Quantum Science 101

Qubits: What are Qubits?

Computation using quantum mechanics

One of the most widely recognised and complex manifestations of a “quantum technology” is the quantum computer [1]. Quantum computers rely on the principles of quantum mechanics and unlike conventional “classical” computers available today which process information using bits consisting of a one or a zero, a quantum computer will require the use of qubits. A qubit, like a bit, represents a one or a zero, however the distinguishing feature is that it can represent any quantum superposition of the two values, which means the qubit can simultaneously be a zero and a one and everything in between. The implication for computational power is that in a quantum processor, the number of computations that can be achieved simultaneously is 2 to the power of n (n being the number of qubits). Processing power thus potentially scales exponentially with the number of qubits, instead of linearly like a classical computer.


The principle of quantum superposition in individual qubits is the first key to achieving truly parallel processing. Superposition says that while the state of a qubit remains “unknown” or unmeasured, it is not confined to exist in any one state but rather exists in all possible states at the same time. It is only by measuring or disturbing the qubit that the superposition is destroyed and the state becomes well defined as a zero or a one. The thought experiment by Erwin Schrödinger illustrates this conceptually with the analogy of a cat in a box (Schrödinger’s Cat).


Entanglement is a distinctly “quantum” phenomenon used to describe the coupling which can occur between individual quantum systems or qubits [2]. Entanglement between groups of qubits, which are in a superposition of states, forms the basis of a quantum device and may ultimately enable the highly desirable parallel processing power offered by a quantum computer.


Identifying and understanding the best type of qubit for implementation in practical quantum technologies comprises a huge fraction of experimental and theoretical research taking place today.

A qubit must consist of a 2-state quantum system which can be manipulated (i.e. set to 0 or 1), coupled (entangled with other qubits) and measured. The qubit must be as robust against fluctuations in the local environment as possible so as to not interfere with the fragile state of superposition until necessary computations have been completed. Some of the leading alternatives currently being investigated are electron/molecular spins, charge of a single electron, the state of a trapped atom or ion, the mode of a single photon or the phase or charge of a superconducting circuit. Specific materials and systems under study at present include single photons from a spontaneous parametric down-conversion source [3], phosphorous spins in silicon [4], Josephson junction superconducting qubits [5] (including a commercial enterprise [6]), nitrogen-vacancy spins in diamond [7], semiconductor quantum dots [8] and others.

Illustration of coupled spin qubits

Superconducting qubit (Redrawn from: http://www.eng.yale.edu/rslab/projects/cQED-layman.html)

Artist’s impression of a hybrid optical and spin based quantum chip. (C. Bradac)


[1] R. Feynman, Int. J. Theor. Phys. 21, 467 (1982).

[2] A. Einstein, B. Podolsky, N. Rosen, Phys. Rev 47 p. 777 (1935).

[3] J.L. O’Brien, Science 318 pp. 1567-1570 (2007).

[4] B.E. Kane, Nature 393 pp. 133-137 (1998).

[5] Y. Nakamura, Y.A. Pashkin, J.S. Tsai, Nature 398 pp. 786-788 (1999).

[6] www.dwavesys.com

[7] J. Wrachtrup, F. Jelezko, J. Phys. Cond. Mat. 18 pp. 807-824 (2006).

[8] D. Loss, D.P. DiVincenzo Phys. Rev. A 57 pp. 120-126 (1998).

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