Types of quantum information
In
quantum computing , a
qubit is a unit of information analogous to a
bit (binary digit) in
classical computing , but it is affected by
quantum mechanical properties such as
superposition and
entanglement which allow qubits to be in some ways more powerful than classical bits for some
tasks . Qubits are used in
quantum circuits and
quantum algorithms composed of
quantum logic gates to solve
computational problems , where they are used for
input/output and intermediate computations.
A physical qubit is a physical device that behaves as a
two-state quantum system , used as a component of a
computer system .
[1]
[2] A logical qubit is a physical or abstract qubit that performs as specified in a
quantum algorithm or quantum circuit
[3] subject to
unitary transformations , has a long enough
coherence time to be usable by quantum logic gates (c.f.
propagation delay for classical logic gates).
[1]
[4]
[5]
As of September 2018
[update] , most technologies used to implement qubits face issues of stability,
decoherence ,
[6]
[7]
fault tolerance
[8]
[9] and
scalability .
[6]
[9]
[10] Because of this, many physical qubits are needed for the purposes of
error-correction to produce an entity which behaves logically as a single qubit would in a quantum circuit or algorithm; this is the subject of
quantum error correction .
[3]
[11] Thus, contemporary logical qubits
typically consist of many physical qubits to provide stability, error-correction and fault tolerance needed to perform useful computations.
[1]
[7]
[11]
Overview
1-bit and 2-bit
quantum gate operations have been shown to be universal.
[12]
[13]
[14]
[15] A quantum algorithm can be instantiated as a
quantum circuit .
[16]
[17]
A logical qubit specifies how a single qubit should behave in a quantum algorithm, subject to quantum logic operations which can be built out of quantum logic gates. However, issues in current technologies preclude single
two-state quantum systems , which can be used as physical qubits, from reliably encoding and retaining this information for long enough to be useful. Therefore, current attempts to produce scalable quantum computers require quantum error correction, and multiple (currently many) physical qubits must be used to create a single, error-tolerant logical qubit. Depending on the error-correction scheme used, and the error rates of each physical qubit, a single logical qubit could be formed of up to 1,000 physical qubits.
[18]
Topological quantum computing
The approach of
topological qubits , which takes advantage of
topological effects in quantum mechanics , has been proposed as needing many fewer or even a single physical qubit per logical qubit.
[10] Topological qubits rely on a class of particles called
anyons which have
spin that is neither
half-integral (
fermions ) nor
integral (
bosons ), and therefore obey neither the
Fermi–Dirac statistics nor the
Bose–Einstein statistics of particle behavior.
[19] Anyons exhibit
braid symmetry in their
world lines , which has desirable properties for the stability of qubits. Notably, anyons must exist in systems constrained to two spatial dimensions or fewer, according to the
spin–statistics theorem , which states that in 3 or more spatial dimensions, only fermions and bosons are possible.
[19]
See also
References
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