Computer Science; System-Level Programming; Computer Engineering

Quantum computing is an emerging field of computer engineering that uses charged particles rather than silicon electrical circuitry to process signals. Engineers believe that quantum computing has the potential to advance far beyond the limitations of traditional computing technology.

**entanglement:**the phenomenon in which two or more particles’ quantum states remain linked even if the particles are later separated and become part of distinct systems.**quantum logic gate:**a device that alters the behavior or state of a small number of qubits.**quantum bit (qubit):**a basic unit of quantum computation that can exist in multiple states at the same time, and can therefore have multiple values simultaneously.**state:**a complete description of a physical system at a specific point in time, including such factors as energy, momentum, position, and spin.**superposition:**the principle that two or more waves, including waves describing quantum states, can be combined to give rise to a new wave state with unique properties. This allows a qubit to potentially be in two states at once.

Quantum computing is an emerging field of computing that uses subatomic particles rather than silicon circuitry to transmit signals and perform calculations. Quantum physics studies particle behavior at the subatomic scale. At extremely small scales, subatomic particles such as photons (the basic unit of light) exhibit properties of both particles and waves. This phenomenon, called wave-particle duality, gives subatomic particles unique properties. Traditional computer algorithms are constrained by the physical properties of digital electrical signals. Engineers working on quantum computing hope that quantum algorithms, based on the unique properties of quantum mechanics, will be able to complete computations faster and more efficiently.

Digital computing uses electrical signals to create binary data. Binary digits, called bits, have two possible values: 0 or 1. Digital computers also use logic gates. These are electronic circuits that process bits of data by amplifying or changing signals. Logic gates in digital computers accept one or more inputs and produce only one output. In quantum computing, digital bits are replaced by quantum bits (qubits). Qubits are created by manipulating subatomic particles.

The value of a qubit represents its current quantum state. A quantum state is simply all known data about a particle, including its momentum, physical location, and energetic properties. To be used as a qubit, a particle should have two distinct states, representing the binary 0 and 1. For example, if the qubit is a photon, the two states would be horizontal polarization (0) and vertical polarization (1). However, a particle in a quantum system can exist in two or more states at the same time. This principle is called superposition. Thus, a qubit is not limited to binary values of either 0 or 1. Instead, it can have a value of 0, 1, or any superposition of both 0 and 1 at the same time.

Quantum particles also display a property known as entanglement. This is when two or more particles are linked in such a way that changing the state of one particle changes the state of the other(s), even after they are physically separated. Entanglement could potentially allow for the development of quantum computers that can instantly transmit information across great distances and physical barriers.

Current designs for quantum computers use energetic particles such as electrons or photons as qubits. The states of these particles are altered using quantum logic gates, much like digital logic gates alter electrical signals. A quantum gate may operate using energy from lasers, electromagnetic fields, or several other methods. These state changes can then be used to calculate data.

One avenue of research is the potential derivation of qubits from ion traps. Ions are atoms that have lost or gained one or more electrons. Ion traps use electric and magnetic fields to catch, keep, and arrange ions.

As of 2016, the practical value of quantum computing had only been demonstrated for a small set of potential applications. One such application is Shor's algorithm, created by mathematician Peter Shor, which involves the mathematical process of factorization. Factorization is used to find two unknown prime numbers that, when multiplied together, give a third known number. Shor's algorithm uses the properties of quantum physics to speed up factorization. It can perform the calculation twice as fast as a standard algorithm. Researchers have also demonstrated that quantum algorithms might improve the speed and accuracy of search engines. However, research in this area is incomplete, and the potential benefits remain unclear.

There are significant challenges to overcome before quantum computing could become mainstream. Existing methods for controlling quantum states and manipulating particles require highly sensitive materials and equipment. Scientists working on quantum computers argue that they may make the biggest impact in technical sciences, where certain math and physics problems require calculations so extensive that solutions could not be found even with all the computer resources on the planet. Special quantum properties, such as entanglement and superposition, mean that qubits may be able to perform parallel computing processes that would be impractical or improbable with traditional computer technology.

*—Micah L. Issitt*

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