qubit (rus. кубит otherwise q-бит) — quantum bit, or the smallest unit of information storage in a quantum computer.


Information in a quantum computer is encoded in quantum bits, or qubits. Like a bit, a qubit can be in one of the two eigenstates (conventionally |0> and |1>), but unlike the first, the qubit allows superposition of these states (i.e. it can be in the state A*|0>+B*|1>, where A and B are complex numbers satisfying the normalisation condition | A |2 + | B |2 = 1), and thus is more "informative".

A qubit accidentally turns into one of its eigenstates in case of any measurement. Qubits can be entangled with each other (i.e. have non-observed bonds with each other); and any impact on one qubit of such an entangled system causes the other qubits of the system to change their states consistently. The set of qubits entangled this way can be interpreted as a loaded quantum register.

Any two-level system (spin, photon, atom, molecule, ion) with a wave function determining its values can be used for physical implementation of qubit. A message is a sequence of N qubits, i.e. it corresponds to the wave function of N variables [1].

It is necessary, first of all, while developing of a quantum processor, to choose a physical system (physical basis of the processor); today, the most widely discussed options of such systems are the following:

- spin-1/2 nuclei bonded by indirect spin-spin interactions (logical operations on qubits (using radio-frequency pulses) and result output are performed using standard methods of nuclear magnetic resonance);

- energy levels of ions captured in ion traps (such ion traps are created in a vacuum by a certain configuration of electric field) in the conditions of laser cooling of the ions to micro-Kelvin temperatures (individual control of the ions is carried out using infra-red lasers);

- charge states of superconducting quantum dots connected by Josephson junctions (charge qubit);

- quantum states of superconducting quantum interferometers (SQUIDs) with Josephson junctions in a superconducting ring (phase qubit).


  • Razumovsky Alexey S.


  1. Zvezdin A.K. Magnetic molecules and quantum mechanics (in Russian) // Priroda. — №12, 2000 — 11—19 pp.

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