M1L7n.txt

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I just want to share something I saw in my visit
to the Netherlands.
When I visited the University of Delft and the Kavli Institute,
they had just accomplished the entanglement of two NV centers.
And we had just talked in class about the entanglement
of two of ions.
So I'm excited to show you that people have just
taken the next step.
And what often the next step means
is that, in atomic physics we use pristine systems.
Ions, neutral atoms, in [INAUDIBLE] vacuum chamber.
And we create new forms of entanglement
with quantum gases, new forms of quantum matter.
But ultimately, we hope that those concepts, those methods,
and this knowledge translates to some room temperature materials
or solid state materials, which can be handled more easily,
and therefore much closer to applications.
So NV centers are nature's natural ion trap, or nature's
natural neutral atom trap.
Let's not discuss whether this is neutral or ionized.
It's a vacancy in nitrogen. And it has a spectrum
which looks like an atom.
So you can say, once you have such a defect in nitrogen,
you have an atom, a single atom, in an atomic trap or an ion
trap.
And you don't have to create the vacuum.
It's there.
Every time you look at it, it's there.
And you can excite with a laser.
It has a spectrum similar to atoms.
It has spin structure.
So you have these vacancies in diamond.
And so these are little quantum dots.
But now you have two problems.
One is you want to collect the light emitted by them
and what is best is to mill a lens right into the material.
So this is the diamond material, lens is milled,
and that already gives some collimation
of the light emitted by it.
But the big problem, until recently, has been,
when you create those, some people
call it artificial atoms.
Every atom is a little bit different
because it experiences a slightly different environment.
Crystal is strain, so if you have seemingly two identical
defects in diamond crystal, the two defects
will have a resonance line, which
is a few gigahertz different.
You would say, well, maybe it's just the part in 10 to the 5.
But it means the photons are distinguishable.
So if you want to do entanglement
by having two such artificial atoms emitting
a photon onto a beam splitter, and then by performing
a measurement, we project check the atoms
into a balanced state.
I hope you all remember what we discussed for the trapped ions.
Then you have to make sure that, fundamentally,
those photons cannot be distinguished.
And the trick here is that they put on some electrodes
and, by adding an electric field,
they can change the relative frequency.
And therefore, within the frequency uncertainty
given by Heisenberg's uncertainty relation,
they can make the two photons identical.
And then, what's the experiment?
Well you have two NV centers.

Defects in diamond.
You can manipulate it with microwaves
coherently with spin.
You need lasers for initialization and readout.
You need, but then closer to what we want to discuss,
you need laser beams, which excite the NV center,
and then the two NV centers emit photons.
And what you see now is exactly what we discussed schematically
and in context of the ions, that the two NV centers now
emit photons.
And by using polarization tricks in the beam splitter,
you do a measurement after the beam splitter.
And based on the outcome of the measurement,
you have successfully projected the two NV centers
into a balanced state.