[time-nuts] laser as clock source

[Michael Wouters]

Dear Ilia

Emission of light is a quantum mechanical process. It is fundamentally
statistical in nature and as someone commented earlier, makes a good
random number generator. Here's one, for example:

http://www.nature.com/articles/srep10214

If you attenuate any light source, lasers included, to the point that
you can count individual photons, it will just be noise.

Your professor is right about a laser having a narrow linewidth
compared to other sources. Its essential property though is that its
light is coherent. When viewed as a particle (photon), this means that
the wave functions of the photons are phase coherent. When viewed as
an electromagnetic wave, this means that there is phase coherence in
the electromagnetic wave.

Here's a simple picture that may help:

A laser consists of some active medium, usually placed in an optical
resonator to increase the overall gain of the lasing process. You
'pump' the medium, putting energy into it via an electric current, a
flashlamp, or another laser. Excited atoms start to emit this absorbed
energy  via spontaneous emission - this is random and is not laser
light. However, there is another process that takes place (this is one
of Einstein's insights), stimulated emission. A photon passing by an
excited atom will cause that atom to de-excite (with a certain
probability), emitting a photon whose wave function is coherent with
the incident photon. These photons circulate around the cavity,
causing other phase coherent photons to be emitted, in a kind of
avalanche - this is the laser light. You make one part of the cavity
slight transparent so that some light leaks out for you to use.

Imagine now that your active medium only has a few atoms in it,
randomly scattered along the length of the cavity. As a photon travels
along the cavity, the photons that are caused by stimulated emission
will be emitted at random times determined by their random positions
(and you don't have to make any assumptions about probabilistic
emission to see this). The light that we see coming out of the cavity
is therefore emitted at random times.

You may be thinking, OK that's a gas laser where the atoms are moving
around. What about a solid state laser like a diode laser ? The
crystal is of course not perfect, but really it comes down to my
initial statements that emission and absorption of light is a
probabilistic process. So the circulating photon effectively causes a
sequence of emissions that are random in time.

The only way to use a laser as a clock is to lock it to some reference
like an atomic transition or stable cavity and then use that as source
to heterodyne other lasers suitably close in frequency with, or to
lock an optical frequency comb to it, which transfers the optical
frequency back into the RF domain.


Cheers
Michael


On Sat, May 7, 2016 at 3:14 PM, Ilia Platone <info at iliaplatone.com> wrote:
> Wait... no telescopes, very close distances...
>
> only a laser, with a photon limiter like a dark window, "close" like 10mm or
> so... just the space required for the laser optics plus the "limiter", and a
> photon counting detector that can be an APD or a PMT, it depends on the size
> required and scale of integration.
>
> The "idea" came because my professor told me that laser is a light source
> composed by a limited number of harmonics, so close the ones as some nm
> wavelengths, to get these lights can be directional and manouverable: if
> these should be the carachteristics of lasers (a laser expert can correct
> me), photons emitted by this type of light source should hit the detector at
> a constant rate. The (very dark) limiter serves to regulate the photon flux
> so a very limited number of photons reach the sensor part.
>
> The question was if the photodetector could use the individual photon
> detection as clock tick, and if these ticks can be regular in frequency.
> Many have replied that it would be noisy: phase noise? I don't think a
> single photon can cause AM noise, because I was talking about single photon
> pulses into the photon counting region, not into the analog region. Please
> correct me if I'm wrong.
>
> Ilia.
>
>
> Il 05/05/2016 21:22, Hal Murray ha scritto:
>>
>> jimlux at earthlink.net said:
>>>
>>> Well, in deep space optical comm, we send many photons with a laser, and
>>> we
>>> use pulse position modulation at the receiver detecting single  photons
>>> (or
>>> "few photons"), by which we can send "many bits/photon"  (e.g. if you
>>> have
>>> 256 possible time slots in which the photon can  arrive, you have 8 bits/
>>> photon)
>>
>> Neat.  Could you please say a bit more.
>>
>> What sort of distance?  Bandwidth?  Error rate?
>>
>> How big is the laser and telescope?   What sort of optics on the receiver?
>> How hard is it to point the receiver in the right direction?  How hard is
>> it
>> to point the transmitter telescope?  ...
>>
>> How does the receiver get timing?
>>
>>
>
> --
> Ilia Platone
> via Ferrara 54
> 47841
> Cattolica (RN), Italy
> Cell +39 349 1075999
>
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