## The Right Kind of Light

December 13, 2016 23 Comments

The physicist Seth Lloyd said that “Almost anything becomes a quantum computer if you shine the right kind of light on it.”

This is related to computational gates. A computational gate is an operation performed on some fixed finite number of bits as input and gives a fixed finite number of bits as output after a fixed finite amount of time. The not gate takes one bit as input and changes its value from 1 to 0 or 0 to 1. A controlled not gate takes two bits as input and flips the second bit if the first bit is 1 and leaves it alone otherwise so it would change the bits as follows

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Classical computers like the laptop I am using to write this post can do any classical computation. A classical computation takes bits with some definite set of values as input and changes them to produce some bits as output.

A quantum computer uses qubits – the closest quantum mechanical equivalent of a classical bit. A qubit need not have only a single value, it can have multiple values at the same time: the qubit exists in different versions that have different values. Those different values can undergo a process called interference that pushes both versions into the same state in a way that depends on what happened to both of them while they were different. If you have a set of qubits you can prepare them in all of the possible values of the bits at the same time. You can then do computations on all of the possible states of the qubits and combine those values to get solutions to problems that would be solved far more slowly with a single computation on a single set of values.

It is possible to construct a quantum computer that can do any computation another quantum computer can do – a universal quantum computer. A universal quantum computer would be able to simulate any finite physical system if you give it enough qubits and enough time.

And it is possible to do any computation the universal quantum computer could do by combining computational gates that act on qubits instead of bits. This might not sound too impressive since you might need really huge gates to do big computations. But in reality you can do all possible computations to any accuracy you like by composing gates out of a particular set of gates. Any possible gate for a single single qubit can be described by a set of three numbers all in the range . The set of single qubit gates and the controlled not gate form a universal set of gates.

An atom can be isolated in various ways, e.g. – putting the atom in a specially chosen magnetic field. The atom’s outer electron can be moved between its lowest possible state and the next highest energy state by shining light of the right energy on it. The energy of each photon has to match the energy of the difference between the states. By shining the light at a controlled intensity for a controlled amount of time you can control the electron’s state by giving it a controlled probability of moving from one level to another. You can also control the interference properties of the different versions of the electron. This allows you to do any single qubit gate on the electron by treating what energy level it is on as a qubit. You can also get the atoms to interact by sending light signals between them and in particular you can do a controlled not gate. So by shining the right kind of light on atoms you can make a universal quantum computer. The property of having two or more possible states for an electron in an atom is common. “Almost anything becomes a quantum computer if you shine the right kind of light on it.”

This sounds very complicated. Perhaps all of the work and information is stored in the apparatus for manipulating the qubits. This is the wrong way of looking at the issue. That equipment is needed to set up the atoms to do a computation but it won’t do any computation without the atoms. A large part of your ordinary desktop or laptop computer is not doing computation. Rather, some of the equipment provides ways to put information into the computer, e.g. – the keyboard. Other parts of the equipment supply power to the computer or cool parts that get hot. But without the chips that do the computation, this equipment can’t do much for you. The same is true for the quantum computer. You can shift some of the storage of information out of the qubits into the surrounding apparatus, but you can’t do any quantum computation without the atoms.