# Can a beam of light be modulated close to it's frequency?

I'm wondering if it would be possible to modulate a signal using photon pulses - where the frequency of the signal would be close to the frequency of the light. Also would it be possible to transmit multiple of these signals in parallel and have them not interfere with each other.

Thought experiment A: I have a laser: green(500nm, 600kGhz) which can be controlled to emit individual photons. I feed a binary signal into this laser with the same frequency as the light frequency (600kGhz). For each 1 in the signal, the laser will emit a photon and for each 0 the laser will not emit a photon. Example: For an '1 1 0 1' bit stream - the laser is emitting photon A, waiting for the photon A to travel it's wavelength, it emits photon B, waits for photons A+B to travel their wavelength, does nothing, waits for photons A+B to travel another wavelength and then emits photon C. Using this scheme, I can transmit 600,000 GigaBits of information / sec to an observer 1 mile away by having a light sensor which would either sense a photon or not sense a photon at each wavelength interval. Would this be possible ? How about if I emit/not emit a photon at 2*wavelength period ? What's the maximum signal frequency for which this would work ?

Thought experiment B:

Assuming the previous scenario works I have 2 signals + 2 lasers as described above :green(500nm, 600kGhz) & red(700nm, ~430kGhz). Can I pass the two photon 'beams' through an optical fiber (ignoring material properties of the fiber such as loss & lower light speed), have the mixed beams emerge 1 mile away, separate them using a prism and measure the original signal on each beam using a photon sensor ? Basically Would I be able to transmit a 600,000 GigaBit/s signal + a 430,000 GigaBit/s signal through an optical fiber using this method ? Or would the two signals somehow interfere with each other ?

Thanks!

## 1 Answer

There are (at least) a couple problems with your proposals:

In experiment A, you start with "a laser ... which can be controlled to emit individual photons." However, such a laser is, as far as I know, impossible to make, because the stimulated emission process that generates the output in the laser is a random process, emitting photons at random times. This "shot noise" problem (although it's normally looked at as a receiver problem rather than a source problem) gives a well-known result in optical communications that it requires about 9 photons per bit to transmit a signal reliably.

As for experiment B, when you modulate a signal, the signal bandwidth is spread by the bandwidth of the modulation. So if you modulate a 600 THz signal with a 600 THz signal, the bandwidth of the modulated signal is now 600 THz. The initially "green" beam is now spread across the whole spectrum (from 300 - 900 THz). This effect will prevent demultiplexing your 600 THz signal (modulated at more than 50 THz) from your 650 THz signal at the receiving end.

• Experiment A: Yes, it is only supposed to be a thought experiment, I wouldn't expect for it to be able to be reproduced with current technology. I don't see any reason why you wouldn't be able to have a stream of photons spaced 1 wavelength across. Producing them with a laser is just an example - another way to produce them that might be possible even today is to split a single photon into let's say 8 photons with multiple steps of spontaneous parametric down-conversion, then arrange the photons into a beam, delaying each photon by making it travel a longer path until it merges into the beam. – JonSnow Aug 22 '15 at 16:47
• @JonSnow, it's not a technology problem, it's a physics problem. All the ways I know to produce light have randomness of the timing the photons are produced. As for making the photons randomly and sorting them out later, how do you propose to keep track of them to know which ones to delay more and less without messing them up (Heisenberg problem)? – The Photon Aug 22 '15 at 16:53
• Experiment B: I understand that that is what signal theory probably says. But what does it mean practically ? Will the photons merge together, will they change color ? Will they not follow the expected path through the prism ? How do individual photons 'know' that i'm modulating a signal. From their perspective they might as well be alone in the universe - they can't know / interact with anything 1 wavelength behind them, right ? – JonSnow Aug 22 '15 at 16:56
• It means if you try to control the time the photon is emitted very tightly, the energy of the photon is very uncertain ($\Delta{}E\Delta{}t\approx{}h/4\pi$), another statement of the uncertainty principle. – The Photon Aug 22 '15 at 16:59
• FYI: hyperphysics.phy-astr.gsu.edu/hbase/uncer.html If the photon is localized in space (say to a region 1 wavelength in length), then there must be uncertainty about its momentum/energy/wavelength. – The Photon Aug 22 '15 at 17:02