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M1L3i.txt
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#
# File: content-mit-8422-1x-captions/M1L3i.txt
#
# Captions for 8.422x module
#
# This file has 166 caption lines.
#
# Do not add or delete any lines. If there is text missing at the end, please add it to the last line.
#
#----------------------------------------
Since the creation of single photos
is essential for starting on classical light,
but also since it's a very active frontier of our field,
actually one of the leaders of this field
is Professor Vladan Vuletic here at MIT,
let me at least give you a taste how to create single photons.
I already gave you the major ingredient.
Namely, it involves single atoms.
But it's a little bit more demanding like that.
So you want to take one atom or one ion,
but the problem is if the atom or ion emits a light,
it can emit the light into all directions.
And therefore, you have a single photon afterwards,
but you have many, many different spatial modes.
And in any given mode, it will not have a single photon.
So therefore, what you have to add to this single atom
or single ion is you have to put it in a cavity.
And in the limit of a very, very high-finesse cavity,
the probability will be very, very high
that your photon is emitted into the mode of the cavity
and it is not emitted perpendicular into other modes.
But that itself is not yet sufficient,
because you have a single photon,
but you also want to know when does the single photo arrive?
You want to do experiments.
You want single photons on demand.
So one option is that you prepare your atoms
in the ground state.
You take the pi pulse, which with 100% probability
excites the atom to the excited state.
And then within a natural lifetime, or maybe even
in resonator-enhanced inverse nature line
in which the single photon is emitted.
So therefore, you pump the system,
and then you know within the next few nanoseconds
your cavity mode will have a single photon.
The problem here is that you have to prepare single atoms,
which is difficult. You have to couple them
to a high-finesse cavity.
There is technically another approach
where you use many atoms.
Since you have many atoms, you no longer have a single gun.
So therefore, you can get several photos.
But the singleness in photons comes
because the photons are now heralded, they are announced.
And the idea is the following.
If you start with a state 1, and you have an excited state,
and you have a pump pulse, you are now--
if you have many atoms, you have much higher efficiency
of pumping atoms to the excited state,
because if you have n atoms, it's n times more efficient.
But now you take the following situation.
You wait until there is a Raman transition
until you detect a photon for Raman transition,
where the excited state decays to the state 2.
At this moment, you know I have now one atom in state 2.
So in other words, you're not starting
with single atoms, which is sometimes more demanding--
how can you prepare the system-- because other disadvantages
that n atoms have.
I'm not sure if I should mention, but with n atoms,
they have a super-radiant factor n with n atoms,
you can get an n times enhancement of emission
into a single mode.
So there are all real massive advantages
in working with many atoms.
But now you know that one atom is in state 2
when you detect the first photon.
And then you have the same situation which we had above.
You can now take your system-- we'll just
redraw the level diagram-- excited
state, state 1, state 2.
You know now that you have one single atom here.
And by using a laser pulse, you can excite it
to the excited state, and then you
observe the single photon 2.
So in other words, the observation of the first photon
tells you that your system is prepared
with one atom in state 2.
And then you can get a single photon out
of it, which is the photon for the inverse Raman
transition when you pump the system back to state 1.
If I have prepared a single atom [INAUDIBLE],
won't there always be some uncertainty
as to how many atoms [INAUDIBLE]?
There are some uncertainties.
And what you are saying is correct.
We don't have perfect single photon sources.
And people characterize the fidelity of the single photon
source.
For instance, if you detect this single photon, you would say,
now my system is ready to emit a single photon triggered
by this pump pulse.
And if you can get now a single photon in 90% of the cases,
you publish a wonderful paper because you've
set a new record for the fidelity of a single photon
source.
So people are really struggling with some
of those uncertainties.
But to involve, for instance, three levels
is sort of an advantage because you're not
limited by the preparation of a single atom, for instance.
You can have many atoms.
The atoms are always there.
And the moment one atom is prepared in state 2,
this atom announces itself with a photon.
So it takes a lot of uncertainty out that the system says
with the first photon, I'm ready now.
I can emit a single photon.
And then you get your single photon.
Then you can gate your whole experiment
to a time following the detection of the first photon.
And for your gated time afterwards, you
have very, very high probability of finding this photon.
Or if you want, you can now do experiments with two photons.
If you keep this laser on and pump the atom immediately back,
you have actually a single photon here
followed by a single photon here.
It's sort of click-clack.
And now you can do correlations between two
different single photon states and such.
It's a very rich frontier of our field.
You had another question?
This is also within a cavity now?
Like the single photon detection?
Yes.
Actually, all that-- when these photons, the wavy lines,
are emitted spontaneously, spontaneous emission,
pretty much by definition, goes into
all possible spatial modes.
And the only way to control the spatial mode is with a cavity.
What I don't want to go into details,
but those who have an understanding of that
is the following.
If you have n atoms, and you prepare one atom here
in stage 2, you do not know which of your n atoms
is prepared.
So you have n indistinguishable possibilities.
And if you have n indistinguishable probabilities
which atom you have prepared, the emission of the photon bank
is n times enhanced.
So therefore, you have actually a system
which has an n times stronger coupling to the cavity.
So having n atoms makes it much, much easier
to construct a high-finesse cavity.
You get sort of this super-radiance increase
of the strong coupling for free.
And this is why there are many reasons why
you do not want to work with a single ion or a single atom.
It's possible for very fundamental experiments.
People have shown that it can be done.
But technologically, it's much easier and much
more robust to work with many atoms.
But then you need sort of the many atoms
have to tell you when one atom is prepared to emit now
a single photon.
If you are interested in this subject,
talk to the students in Vuletic's group.
They're really the world experts in that.