Superconductors with a focus upon quantum computing and microprocessors

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How does a superconductor work?

A superconductor is a substance that can conduct electricity or transport electrons from one atom to another with no resistance resulting in no energy dissipation when the temperature that causes the substance to become superconductive is reached (critical temperature).

In 1957, John Bardeen, Leon Cooper and Robert Schrieffer proposed a theory (BCS theory) that could explain all phenomena related to type 1 superconductivity. The critical points in this BCS theory include:

• Electrons operate in pairs – Cooper pairs.
• Electrons faced resistance in materials causing a loss of energy due to collisions with positive ions within the lattice.
• The interactions between electrons and the lattice produced vibrations of the lattice in packets – phonons.
• In a superconducting state, upon a collision of an electron with the lattice, the produced phonons briefly travel through lattice before being absorbed by another electrons. This caused the electrons to exchange momentum and energy. This transfer of energy allows the electrostatic repulsion existing between the Cooper pairs to be overcome.
• Cooper pairs are continually formed, broken and reformed between different electrons, allowing them to move through the lattice coherently, without collision and thus, without resistance.

Effectively, the BCS theory accounted for the process of superconductivity – the state in which resistance to current within certain conductors is negligible or non-existent. 

Description of device

Microprocessors, the core component in computer systems, such as the Josephson junction chips use the principle of superconductivity in the circuits. The circuits are fabricated with superconducting elements such as niobium nitride or silicon laced diamonds. These microprocessors and any other chips that use the principle of superconductivity are far superior to standard chips, as they contain the robustness of superconductivity and zero electrical resistance, however, at times a classical microprocessor would be a better choice.

A Josephson junction is created by separating two superconductors by placing an insulator in between, in which a supercurrent (the Josephson effect) can ‘tunnel’ through the insulator. When the current passing through reaches a critical level, an AC voltage is developed with a frequency of up to 500 Gigahertz. These Josephson junctions are used to construct quantum bits (qubits, a generalisation of a bit – a system with two possible states) in superconducting quantum computing.

Discussion of the principle of superconductivity

In the nanocryotron (nTron), a more advanced form of a Josephson junction chip which showcases superconductors in microprocessors, liquid helium is used to achieve superconductivity which can circulate through pipes as a quick and efficient cooling system. The superconductor that is used in the nTron is niobium nitride with the required temperature to achieve superconductivity being 16 Kelvin or -257 degrees Celsius.

The nTron has a ‘T’ shape, in which when a layer of niobium nitride is inserted into the chip. This shape ensures that when electrons pass through, an excess amount inputted through a gate causes a hotspot within the shape to bleed energy, effectively cancelling out the superconductivity of the niobium nitride. This process is known as electrothermal suppression. Hence, when a current is applied to the ‘T’, it can turn off the current flowing through the microprocessor acting as a switch, a key component in a computer system.

The importance of the superconducting circuit is a switch, is that microprocessors use binary to develop an instruction set that enables it to do computations. For example, when switches are turned on it can correspond to 0 and when off it can correspond to 1. All computer instructions at a barebone level are constructed using a series of 0s and 1s. Effectively, superconductors provide a new method to construct microprocessors that perform more efficiently. However, since the technology is new, its applications are minimal, and it is in a very raw state, expensive to create and will require time to be widely accepted.

Application

Since quantum computing technology is not advanced and has many limitations, there are not many advantages over classical microprocessors. For example, the nTron is unable to reach the speeds of modern day microprocessors (>1 GHz) because the junction must be cooled to 16 Kelvin before resuming operation. Although liquid helium does this relatively fast, it still causes a loss in speed for the microprocessor.

However, this lack of speed relates to functions that a typical computer completes, such as opening a web browser or refreshing the desktop. Superconductors in microprocessors enable extreme tasks to be completed in a much less time span and efficient manner than standard processors. An example is the generation of prime numbers past ten billion or finding the trillionth digit of pi. This ability enabled quantum computers to crack top-tier security and led to the development of quantum encryption, a more superior form of encryption than current methods such as SHA256 or others that multiply two large prime numbers together for the encryption of data.

Furthermore, theoretically, the basic functionality of the microprocessors that use a superconducting circuit can enable zero waste heat dissipation. These microprocessors in quantum computers are much more energy efficient than classical microprocessors, which is also a factor that enables it to perform extreme tasks.

Bibliography

•   First two images are from http://hyperphysics.phy-astr.gsu.edu/.
•   Office, L. (2018). Superconducting circuits, simplified. Retrieved from http://news.mit.edu/2014/cheaper-superconducting-computer-chips-1017.
•   M. Martinis, J., & Osborne, K. (2018). Retrieved from https://web.physics.ucsb.edu/~martinisgroup/classnotes/finland/LesHouchesJunctionPhysics.pdf.[/list]