Quantum computing: qubits

December 19, 2005
Quantum bits, or qubits, are the quantum equivalent of the transistors that make up todayís computers. In order to carry out the logic of computing, there must be some way to represent the 1s and 0s of computer information. The many candidate qubits all have one thing in common - the ability to switch from one state to a second state. These states are used to represent binary information.

Qubits use properties of one of four types of quantum particles: photons, electrons, atoms and ions. Photons do not interact with each other very well but they travel easily from one place to another, which makes them appropriate for transmitting quantum information. Electrons, atoms and ions don't travel well but readily interact, which makes them appropriate for storing and processing quantum information.

Photons

The electric field of unpolarized photons vibrates in a plane perpendicular to the photonís course. Polarized photonsí electric fields, however, vibrate in only one of four directions within that plane: vertical, horizontal and the two diagonals. Two pairs of polarizations can represent 1 and 0 respectively.

Photons can be controlled by mirrors and polarizing filters, which block all photons but those with one particular polarization orientation.

The phase, or wave cycle, of photons and their times of arrival can also be used as qubits.

Electrons

Electrons are oriented in one of two directions, spin up and spin down, which are akin to the two poles of a magnet. Electrons can be switched between the two states using electric, magnetic or optical fields. An electronís position within a quantum dot can also be used to represent a binary number.

Atoms and ions

Atoms and ions are more complicated objects than electrons and have several ways of representing information. Ions are atoms that contain a charge because they have an extra or missing electron.

Like electrons, atoms have a spin orientation that can be used to represent binary numbers in a qubit. The position of an atomís outer electron - at the low-energy level or at a higher-energy level - can also be used to represent 1s and 0s. Atoms that are trapped and cooled vibrate in discrete quantum steps that can also be used in a qubit. A fourth type of atomic qubit is based on hyperfine levels, or subtle variations in electron orbital levels caused by the magnetic interactions between the nucleus and electrons.

Qubits

Qubits are made up of controlled particles and the means of control - devices that trap particles and switch them from one state to another. There are four established qubit candidates: ion traps, quantum dots, semiconductor impurities, and superconducting circuits.

Ion traps

Ion traps use optical and/or magnetic fields to contain individual ions. Researchers have entangled as many as six ions in a single ion trap. Ion trap technology is well-established and is likely to be able to scale up to large numbers of qubits. Because ions are charged, they are more vulnerable to environmental noise than neutral atoms.

Quantum dots

Quantum dots are bits of semiconductor material that contain one or a few electrons. Quantum dots can be reliably loaded with individual electrons, and they can be readily integrated into electronic devices. Current prototypes, however, work only at extremely low temperatures.

Semiconductor impurities

Atoms embedded in semiconductor materials are commonly found as impurities, or flaws in computer chips. It is difficult to make a pure chip - there tends to be an unwanted atom of some kind in every few billion semiconductor atoms. Semiconductor impurity qubits use electrons contained in phosphorus or other atoms intentionally introduced into semiconductor materials; the electron states can be controlled using lasers or electric fields.

Superconducting circuits

Superconducting circuits are electrical circuits made of superconducting material, which allows electrons to flow with almost no resistance at extremely low temperatures. Superconducting circuits can form qubits in several ways, including the flow of current itself, which can be made to flow in both directions at once in the quantum state of superposition.

Electrons pair up to flow through a superconductor, and billions of these pairs form a single entity that behaves as one giant subatomic particle when the superconductor contains a tiny break. When one of the circuits, dubbed Josephson junctions, is connected to a reservoir of electron pairs, the number of pairs in the reservoir can be changed by exactly one, and this change can be reliably measured.

Superconducting circuits can be made using semiconductor manufacturing techniques. The principal advantage is that they use millions or billions of electrons rather than requiring control over individual particles. The drawback is that they operate at extremely low temperatures.

Optical traps

Neutral atoms in optical traps are another candidate type of qubit. Optical traps work because light waves are strong enough at the atomic level to trap and control particles, much like wind pushing a windmill. Atoms are less vulnerable to noise than ions, but itís harder to make atoms interact.


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