|Unsolved problem in physics:
In quantum computing, a qubit (//) or quantum bit (sometimes qbit) 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. However, quantum mechanics allows the qubit to be in a superposition of both states at the same time, a property that is fundamental to quantum computing.
Origin of the name
The coining of the term "qubit" is attributed to Benjamin Schumacher. In the acknowledgments of his paper, Schumacher states that the term qubit was invented in jest, during a conversation with William Wootters. The paper describes a way of compressing states emitted by a quantum source of information so that they require fewer physical resources to store. This procedure is now known as Schumacher compression.
Bit versus qubit
The bit is the basic unit of information. It is used to represent information by computers. Regardless of its physical realization, a bit has two possible states typically thought of as 0 and 1, but more generally—and according to applications—interpretable as true and false, or any other dichotomous choice. An analogy to this is a light switch—its OFF position can be thought of as 0 and its ON position as 1.
A qubit has a few similarities to a classical bit, but is overall very different. There are two possible outcomes for the measurement of a qubit—usually 0 and 1, like a bit. The difference is that whereas the state of a bit is either 0 or 1, the state of a qubit can also be a superposition of both. It is possible to fully encode one bit in one qubit. However, a qubit can hold even 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.
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, they are represented by Dirac—or "bra–ket"—notation. This means that the two computational basis states are conventionally written as and (pronounced "ket 0" and "ket 1").
When we measure this qubit in the standard basis, the probability of outcome is and the probability of outcome is . Because the absolute squares of the amplitudes equate to probabilities, it follows that and must be constrained by the equation
It might, at first sight, seem that there should be four degrees of freedom, as and are complex numbers with two degrees of freedom each. However, one degree of freedom is removed by the normalization constraint |α|2 + |β|2 = 1, which can be treated as the equation for a 3-sphere embedded in 4-dimensional space with a radius of 1 (unit sphere). This means, with a suitable change of coordinates, one can eliminate one of the degrees of freedom. One possible choice is that of Hopf coordinates:
Additionally, for a single qubit the overall phase of the state ei ψ has no physically observable consequences, so we can arbitrarily choose α to be real (or β in the case that α is zero), leaving just two degrees of freedom:
The possible states for a single qubit can be visualised using a Bloch sphere (see diagram). Represented on such a sphere, a classical bit could only be at the "North Pole" or the "South Pole", in the locations where and are respectively. The rest of the surface of the sphere is inaccessible to a classical bit, but a pure qubit state can be represented by any point on the surface. For example, the pure qubit state would lie on the equator of the sphere, on the positive y axis.
The surface of the sphere is a two-dimensional space, which represents the state space of the pure qubit states. This state space has two local degrees of freedom.
It is possible to put the qubit in a mixed state, a statistical combination of different pure states. Mixed states can be represented by points inside the Bloch sphere. A mixed qubit state has three degrees of freedom: the angles and , as well as the length of the vector that represents the mixed state.
Operations on pure qubit states
There are various kinds of physical operations that can be performed on pure qubit states.
- A quantum logic gate can operate on a qubit: mathematically speaking, the qubit undergoes a unitary transformation. Unitary transformations correspond to rotations of the qubit vector in the Bloch sphere.
- Standard basis measurement is an operation in which information is gained about the state of the qubit. The result of the measurement will be either , with probability , or , with probability . Measurement of the state of the qubit alters the values of α and β. For instance, if the result of the measurement is , α is changed to 1 (up to phase) and β is changed to 0. Note that a measurement of a qubit state entangled with another quantum system transforms a pure state into a mixed state.
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. Take, for example, two entangled qubits in the Bell state
In this state, called an equal superposition, there are equal probabilities of measuring either or , as .
Imagine that these two entangled qubits are separated, with one each given to Alice and Bob. Alice makes a measurement of her qubit, obtaining—with equal probabilities—either or . Because of the qubits' entanglement, Bob must now get exactly the same measurement as Alice; i.e., if she measures a , Bob must measure the same, as is the only state where Alice's qubit is a . Entanglement also allows multiple states (such as the Bell state mentioned above) to be acted on simultaneously, unlike classical bits that can only have one value at a time. Entanglement is a necessary ingredient of any quantum computation that cannot be done efficiently on a classical computer. Many of the successes of quantum computation and communication, such as quantum teleportation and superdense coding, make use of entanglement, suggesting that entanglement is a resource that is unique to quantum computation.
Variations of the qubit
Similar to the qubit, the qutrit is the unit of quantum information in 3-level quantum systems. This is analogous to the unit of classical information trit. The term "qudit" (quantum digit) denotes the unit of quantum information in d-level quantum systems.
Any two-level 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 that 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.
The following is an incomplete list of physical implementations of qubits, and the choices of basis are by convention only.
|Physical support||Name||Information support|
|Photon||Polarization encoding||Polarization of light||Horizontal||Vertical|
|Number of photons||Fock state||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|
|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 superconducting island (Q=0)||Charged superconducting island (Q=2e, one extra Cooper pair)|
|Superconducting flux qubit||Current||Clockwise current||Counterclockwise current|
|Superconducting phase qubit||Energy||Ground state||First excited state|
|Singly charged quantum dot pair||Electron localization||Charge||Electron on left dot||Electron on right dot|
|Quantum dot||Dot spin||Spin||Down||Up|
In a paper entitled: "Solid-state quantum memory using the 31P nuclear spin", published in the October 23, 2008 issue of the journal Nature, 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. Recently, 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.
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