What is a quantum computer?

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Quantum computers have been a theory that has just been evaluated recently. It is to develop a computer that is faster than a giga hertz. What is a quantum computer? A normal computer codes information using a binary code. A bit can therefore be found either in the form of 0 or 1. We can picture a bit as an electron where the spin of an electron can be up which can be represented as a 0 and the spin of an electron down is 1. If we look at quantum mechanics, than we can deduce from the law of superposition that both states could occur simultaneously. This means a electron can spin up and down at the same time.

Therefore if one qubit has two states then two qubits could be in four possible states i. e. 00, 01, 10 and 11. This means that you can do four numbers can be stored at once. So if there were 3 qubits then 8 different numbers can be stored simultaneously1. So we can see that there is an exponential increase so if n qubits were used to make a computer then 2n calculations can be done at once. Physicists are excited by the prospect because this means that unsolved problems and calculation could be solved by a quantum computer and the possibility of simulations being created that look very real.

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Problems faced making a quantum computer? One of the problems is how to get the qubits in two states simultaneously because the quantum states of a qubit can easily be disturbed by interactions with the environment which is called ‘decoherence’. This means that for both quantum states to occur the qubits have to be isolated but to function as a computer there has to be some sort of link between the qubits. A quantum computer requires many qubits to interact with each other and without decoherence occurring.

We can look at nuclei and see that if an electron remains in its ground state then it is 0 state and when the electron is excited to a higher energy level it is said to be in state 1. If we try to create a NOT function like in a NOT logic gate then we can flip the 0 state to 1 state by firing a photon with a particular frequency and therefore a certain energy (E + hf) which is exactly the same amount of energy required to excite the electron no less no more (deduced from photoelectric principle).

It is also used to flip from state 1 to 0 by again firing a single photon with the same frequency. Therefore for the nuclei using quantum mechanics to be in two states the electron should neither be present in the ground state or the excited state This is done by zapping the nuclei with a photon pulse of half the duration so that it is in the process of being in state 1 from state 0 but isn’t fully excited, so it is a coherent quantum superposition of both states. Because two half flips are required toOne way of doing this is by using NMR (Nuclear magnetic resonance).

This requires using atomic nuclei as qubits and looking at nuclear spin. Nuclear spin hardly interacts with the environment and therefore decoherence does not occur but this doesn’t mean that the nuclei can not be used. Spinning nuclei act as mini magnets which can be manipulated by a magnetic field. If placed in a magnetic field the spinning nuclei will vibrate at a particular frequency depending on its chemical environment. You can control the qubits by bombarding the nuclei with electromagnetic waves set at that particular frequency (resonant frequency).

Existing quantum computers? One of the simplest quantum computers is chloroform (CHCl3). Nuclear spin is a property that is found in certain atoms such as 1H AND 13C. Therefore the three chlorine atoms can be ignored. The atoms with nuclear spins act as tiny magnets which can interact with an external magnetic field. If the nuclear spins line up with the field it is said to be in 0 states but if the nuclei align in the opposite direction of the external field then it is said to be 1 state.

The first qubit operation is easily achieved by zapping the with its resonant frequency but the second qubit operation is much more complicated as it is dependant what nuclei with spin are coupled with it. So in this case the carbon is bond to the hydrogen so the electrons that surround the nuclei which acts a bit like a spring. This coupling affect is manipulated to form a CNOT gate.


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