How is the frequency of a photon/light measured? How long does such a measurement take?

Asked by:
Torbjorn

Answer

It is not actually possible to directly measure the frequency of a single photon of light.
This is because a single photon is going to behave more like a particle than a wave, and
the concept of frequency (cycles or alternations per second) only applies to waves.

A spectrometer is a device that disperses the path of impinging photons through an angle
that is dependent on their wavelength. In this way it is possible to closely estimate the
wavelength of the photons.

The wavelength measurement is then used in a simple equation relating speed of a wave, its
wavelength and frequency: frequency = speed / wavelength.

The speed of light is defined exactly as 299,792,458 m/s. A photon of red-orange light
from a HeNe laser has a wavelength of 632.8 nm. Using the equation gives a frequency of
4.738X10^{14} Hz or about 474 trillion cycle per second.

A much more accurate method directly measures the wavelength of a laser beam by counting
the number of fringes in an interferometer as one of its mirrors is moved over a very
precisely measured distance.

A third and most accurate method measures the frequency of a laser by measuring the
difference-frequencies produced by mixing it with a series of lower and lower frequency
signals. (When two waves of different frequency are mixed, two new waves are produced with
frequencies equal to the sum and the difference of the original frequencies.) The lowest
or reference frequency and each of the difference frequencies is directly measured by
comparing them with a frequency standard such as one of the atomic clocks at NIST.
Described at: http://www.boulder.nist.gov/timefreq/ofm/synthesis/synthesi.htm

The time it takes to make a measurement depends on the method used and the accuracy
desired. For the highest accuracy, measurements may take a second or more. A single
photon wavelength measurement can be completed in a fraction of a microsecond, but the
accuracy will be many orders of magnitude less.
Answered by:
Scott Wilber, President, ComScire - Quantum World Corporation

'One cannot help but be in awe when he contemplates the mysteries of eternity, of life, of the marvelous structure of reality. It is enough if one tries merely to comprehend a little of this mystery every day.'