state, remains largely uncharted territory. An enormous amount of research is currently focused on ways to create or utilize quantum bits ("qubits") to construct quantum mechanical systems that can harness physical events in nature to solve complex computing problems lying outside the practical grasp of classical systems. At the moment, qubit research remains in a relatively nascent state and, as a result, it is not clear which approaches will ultimately prevail.

"There's an enormous push to develop different quantum systems that could prove stable enough to be useful," says Michael Cuthbert, director of the National Quantum Computing Center in Oxfordshire, U.K.

For now, qubit research is heavily focused on a handful of key technologies: superconducting circuits, trapped ions, photonics, ultra-cold atoms, spins in silicon, color spins in diamonds, and an emerging area known as topological insulators.

Key questions and challenges remain, including how to scale devices while reducing noise and errors to the point where qubit devices become useful. Says Travis Humble, deputy director of the U.S. Department of Energy (DOE) Quantum Science Center at Oak Ridge National Laboratory, "We are reaching a critical point where quantum computing technology is advancing rapidly. Over the next few years, we may see actual devices emerge that solve real-world challenges in drug development, financial modeling, cyber-security, and physics."

A Quantum Challenge

Quantum computing devices are emerging in several shapes and forms—and they use radically different methods to process calculations. The common denominator is that all these systems use a quantum circuit to handle calculations. As with bits in classical computing models, qubits operate in a 1 or 0 state. Unlike classical computing, the information is challenging to record; moreover, the physics of the quantum realm unlocks the possibility of the occurrence of both 1s and 0s together. As a result, it is necessary to measure the qubit before and after a change, to understand its state at any precise moment.

Qubits typically maintain useful information for very brief periods of time, in some cases no more than a millisecond, as they are prone to high levels of noise and errors. By comparison, silicon can run a billion classical operations per second for a billion years before a statistical error occurs. "It's possible to mitigate this instability through error correction," Cuthbert says, "but the error correction comes with significant overhead. It's necessary to reach somewhere around 1,000 physical qubits to get to the point where the error correction works effectively, creating a single logical or 'forever' qubit."

So far, researchers and engineers have been able to build qubit devices to a scale of just over 60 qubits. What is needed are devices that can reach at least 1,000 qubits, or for a fully fault-tolerant error-corrected machine, 1 million qubits, experts say. Although better algorithms can address part of the noise and error problem by calculating more efficiently, they cannot address it entirely.

The goal, then, is to build qubit devices that break through today's barriers and serve as components for quantum computers. While digital devices encode electrical signals in a string of ones and zeros, quantum systems require a highly stable two-level system that can establish a superposition of a one or zero, as well as the entanglement of states between different qubits.   .. '