# The Right Kind of Light

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

$(0,0) \to (0,0),$

$(0,1) \to (0,1),$

$(1,0) \to (1,1),$

$(1,1) \to (1,0)$.

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 $[0,2\pi]$. 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.

My name is Alan Forrester. I am interested in science and philosophy: especially David Deutsch, Ayn Rand, Karl Popper and William Godwin.

### 23 Responses to The Right Kind of Light

1. > The not gate takes one bit as input AND changes its value from 1 to 0 or 0 to 1.

typo, missing AND

2. > A classical computation takes bits from one definite set of values to another set of definite values.

this sentence is confusing. i think it’s jargon. i think “takes” has something to do with mapping or changing according to an algorithm, but i don’t think it’ll be understood.

3. it’s also:

> takes bits from one definite set of values

this sounds like it takes SOME bits from one set of values (and puts them into some other set)

but you actually mean more like it takes ALL the bits making up one set and changes them to a different set.

4. you don’t say what qubits are, you just start talking about one attribute of them (that they can have multiple values — a confusing attribute that deserves more attention).

5. > You can then do computations on all of the possible states of the qubits and combine those values to get solutions to problems you could not get with a single computation on a single set of values.

“solutions to problems” is vague here.

can’t classical computers compute all the same stuff, just sometimes they have to use different, slower algorithms? are you trying to sneak being too slow as not counting as a solution to some problems (the problem of doing it at a given speed) in here, but not stating it?

6. > You can then do computations on all of the possible states of the qubits

i think you mean the computer can do computations. it’s confusing to say “you” can do something to talk about a computer doing it.

7. > Any possible gate for a single single qubit

double word

8. you claim there’s a small set of single qubit gates. but there are a very large number of reals from 0 to 2pi. and we’re talking about reals, aren’t we?

9. > If you get an isolated atom

you mean something like a scientist uses machinery and prepares an atom a certain way. “you get” is confusing both for the personalization and because “get” is vague.

10. > If you get an isolated atom its outer electron can be moved

you want a comma after “atom”. you should throw in the word “then” there, too, for extra clarity.

11. > By shining the light at a controlled intensity for a controlled amount of time you can control the electron’s state in such a way that you can do any single qubit gate on it.

what does doing a single qubit gate *on an election* mean? are you saying the electron is or contains a qubit? or some aspect of the electron’s state has a qubit or multiple qubits? not clear.

12. you talk about how to do individual gates by shining light on stuff. don’t you need a series of lots of gates to do interesting computations? so you’d need a bunch of infrastructure to keep reusing the same gates over and over. so basically a lot of external stuff would be doing lots of the work? to keep track of outputs and then feed the right combinations of outputs back in again as inputs and so on. so it’d be the light shining apparatus which is storing data and has most of the complexity?

13. what’s the point of learning about physics? who should learn about it and why? should everyone learn about physics? how should someone decide if they should learn some physics, and which physics, and how to learn it? (maybe you should make a physics open thread post i could leave comments on)

14. > You can also get the atoms to interact by sending light signals between them and in particular you can do a controlled not gate.

what are light signal between atoms?

normally signals with people are like you say “ok go over there when i hold up a red sign” and then they see the signal (red sign) and move. it involves intelligently responding to a signal.

when there isn’t intelligent response, then normally it’s more like the ball moves when i kick it, and kicking the ball is not called a “signal”.

not really clear on what’s going on with the light signals and cnot.

also repeating this question:

> you claim there’s a small set of single qubit gates. but there are a very large number of reals from 0 to 2pi. and we’re talking about reals, aren’t we?

15. > what are light signal between atoms?

You send information from one atom to another in a photon. You arrange the atoms in such a way that the atom stores the content of the information in the photon or interacts with another atom when it receives a signal with particular content or whatever. This is like the signals sent from one gate in a computer chip to another gate.

> you claim there’s a small set of single qubit gates. but there are a very large number of reals from 0 to 2pi. and we’re talking about reals, aren’t we?

The set is small in the sense that it can be approximated well with a single piece of equipment: one laser and maybe a filter that sets the intensity of the light getting through to the atom. You approximate it by shining light on an atom for a longer or shorter time. The longer the time the larger the number in the range 0 to 2pi.

16. what happens if you keep shining the light longer past the time for 2pi? does it stay at 2pi? cycle back to 0? ruin the qubit? other?

want to write a post explaining the light signal stuff? in terms of physical details. how physically does the electron state change depending on information stored (physically where/how?) by the atom? i have a pretty good mental model of how silicon computers work. i know about how to build gates out of wires (the general concepts, materials and designs, enough i can see how it could be done). and i can understand how to take the individual components and build bigger things out of them, e.g. adders. i have a decent idea of what’s going on at every level up to Safari where I’m typing this. i learned a bunch of the lower level bits from Feynman’s computation lectures which i super recommend btw. can you offer the equivalent for these quantum atom computer gates and stuff?

• “what happens if you keep shining the light longer past the time for 2pi?”

cycles back to 0.

• why does it cycle back? got an analogy for what’s going on?

• Each qubit is described by a set of three observables that act like the axes of a three dimensional coordinate system when you act on that qubit alone. So the situation looks kinda similar to rotation in some plane through the origin in such a coordinate system.