The basic element of information in quantum computing; an abbreviation of quantum bit. In conventional computing, the basic element of information is the bit, having values of either 0 or 1. This is a two-state (binary) system. In quantum computing, information is also encoded using two states, but the quantum system is not restricted to existing merely in one or other, and can exist as a combination of both simultaneously. The qubit is in a superposition of two quantum states, a 0 state and a 1 state. Reading a qubit gives a certain probability of finding a 0 state, and a certain probability of finding a 1 state, and in the process destroys the special character of the qubit.
Representation
The states a qubit may be measured in are known as basis states (or vectors).
Qubit states
A pure qubit state is a linear superposition of those two states. Much like Schrödinger's cat, a qubit can exist in more than one state, but measuring that qubit causes that superposition to collapse into one state or the other, according to the probabilities mentioned above.
Entanglement
An important distinguishing feature between a qubit and a classical bit is that multiple qubits can exhibit quantum entanglement. Take, for example, two entangled qubits in the Bell state
(Note that in this state, there are equal probabilities of measuring either or .)
Imagine that these two entangled qubits are separated, with one each given to Alice and Bob.
The use of entanglement in quantum computing has been referred to as "quantum parallelism", and offers a possible explanation for the power of quantum computing: because the state of the computer can be in a quantum superposition of many different classical computational paths, these paths can all proceed concurrently.
Quantum register
A number of entangled qubits taken together is a qubit register.
Variations of the qubit
Similar to the qubit, a qutrit is a unit of quantum information in a 3-level quantum system.
This is an incomplete list of physical implementation of qubits:
| Physical support | Name | Information support | "0" | "1" |
|---|---|---|---|---|
| Single photon (Fock states) | Polarization encoding | Polarization of light | Horizontal | Vertical |
| Photon number | Photon number | Vacuum | Single photon state | |
| Time-bin encoding | Time of arrival | Early | Late | |
| Coherent state of light | Squeezed light | Quadrature | Amplitude-squeezed state | Phase-squeezed state |
| Electrons | Electronic spin | Spin | Up | Down |
| Electron number | Charge | No electron | One electron | |
| Nucleus | Nuclear spin addressed through NMR | Spin | Up | Down |
| Optical lattices | Atomic spin | Spin | Up | Down |
| Josephson junction | Superconducting charge qubit | Charge | Uncharged capacitor | Charged capacitor |
| Josephson junction | Superconducting flux qubit | Current | No current | Current |
| Singly-charged quantum dot pair | Electron localization | Charge | Electron on left dot | Electron on right dot |
User Comments Add a comment…