Qubit
In quantum computing, a qubit (/ˈkjuːbɪt/) or quantum bit is a basic unit of quantum information—the quantum version of the classic binary bit physically realized with a two-state device. A qubit is a two-state (or two-level) quantum-mechanical system, one of the simplest quantum systems displaying the peculiarity of quantum mechanics. Examples include the spin of the electron in which the two levels can be taken as spin up and spin down; or the polarization of a single photon in which the two states can be taken to be the vertical polarization and the horizontal polarization. In a classical system, a bit would have to be in one state or the other. However, quantum mechanics allows the qubit to be in a coherent superposition of both states simultaneously, a property that is fundamental to quantum mechanics and quantum computing.
Etymology
The coining of the term qubit is attributed to Benjamin Schumacher. In the acknowledgments of his 1995 paper, Schumacher states that the term qubit was created in jest during a conversation with William Wootters.
Bit versus qubit
A binary digit, characterized as 0 or 1, is used to represent
information in classical computers. When averaged over both of its
states (0, 1), a binary digit can represent up to one bit of Shannon
information, where a bit is the basic unit of information. However, in
this article, the word bit is synonymous with a binary digit.
In classical computer technologies, a processed bit is implemented by
one of two levels of low DC voltage, and whilst switching from one of
these two levels to the other, a so-called forbidden zone
between
two logic levels must be passed as fast as possible, as electrical
voltage cannot change from one level to another instantaneously.
There are two possible outcomes for the measurement of a qubit—usually
taken to have the value 0 and 1, like a bit or binary digit.
However, whereas the state of a bit can only be either 0 or 1, the
general state of a qubit according to quantum mechanics can be a
coherent superposition of both. Moreover, whereas a measurement of a
classical bit would not disturb its state, a measurement of a qubit
would destroy its coherence and irrevocably disturb the superposition
state. It is possible to fully encode one bit in one qubit. However, a
qubit can hold more information, e.g., up to two bits using superdense
coding.
For a system of n components, a complete description of its state in classical physics requires only n bits, whereas in quantum physics it requires 2n complex numbers (or a single point in a 2n-dimensional vector space).
Quantum register
A number of qubits taken together is a qubit register. Quantum computers perform calculations by manipulating qubits within a register.
Qudits and qutrits
The term qudit denotes the unit of quantum information that can be realized in suitable d-level quantum systems. A qubit register that can be measured to N states is identical to an N-level qudit. A rarely used synonym for qudit is quNit, since both d and N are frequently used to denote the dimension of a quantum system.
Qudits are similar to the integer types in classical computing, and may be mapped to (or realized by) arrays of qubits. Qudits where the d-level system is not an exponent of 2 can not be mapped to arrays of qubits. It is for example possible to have 5-level qudits.
Qubit storage
In 2008 a team of scientists from the U.K. and U.S. reported the first
relatively long (1.75 seconds) and coherent transfer of a
superposition state in an electron spin processing
qubit to a
nuclear spin memory
qubit. This event can be considered the first
relatively consistent quantum data storage, a vital step towards the
development of quantum computing. In 2013, a modification of similar
systems (using charged rather than neutral donors) has dramatically
extended this time, to 3 hours at very low temperatures and 39 minutes
at room temperature. Room temperature preparation of a qubit based on
electron spins instead of nuclear spin was also demonstrated by a team
of scientists from Switzerland and Australia. An increased coherence
of qubits is being explored by researchers who are testing the
limitations of a Ge hole spin-orbit qubit structure.