Time Domain (Oscilloscope) View of a Photon or Light Wave

  1. We can view the amplitude vs time variation of an EM wave using an oscilloscope with the needed bandwidth. Such a view shows us the shape of the wave (sinusoid or other), the number of cycles contained in a given burst, any modulating signal etc. Though it wouldn't be measured with an oscilloscope directly, is there any such measurement data for a photon or light wave to show the shape of the vibrating wave, the number of cycles, etc.? Do we know the time duration of a photon from leading edge to trailing edge?
  2. jcsd
  3. Bill_K

    Bill_K 4,157
    Science Advisor

    You're trying to attribute properties that are classical, Jeff, to an object that is purely quantum. You can't talk about a photon as if it had parts: a front and a back, with wiggles in between! :wink:
  4. mfb

    Staff: Mentor

    If your device is sensitive to individual photons, you can't use it as an oscilloscope. Those two detection mechanisms just don't work together.
  5. I don't understand when you say that that an object is purely quantum. Light is an electromagnetic wave, just like radio waves. Is that different than your understanding?
  6. An oscilloscope with sufficient bandwidth can view the time domain properties of other electromagnetic waves, so why not light? And as contained in my original post, I'm not concerned with viewing light on an actual oscilloscope, but rather I'm interested in knowing what the electromagnetic waveform of a light signal would look like on a time domain display of amplitude vs. time.
  7. mfb

    Staff: Mentor

    Light is like an electromagnetic wave in the classic limit - where you have so many photons that you don't see them individually.

    There is nothing special about light (apart from serious engineering problems to follow the extremely fast oscillations), the problem is that you want to see individual photons.

    It is possible to study this for light pulses (but not for individual photons). The result depends on the source. An example is shown here (blue animation at the right side).
  8. Bill_K

    Bill_K 4,157
    Science Advisor

    Back when the nature of the atom was first being investigated, the first ideas that came along were semiclassical: electrons went around in definite orbits, and an atomic transition happened when an electron jumped from one orbit to another, emitting an electromagnetic wavetrain in the process. In this picture, questions that seemed reasonable turned out to have no answer. Things like, "How long does a transition actually take? How long is the wavetrain that's emitted? Does it last 10 cycles? 20 cycles? What state is the atom in at a moment when the photon is only partway out?" Similarly, when the photon hits the screen, how wide is the spot? And what if you blinked a shutter quickly enough to catch half the photon and shut out the rest?

    The difference between classical and quantum objects is that classical interactions take place continuously, and questions like the above have meaning. The interaction of quantum objects instead are governed by a probability amplitude, and when they do interact, they interact instantaneously, at a single point. A uranium atom may sit for a billion years and there's no way to tell when it will decay. But when it does decay, it does not happen gradually, it happens all at once.

    A photon is indivisible and for all intents and purposes pointlike. It's probability amplitude may be spread out, and may be different for each experiment you do, but it does not represent a photon size or shape. When the photon shows up, it will show up at a single point, and at a single moment.
  9. I think you are saying that, though technically challenging, the time domain signal of a light wave can be measured, but you are saying that a problem occurs when you try to see this for individual photons. So let's say I have this test setup in my lab, with a light wave generator connected to my oscilloscope-like device, and I'm observing this continuous light wave signal. Now I reduce the intensity of the light signal to the point where the generator is emitting individual photons periodically, what do I observe on my oscilloscope-like device?
  10. But doesn't a geiger counter still click when it decays? How is this measurable but the time-domain signal is not?
  11. mfb

    Staff: Mentor

    Just noise.
    To see anything with the high intensity, you need coherent light - like a laser. Then you can get something like that.
    But lasers and single-photon sources don't work well together. And your oscilloscope is not sensitive to individual photons.

    It makes a single click, not a click spread out over several billion years (in case of U238).
  12. UltrafastPED

    UltrafastPED 1,912
    Science Advisor
    Gold Member

    The frequency of visible light is 430 to 750 trillion hertz (red to blue), or about 500 terahertz.

    Tracing a gigahertz wave requires nanosecond response (for 1 GHz); for 1 THz your circuitry would need to respond faster than 1 picosecond ... for visible light (approaching 1,000 THz) the circuit would need to respond in 1 femtosecond.

    Alas, the very fastest electronic circuits operate at about 100 picoseconds per cycle, so they are about 100,000 times to slow for optical frequencies.

    It is possible to measure shorter time intervals, but not this way.

    When I was measuring sub-picosecond changes during ultrafast laser-matter interactions a "stop image" technique was used - an application of the stroboscope idea - just repeat the identical process millions of times, but capturing the image for a specific time delay. Light travels 300 microns in 1 picosecond. The error limit for this type of process is the laser pulse duration ... today that is about 5 femtoseconds, which is too slow to measure a light wave.

    Another very fast technique is called the "streak camera"; modern streak cameras can resolve time down to 1 picosecond; see http://en.wikipedia.org/wiki/Streak_camera

    Measuring fast events requires even faster instrumentation. Researchers are always working on pushing the limit. Nobel prizes are awarded for success:
  13. jtbell

    Staff: Mentor

    If you're thinking of the time-domain details of an individual click, that is determined by the detection process. In this case, one of the decay products enters the Geiger tube (a short time after the decay itself), and triggers a cascade of ionized gas atoms or molecules which arrive at an electrode and produce a pulse of current. They don't arrive simultaneously, so the click has some structure, but this structure does not reflect any time-domain structure in the decay event itself.
  14. sophiecentaur

    sophiecentaur 14,686
    Science Advisor
    Gold Member

    A GM tube reacts to a single photon and the shape of the pulse is determined by the characteristics of the system. A TDR reacts to billions of photons (that number needn't be accurate) of a much lower frequency EM wave in order to show a meaningful trace which may have the resolution to show the nature of the transmission path. The trace tells you nothing about the extent of all those individual photons - or when they were emitted so it is 'looking' in the classical sense. You seem to be still not seeing the essential difference between the quantum world and the classical world.
  15. Drakkith

    Staff: Mentor

    It may help to imagine a photon not as an individual little particle or a small wave itself, but as a single, instantaneous interaction of an EM wave. In other words, as the EM wave comes in and interacts with matter, energy is transferred in "packets" or "quanta" instead of continuously. So when you look at a radio wave on an o-scope, you are seeing trillions of interactions per second that carry the force and energy from the EM field to the o-scope. If you think of it this way, it becomes obvious why you can't ask what the shape or duration of a photon is.
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