This piece is part of a package on the future of quantum computing. Read about the quest to develop these machines here and their most promising applications here.
Around the world researchers are racing to build computers that take advantage of the bizarre rules of quantum mechanics. Such machines could perform calculations impossible for conventional computers and solve certain problems much more quickly.
The magic of quantum computers comes from their quantum bits, or qubits. Classical computer bits can occupy one of two states: 0 and 1. Quantum bits, however, can be placed in a weird state called a superposition, where they simultaneously occupy some combination of 0 and 1. Qubits, therefore, can take on an infinite number of possible states akin to the infinite points on the surface of a sphere. Qubits can also experience entanglement—a special quantum connection that enables operations on one qubit to affect another qubit.
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These abilities enable their special feats of computing. Imagine that a classical computer solves a maze by trying one path at a time. Quantum computers, in contrast, can essentially explore all possible routes at once. When scientists measure a qubit, however, its limitless possibilities collapse into a single option.
Source: “The Qubit,” by Massine Kelai/Center for Quantum Nanoscience (https://qns.science/thequbit) (reference)
But what exactly is a qubit? Qubits can be encoded in many different physical systems, and researchers are still pursuing multiple alternatives. “It is a completely wide-open space right now,” says quantum computing scientist Nathalie de Leon of Princeton University, who is also a visiting research faculty member at Google Quantum AI. “All of these platforms still have open science questions in addition to engineering and scaling risk.” Here are some of the options.
Superconducting qubits are made of tiny circuits of materials that conduct electricity with zero resistance at ultracold temperatures. The energy level of the circuit determines a qubit’s state: when the circuit absorbs a microwave photon, the qubit jumps from the ground state (0) to the first excited state (1). Some scientists favor these qubits because they can perform operations very quickly.
These qubits are based on the spin state of single particles. They include electrons in semiconductors confined by electrostatic traps, electrons and nuclear spins associated with atomic defects in semiconductors, electrons floating on liquid helium, and defect centers in wide-bandgap materials. Chips made with them can be wired using the same technology used in classical semiconductors.
These atoms have no net electric charge. Scientists can make them into qubits by using lasers to trap, manipulate and read them. Their state is determined by the spin of the electron or the spin of the atomic nucleus. They are prized by some researchers for the ease with which they can be combined to scale up into large numbers of qubits.
These qubits are made of particles of light, called photons, and their state is encoded in the direction of the photon’s travel along a spatial pathway called a rail. One advantage of them is that they can be scaled up into larger and larger computers by means of the same techniques that helped to scale up classical optical and electronic chips.
These qubits use the spin states of individual ions (charged atoms) that scientists hold in place with electromagnetic fields and manipulate with lasers. The atoms may be, for example, calcium, magnesium or beryllium. Some early experiments used orbital positions of electrons as qubit states, as shown below. They have demonstrated the lowest error rates for gates between qubits.
Instead of using circuits or individual atoms, topological qubits are made of quasiparticles called anyons. They are theorized to be less error prone than other types of qubits, but scientists are still working on demonstrating them experimentally.
Ben Gilliland
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