qbit (qubit or quantum bit) is the basic unit of quantum information—the quantum version of the classical binary bit physically realized with a two-state device.

Qbit 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;
  • the polarization of a single photon in which the two states can be taken to be the vertical polarization and the horizontal polarization.

Qbit can be in a coherent superposition of both states/levels simultaneously, where a classical system, a bit would have to be in one state or the other which is a property which is fundamental to quantum mechanics and quantum computing.

Bit versus Qbit#

A binary digit, characterized as 0 and 1, is used to represent information in classical computers. A binary digit can represent up to one bit of data entropy, where a bit is the basic unit of data. Typically the word bit is synonymous with binary digit.

In classical computer technologies, a processed bit is implemented by one of two levels of low DC voltage, and switching from one of these two levels to the other, a so-called forbidden zone 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 qbit—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 qbit 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 qbit would destroy its coherence and irrevocably disturb the superposition state. It is possible to fully encode one bit in one qbit. However, a qbit 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−1 complex numbers.

More Information#

There might be more information for this subject on one of the following: