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I Can the wavelength of a single photon be measured?

  1. Feb 11, 2019 #1
    Is there any way to accurately measure the wavelength of a single,
    individual photon? How precise could such a measurement be?

    I will be satisfied if the "measurement" consists only of confirmation
    that a photon from a monochromatic source has the expected value,
    as long as it has sufficient precision to provide useful information
    about the photon. But it would be better if the wavelength could
    actually be measured without knowing beforehand exactly what
    the wavelength should be.

    If the former is possible but the latter is not, I would like to know
    anything you can tell me about what makes the difference.

    -- Jeff, in Minneapolis
  2. jcsd
  3. Feb 11, 2019 #2


    Staff: Mentor

    You could measure its frequency and divide by c. Since measurements of time are so precise that would be more accurate.
  4. Feb 11, 2019 #3
    Can the frequency of a single photon be measured?

    Just replace the word "wavelength" in my original post with
    the word "frequency".

    -- Jeff, in Minneapolis
  5. Feb 12, 2019 #4
    Theoretically; the same way you can measure lots of photons. For example, bounce it off of a grating and see which of several detectors it hits.
    Practically; there are a whole lot of issues to address with each part of the experiment that will effect the accuracy (i.e. how good is your grating) and the chance of not seeing anything (the sensitivity of your detector, etc.).
  6. Feb 12, 2019 #5
    A single photon of a given wavelength and frequency might bounce
    off a grating at any angle, even if the grating is perfect. A statistically
    large number of photons all having that same given wavelength and
    frequency will cluster around a specific angle. Measuring a single
    photon gives a good chance that the angle is appropriate to the
    wavelength and frequency, but there is also a good chance that it is
    wildly different.

    -- Jeff, in Minneapolis
  7. Feb 12, 2019 #6
    Yes, not a good example.
    But now I understand your question better. It is definitely "above my pay grade". You may get better answers from google than here. For example, this was the first hit I got:
    (I didn't read it, but I suspect that measuring a "single photon source" isn't the same as measuring a single photon)
  8. Feb 12, 2019 #7
    Thanks. I think a "single photon source" IS what I'm asking about.
    But it also looks like that article goes into a lot of deep technical
    areas that are only distantly related to what I'm interested in.
    It might answer my question, but I doubt I could understand.

    -- Jeff, in Minneapolis
  9. Feb 12, 2019 #8


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    Yes. How accurately will depend on the frequency.
    Single photon calorimeters are -I believe- routinely used in e.g. X-Ray radio astronomy,
  10. Feb 12, 2019 #9


    Staff: Mentor

    Yes. Just use a very specific atomic transition.
  11. Feb 12, 2019 #10


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    The idea of the spatial nature of photons needs to be treated with care. There is the risk of a picture in the mind of a 'short sperm-like squiggle'. That's really not much better than the 'little bullet' picture. You can only talk about the properties of any photon after the event, when it has been deleted. We can't know where they are or the effective path taken until we see where they turn up.
    One thing that is fairly reasonable, however, would be the Energy (AKA frequency).
    With my RF Engineer's hat on, I have a problem, even with that idea. A photon that's emitted by an atom with one energy transition doesn't need (by logic) to be absorbed / detected at its destination by a precisely identical (tautology on purpose) atom; there has to be a Bandwidth involved - as with your familiar transmitter and receiver set up. That will presumably show itself as a probability distribution of the interaction of the incident photon and the receptor. If, as we believe, all photons are identical except for their Energy then how does that translate to things like coherence length of a photon beam? I find that confusing.
  12. Feb 13, 2019 #11

    Mister T

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    Once you measure the energy you calculate from that what the wavelength would be for a very large collection of such photons. But you cannot observe the wave effects unless you have a very large number of particles. So the very concept of a wavelength for a single particle makes no sense to me.
  13. Feb 13, 2019 #12


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    Let me ask one question in return to clarify: Are you interested in a single photon state, which you can prepare several times and then measure an ensemble of identically prepared photons or are you interested in a single individual photon that cannot be prepared repeatedly? And if you are interested in the second case, do you know roughly when and where to expect the photon? If so, Dale's idea to use a narrow transition and check for absorption will work well.

    If you do not have any prior information, a technique from the attosecond community might work in principle (Goulielmakis et al., Direct Measurement of Light Waves, Science 305, 1267 (2004), https://www.attoworld.de/fileadmin/...ns/paper_Science_Y2004_M08_D27_V305_P1267.pdf ).
    The details are probably too technical, but the basic idea is as follows: You fire a strong ultraviolet light field at a gas, which knocks some electrons loose, so they are removed from their atoms. The light field to be measured then passes this gas at the same time. As light is an electromagnetic field, the loose electrons feel the field and their momentum changes slightly due to the presence of the light field. Measuring the spatially resolved momentum distribution of the electrons reproduces the spatially resolved electromagnetic field of the light field of interest, which also allows one to deduce the wavelength. Of course the sensitivity achieved is usually far from the single photon level, but the principle should work as well.
  14. Feb 13, 2019 #13
    OK, another suggestion:
    Find out who makes commercial single photon sources. Plus I'll toss in my former employer Coherent and other scientific laser manufacturers. Then call and ask to talk with an applications engineer. You won't get a PHD but you probably will get some useful free advice about what is out there and how people do this in practice. Big laser companies like Coherent, Newport, etc. will have a staff of people that do this. Smaller companies will probably refer you to their R&D staff which might be better, if you can get them to talk with you. These companies sell scientific instruments by networking with their customers, so you will get some information, how useful it will be depends on who you get on the phone. Sound like somebody who might buy one of their instruments. Or just ask who you should call next.
  15. Feb 14, 2019 at 8:53 PM #14
    Are you suggesting that individual particles (photons,electrons, helium atoms) are not diffracted according to their wavelength by an appropriate grating???
  16. Feb 14, 2019 at 9:29 PM #15
    I am late to this conversation but has anyone recommended a "filter"?. There are interference filters for light with passband widths of ~1 Angstrom and transmittance of >90% midband. They are multiple layer interference devices and are remarkably effective.
  17. Feb 14, 2019 at 11:21 PM #16

    Mister T

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    If you have enough of those individual particles, then you can see a display of a diffraction.pattern.

    One particle is not anywhere near enough to make such a pattern.
  18. Feb 15, 2019 at 1:11 AM #17
    All else being equal, I would expect the precision to be
    directly proportional to the frequency, so that-- making
    up some play values-- a measurement of 500 nm might be
    precise to +/- 1 nm, while a measurement of 500 cm might
    be precise to +/- 1 cm. I wouldn't expect the *accuracy*
    to be frequency-dependant.
    Interesting. I'll look that up, but the idea of a calorimeter
    seems inappropriate for answering my question. Measurement of
    heat energy resulting from a photon-particle interaction is a
    roundabout way of determining the wavelength or frequency.
    Heat is essentially a property of large numbers of particles,
    and I'd expect any measurement of heat energy to have a large
    uncertainty. I think I need something more direct.

    -- Jeff, in Minneapolis
  19. Feb 15, 2019 at 1:14 AM #18
    That must be what was meant by others I talked with.

    My belief has been that the wavelength or frequency of
    light can only be measured with a statistically large
    population of photons, due mainly to the argument I gave
    above that a single photon might bounce off a grating at
    any angle. It is most likely to be close to a specific
    angle, but a largish number of photons are required to
    define that angle precisely, whether each photon hit is
    measured separately or the effect of the whole beam is

    I didn't think of the possibility of detecting the effect
    of photons inducing a very specific atomic transition.
    So my question may be: How precise is such a measurement?
    More precise than the position of a spectrograph line?

    -- Jeff, in Minneapolis
  20. Feb 15, 2019 at 1:16 AM #19
    An interference pattern can be made on a screen by passing a
    beam of monochromatic light through a barrier with two parallel
    slits of appropriate size and spacing for the wavelength of the
    light. This works even if the beam is so sparse that only one
    photon hits the screen at a time. I find this a convincing
    argument that each individual photon has a wavelength.

    Although the interference pattern only emerges from a large
    number of photons hitting the screen, each photon must have a
    wavelength that contributes to the pattern. Although each
    photon in a monochromatic beam has the same wavelength, the
    path of any individual photon to the screen is unpredictable,
    untraceable, and unknowable, but the wavelength is inferred
    from the angle between fringes.

    -- Jeff, in Minneapolis
  21. Feb 15, 2019 at 1:22 AM #20
    I think of photons as short sperm-like squiggles.

    Although the squiggles are electromagnetic, not spatial in the
    transverse direction. But ultimately I'm trying to find out
    whether photons have length. I know they are often treated as
    pointlike. I'm going to use an analogy with the sperm-like
    squiggle to describe my mental picture. I'm not asking whether
    this picture is correct or not. If it has problems I will be
    happy to hear them. But at the moment I'm just trying to find
    out whether the wavelength and frequency of individual photons
    can be measured, because it was previously my belief that they
    cannot. I am surprised to learn that they may be measurable.
    That would be awesome!

    A photon cannot change over time. Time does not exist in the
    photon's reference frame. That means the wave nature of the
    photon cannot be caused by the photon changing, such as pulsing
    or moving up and down. Instead, the wave must be a fixed form
    which moves as a whole, unchanging unit.

    Here's the analogy. Draw a waveform on a scrap of paper, then
    move the paper past you. You see the point on the wave that is
    directly in front of you move up and down, while the wave itself
    is unchanging. With light, made of photons, the changes in the
    electromagnetic field represented by this up and down motion
    cannot be observed. Even if the light comes straight at you,
    and reaches your eye, you can't see the changes because a photon
    can only be detected as a single interaction at a single point
    in time, at a single point in space. The entire photon must reach
    and be absorbed by the detector, or it is not detected. There is no
    such thing as a part of a photon. But since we know the photon
    must have a wavelength, I argue that this wavelength can only be
    a property of the photon if the waveform has length.

    That definite length is relative to the observer. Depending on
    the relative motion of the light source and the observer, the
    photon's wavelength will be longer or shorter, and so the photon
    itself must be longer or shorter.

    That's the idea I'm ultimately exploring, but again, at the
    moment, I'm just trying to find out whether the wavelength can
    be measured.

    I think we can talk about the properties at any time, but
    we can only know what they are after they have been observed.

    Did you mean "detected" rather than "deleted"?
    A photon is deleted when it is detected, so both are true.

    Can we know the path even then? I thought that was also
    unknowable at the level of individual photons.

    Saying "all photons are identical" raises another question:

    Can the polarization of a single photon be measured?

    -- Jeff, in Minneapolis
    Last edited: Feb 15, 2019 at 1:38 AM
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