M5L22k.txt

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Let me talk about another aspect of coherence.
And of course, they are all related.
Coherence is always related to the phase, to beat notes,
superposition, but let me now talk about one aspect, which is
related to delayed detection.

When I was a graduate student and I
learned about spectroscopic techniques, I don't know,
somehow I was so fascinated by techniques
which could measure spectroscopically transitions
better than the natural line widths.
I don't know, maybe from what I had read before,
what I learned as an undergraduate,
the natural line widths appear to be
the natural or fundamental limit.
So the topic I'm teaching right now sort of
has always had a certain fascination for me.
But you will, of course, also realize that in the end,
the answer is rather simple.
Well, once you know the answer, most answers are very simple.
So we want to talk about delayed detection.
So let's say we excite the system.
You can think about a quantum beat experiment.
You have a short pulse, and then your quantum beats happen.

And now the question is, normally,
when you do a measurement on a decaying system,
you're always limited by the natural line widths,
by the inverse of the lifetime.
But now, maybe you want to be smart
and just say, well, I start the detection, I only detect atoms
after a time t 0 which is much, much larger