Quantum Science 101

Qubits: What are Qubits?

This article was updated on the 3rd January 2019.

Quantum bits or “qubits” are artificial atoms that use numerous methods to produce quantum information. Such information is the building blocks of quantum computers. Like most binary processing units in computers, qubits are able to process one of two binary states of one and zero at a time ; however, it could also utilize both states simultaneously, enabling quantum computers to follow more complex operations that is virtually impossible for any traditional computer.

Computation using Quantum Mechanics

One of the most widely recognized and complex manifestation of a “quantum technology” is the quantum computer [1]. Quantum computers rely on the principles of quantum mechanics, and unlike conventional 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, a one, or everything in between. Its 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, with n being the number of qubits. This would mean that processing power potentially scales exponentially with the number of qubits, instead of linearly like a classical computer.

Superposition

The principle of quantum superposition in individual qubits is the first key to achieving truly parallel processing. The concept of superposition implies that while the state of a qubit remains “unknown” or unmeasured, it is not confined to exist in any one state but 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

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.

Developments

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. Over the years, significant developments in the study of qubits have been made, catering to more available quantum technologies in the market.

Earlier contemporary researches have already found characteristic requirements for a qubit to efficiently function. It 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 also 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. Alternatives to qubits, such as 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, have also been investigated. Meanwhile, researchers have also tried to establish a relationship with qubits and 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.

Current research on qubits are more focused on creating avenues to count qubits. A recent study was able to successfully recorded the temporal coherence—the maximum time that a qubit could hold two binary states at the same time—of qubits [9]. The researchers utilized graphene and exotic materials, such as van der Waals materials, to create their qubits. When applied with voltage, a more efficient usage and measurements of qubits are made. In another study about qubit counting, researchers were able to conclude that another method for counting qubits would be through the use of quantum computers [10]. Such computers are believed to be strong and powerful enough to handle qubits, and thus be able to facilitate an operation where the total utility and number of qubits could be assessed.

While these researches are in their initial stages, the results have already provide promise for the future of quantum computing. Understanding the functional capability of qubits would allow more complex technological functions to take place. Drastic technological improvements in many industries are expected to take place when such researches have been fully concluded.

Illustration of coupled spin qubits

Illustration of coupled spin qubits

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

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)

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

References

[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).

[9] R. Matheson, MIT News, “Physicists record “lifetime” of graphene qubits”, http://news.mit.edu/2018/physicists-graphene-qubits-1231

[10] Opremcak et al., Science, 361 p. 1239 (2018).

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