Quantum Computing

Background

Long a "Holy Grail" of scientists and engineers, quantum computers that can be used in practical applications are steadily inching closer to becoming reality. 

Built on the mind-bending properties of quantum physics—how things behave at very, very small scales—quantum computers, in principle, can solve problems and perform tasks that would take centuries for classical computers to perform. 

How it works

Traditional computers store and process information using fundamental units known as bits (see overview). A single bit exists in only two states, either on (1) or off (0). Every complex task your computer or smartphone performs, from word processing to streaming videos, is built off the storage and rapid processing of trillions of bits in different configurations.    

The fundamental units of information in quantum computers are qubits. Like bits, they have distinct states, but they behave much differently than classical bits. Two principles of quantum mechanics are helpful to illustrate why. 

The first is superposition. At very small scales, particles and matter are described by probability waves. This means qubits can simultaneously exist in two states—both 0 and 1—simultaneously, whereas classical bits can only be 0 or 1 at all times. 

The second is entanglement. Not only do tiny particles exist in multiple states at once, but in certain configurations, their probability waves become linked (or entangled). Now, not only is each particle simultaneously in multiple states, but the whole group exists in multiple connected states that can be manipulated all at once.

Together, these properties allow quantum computers to carry out complex operations orders of magnitudes faster than traditional computers. Roughly speaking, the speed increases exponentially with the number of qubits—a Google prototype recently solved a math problem that would take a supercomputer 47 years using just 70 qubits. 

Check out a deeper technical dive into the mechanics of quantum computers here.

Different approaches

Scientists have used a variety of techniques to create working quantum computers. Each attempts to balance stability, coherence, practicality, and the ability to manufacture at scale.   

The most common approach relies on superconducting qubits. These are microscopic circuits that behave like artificial atoms. They’re relatively easy to manufacture but must be operated close to absolute zero (almost minus 460 degrees Fahrenheit). Both Google and IBM have demonstrated such devices. 

Other approaches try to make use of light particles (photons), particles suspended in electric fields (trapped ions), quantum dots, spin-based qubits, exotic states of matter known as Majorana particles, and more. See various advantages of each here. 

A key requirement among all types is that it must maintain quantum coherence. Most existing qubits lose their connected states—or decohere—in a millisecond or less. 

Current state and beyond

A number of established tech companies and newer start-ups have demonstrated operational quantum computers. As of early 2024, two groups (IBM and Atom Computing) had broken the 1,000-qubit milestone.

However, unlike classical computers, the sheer number of qubits is less important than maintaining coherence and lowering error rates. Researchers have developed a metric known as quantum volume (see definition), which attempts to encapsulate overall performance and allows comparison between different computing approaches. 

So far, quantum computers have been used to solve representative theoretical problems, and companies like Microsoft provide tools allowing companies to prepare for fault-tolerant quantum computing on the cloud.