** Q-BIT**

Attention is increasingly focused on quantum computing as a path to the continued rapidgrowth of information-processing technology. But like other physical circuitry, quantumcomputers must face the uncomfortable fact that man-made objects aren’t exactreproductions of idealized devices and aren’t invariably perfectly reproducible. The consequences of this imperfection threaten the future of quantum computing.

Electronic technology using solid-state devices has advanced us toward increasingly faster and more powerful systems for half a century. The invention of the integrated circuit opened the door to miniaturization, which, by making capacitances smaller and reducing distances between devices,has proven to be the key to faster operation. However, some of the problems associated withfurther miniaturization are beginning to appear insoluble:

• More devices per unit area have raised the density of power dissipation to higher than stove-top densities, making heat removal a major problem.

• Greater component densities call for greater wiring densities, taxing interconnection technology.

• Smaller separations between a device’s elements increase the tunneling and leakage currents that add to power usage, hurting device performance.The formidable barriers to the continued improvements in silicon technology have intensiﬁedthe attention paid to alternatives such as quantumcomputing. Unfortunately, troubling obstacles stillstand in the way of physically implementing useful quantum computers.

In quantum computing, a **qubit** or **quantum bit** is a unit of quantum information—the quantum analogue of the classical bit.

A qubit is a two state quantum-mechanical system such as the polarization of a single photon: here the two states are vertical polarization and horizontal polarization. In a classical system, a bit would have to be in one state or the other, but quantum mechanics allows the qubit to be in a superposition of both states at the same time, a property which is fundamental to quantum computing.

Representation

The two states in which a qubit may be measured are known as basis states (or basis vectors). As is the tradition with any sort of quantum states, Dirac, or bra-ket notation, is used to represent them. This means that the two computational basis states are conventionally written as |0> and |1> .

Entanglement

An important distinguishing feature between a qubit and a classical bit is that multiple qubits can exhibit quantum entanglement. Entanglement is a nonlocal property that allows a set of qubits to express higher correlation than is possible in classical systems.

**Quantum register**

A number of entangled qubits taken together is a qubit register. Quantum computers perform calculations by manipulating qubits within a register. A *qubyte* is a collection of eight entangledqubits. It was first demonstrated by a team at the Institute of Quantum Optics and Quantum Information at the University of Innsbruck in Austria in December 2005.^{[1]}

Variations of the qubit

Similar to the qubit, a qutrit is a unit of quantum information in a 3-level quantum system. This is analogous to the unit of classical information trit. The term “**qudit**” is used to denote a unit of quantum information in a *d*-level quantum system.

Physical Representation

Any two-level quantum system can be used as a qubit. Multilevel systems can be used as well, if they possess two states that can be effectively decoupled from the rest (e.g., ground state and first excited state of a nonlinear oscillator). There are various proposals. Several physical implementations which approximate two-level systems to various degrees were successfully realized. Similarly to a classical bit where the state of a transistor in a processor, the magnetization of a surface in a hard disk and the presence of current in a cable can all be used to represent bits in the same computer, an eventual quantum computer is likely to use various combinations of qubits in its design.

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