and become horizontal
and some of them into that beam and become vertical.
That's something that disturbed Einstein a lot.
Hesaid,"How can something that is identically prepared
and then subjected to an identical treatment behave randomly?"
And that's what he said,
he didn't like the fact that god seems to be playing dice with the universe.
We'll explain that a little bit later.
Nowlet's say we rotate the apparatus.
Before I rotate the apparatus,
I'm just going to talk about this this strange behavior, thisprobabilistic behavior.
And the best metaphor for that is what I've heard fromourcolleague
BillWooders who’s taught undergraduates for many decades.
Hesaid, "Well, it's actually an educational metaphor."
Remember the old fashioned kind of school where the student was not supposed to speak upin class,
and was just supposed to answer the questions the teacher asks.
The quantum system being measured is like that.
So the teacher is the measuring improvement,
in this case, consisting of the crystal and the detectors.
And the student is like the photons.
So the teacher says,"Are you vertical or horizontal?"
And the student says, "I'm polarized about 55 degree angle from horizontal."
(The teacher says) "I believeI asked you a question, are you vertical or horizontal?"
(The student says)"Horizontal, sir."
(The student says) "No, sir, Iwas always horizontal."
So that's the way quantum measurement behaves.
That's not making it seems any less mysterious,
just makes it seems more familiar.
But before we explain why that happens,
this is how it can be used for cryptography.
I think it's been pretty well explained already.
You can prepare a stream of vertical and horizontal photons
if you know that’s what they are.
And then it can carry one bit in each one.
And they go reliably into one path of the other.
If you put in diagonal photons, you can also reliably distinguish those.
But you have to do it with a rotated measuring apparatus.
And no measuring can distinguish all four kinds.
And this is the fundamental limitation that gives rise to quantum moneyand quantum cryptography.
And then they're in particular procedures that Gilles and I developed
where we send these four kinds of photons from Alice to Bob.
And then we have a classical discussion of ordinary messages,
that eavesdroppers allowed to listen to.
And the end result is that they have a shared secret bit string, a key,
that if there has been too much eavesdropping, they will reject it.
But if there is not with high confidence,
they know that it is secret from everyone, but the two of them.
But I have to say what entanglement is.
But there's no escape from this uncertainty principle.
If you try to measure a photon, you can't measure its polarization exactly
if you don't know what it is to begin with.
So there's no equipment that you can put a photon in
thatit will come out and tell you what its polarization is.
You can't clone a photon, because if you could clone it,
you could make millions of copies.
And then measure and from the statistics, determine the polarization with arbitrary accuracy.
There is a device whichamplifies photons,
and holding one in my hand is called laser.
Butlaser’s work.
You know the word "laser" stands for light amplification by stimulatedemission of radiation.
That was the idea that you send one photon and you get several photons out.
They'r enormally used in the oscillatory modes.
So itcould be a light oscillator for involving stimulated emission of radiation.
But they thought that the acronym "loser" would be not very good.
So anyway, it's a light amplifier.
But the thing about laser,
this light amplifiers, is that if you put in a very weak signal,
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