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Quantum computing is seen as the next logical step in the production of faster and more efficient computers. Whilst it was theorized a long time ago, advances in nanotechnology and semiconductors has helped to bring quantum computing closer to mainstream use in recent years. In this article, we look at the role by semiconductors in the realization of quantum computing.
Normal computing operations use a bit—otherwise known as a binary bit— which the smallest unit of data in a computer and it adopts a 0 or 1 value. Quantum computing uses a similar bit system known as a qubit. However, qubit values can be superimposed, so instead of being able to adopt two values, they can, in fact, adopt one of three— a 0, 1, and 0 or 1 values. So, unlike conventional computers, this enables operations to be carried out in both values simultaneously.
Principles of Building Quantum Computing Systems
Just like bits in conventional computing, qubits are the building block and all functions arise from how the qubits behave. Even though there are three values, the mathematics behind the function is complex, but in short, these individual qubits can have an infinite value, and this leads to a continuum of states. Qubits store information quantum mechanically and is a two-state system and utilizes the ½ spin state of electrons, which take the form of up or down, and through the polarization of a photon which can be read as either a vertical or horizontal polarization. Qubits also undergo quantum entanglement, which means that the qubits can become one and cannot be described independently—i.e. the outcomes of the operation are correlated, and the quantum state must be described as a whole system rather than as individual qubits.
Quantum computing relies on certain physical characteristics and physical orientations to provide the desired computing power. It is not software, it is hardware technology, therefore the types of materials used to construct the quantum computer is important. There are a few requirements that are necessary for a quantum computer to be efficient. The materials used typically need to possess long-lived spin states, be able to control spin operations with an external field, be able to control the interactions between spins, be able to control the interactions between the spin and external reservoir and have the ability to perform operations on several qubit systems in parallel. Additionally, the quantum systems must be almost completely isolated from their environment, the coherent quantum states need to be preserved, decoherence times need to be long, and the readout needs to be able to measure the single spin states and the bulk spin resonance of the electrons.
Semiconductors in Quantum Computing
Because quantum computing is based on physical materials, the choice of material is important and semiconductor materials have become a widely trialed choice in recent years. A lot of initial research into quantum computing only manifested systems that could operate at temperatures close to absolute zero. However, because semiconductors are now used in the construct of these systems, it has enabled researchers to utilize quantum computers in room temperature conditions. This is one of the major reasons why quantum computing is close to commercial realization, so the role played by semiconductors is of major importance.
It is true that the implementation of semiconductors has been challenging in its own right. Many semiconducting materials can exhibit many quantum degrees of freedom and this causes the qubits to interact with each and decohere quickly. However, advances in atomic engineering and advanced semiconductor fabrication technologies have reduced these effects.
Semiconductor Designs in Quantum Computing
There are three main ways in which semiconductors are fabricated to build quantum computing systems. These are heterostructures, SRT-embedded heterostructures, and quantum dot arrays. The heterostructures often contain many different semiconducting layers, all of which have the same elemental composition, but with varying elemental ratios so that the properties of each layer is slightly different. This is often capped with a semiconducting barrier layer on top, followed by a series of doped gates. The qubits exist within the heterostructures and work in conjunction with a magnetized layer and a heterostructure quantum well layer. The heterostructure is laid on top of a semiconducting ground plane and a buffer layer, all on top of a silicon substrate. In these systems, the qubits are individual but indistinguishable, and capacitance measurements are taken to see whether the nuclear spin of a qubit evolves into a singlet or triplet state. This is the most basic configuration, but many others are adapted from it.
Some semiconducting heterostructures can be fabricated with spin resonance transistors (SRT) embedded within the heterostructure. These show a lot more promise than heterostructure systems alone, as they enable quantum entanglement to occur between qubits. The incorporation of SRTs forms a CNOT gate which enables a more precise control of the interactions between the qubits and removes the need for multiple gates on the surface of the heterostructure. These systems measure the singlet and triplet states of qubits in a different way. Because the qubit is located between the transistor and gate electrode, the channel current is sensitive to charged states. This translates to being able to determine the electronic state by current alone, and if there is a change in the channel current, the qubit is in a singlet state, whereas it is in a triplet state if the current is constant.
The other way is through quantum dot arrays. Electrostatically-defined semiconducting quantum dots can be used to lower the electron tunneling barrier when two qubits couple. Quantum dot arrays are placed on top of a semiconductor heterostructure. In these arrays, the quantum dots are coupled, and a vertical electrical field causes the quantum dots to optically excite and localize the charge carriers at defined distances, prompting the different states. The electrical field also causes the holes to migrate to the buffer and the electron to remain in their ground state. When the electrical field is turned off, the quantum dot qubits entangle. The readout is determined by the localized spin states of the qubits because the electron will leave when the spin state is in a ‘down’ orientation causing the qubit to interact with the reservoir via quantum tunneling. When the electronic spin state is in the ‘up’ orientation, the electron will stay within the quantum dot and no tunneling is observed. Zeeman splitting, i.e. the splitting of the electronic energy levels, of the excitons causes the quantum dot to adopt the superimposed ‘0 and 1’ states. Therefore, all the states are easily distinguishable from their voltage and ampere outputs.
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