Okay, so today I want to dwell on another solid state emitter of single photons, and

that is a defect, and in particular the kind of defect we are going to talk about is quite

a nitrogen vacancy center in time.

That has become quite important in quantum information processing over the past few years.

So, very roughly speaking, it means you can manipulate a single spin and you can store

quantum information in there even at room temperature and even up to coherence times

of milliseconds at room temperature, and that is really impressive.

Now first, in order to set the stage, I want to say something about a similar system, and

that would be ion traps, interestingly.

So if you want to perfectly isolate a single atom, you somehow have to trap it.

There are different ways of doing it.

For example, you can have neutral atoms trapped in the light field of an optical stem cell

lattice, but if the atom is charged, in other words, if you are dealing with an ion, then

it is very easy to be trapped by some clever combination of electric and magnetic and other

time dependent fields.

So there will be some time dependent fields, and they are sufficient to trap the single

ion.

And then typically what you do is you also manipulate the quantum state of the ion in

particular, you manipulate internal spin states of the ion.

And you do this and you weed out the state by optical access.

So you send in photons and you record their absorption, and you record the fluorescence

of photons.

Now this is very nice, but there is quite some effort involved with tracking the ions,

and so it would be really good to have a similar situation, but where you get the trapping

for free, and in particular also a situation where you somehow combine this with a solid

state environment.

And it turns out you can have both of these things at the same time.

You can have them because if you take some crystal, and in this case it would be diamond,

then sometimes there are defects inside the crystal.

In fact this is what gives color to some crystals.

And each of these defect centers then can say have a single electron or so where the

spin could be manipulated.

So I'm trying to indicate a diamond crystal.

I'm sure this is not quite correct, but somewhere in between there sits a single NV center,

and we will learn something about this structure later.

But the idea is that this is then so to speak naturally trapped, and the kind of NV center

we are going to talk about is even negatively charged.

This is naturally trapped sitting inside the solid matrix of the diamond.

And so there again, since this is diamond and it's transparent, you have optical access.

You can use optics to read out the state of the spin, and in addition it turns out that

you can have some time dependent magnetic fields in order to manipulate the spin by

microwaves.

Now let's zoom in and let's look in some more detail at the structure of a single

such defect.

Why is it called a nitrogen vacancy center?

Now first of all, in diamond you have carbon atoms.

Diamond is a form of elemental carbon, and they are arranged such that each carbon atom

has just four nearest angles.

They are equivalent.

So that would be a carbon atom that has four nearest neighbors.