Can a microwave antenna detect single microwave photons?

Question

In a lecture about satellite sounding our professor explained how microwave radiance is measured typically on satellites: There is an antenna behind a parabolic mirror and the received signal is further processed.

Up so far I thought that the receiver just filters the temperature-noise signal for a certain band, lets say with a bandwidth of 1GHz around a middle frequency of 60GHz (by whatever means). The radiation (antenna) temperature is then proportional to the measured power P within that band:

$$P = kT \cdot \Delta f$$

Without being an expert in microwave technology that was my picture in mind for a long time. But then he explained that the antenna counts photons: In a noisy signal from time to time we have peaks above the background noise level- and those peaks are photons, which are counted like in a Geiger Counter. For me this "counting of photons" is totally weird in the context of microwaves and it contradicts my first view, where just spectral noise-power is registered continuously. I' very (!) surprised, that we can detect single microwave photons in such way and couldn't find any literature where it is described.

Is it really true, that such a simple antenna system can detect single Photons?

Answer 1

Whether radiation is treated classically or from the quantum point of view depends not on its range (microwave, infrared, visible, etc.), but from the object of study and the technology employed. In quantum regime there are photon counters available, although there may be some caveats as to what it means to detect a single photon (that is, even a single photon may trigger the device or we count a thousand of photons with precision of 1 photon.)

What may be surprizing about microwave antenna is that it looks very different from an optical (refractor) device - focusing not with lenses, but using a parabolic or hyperbolic mirror... which is actually not very different from an optical reflector telescope.

The relatively low energy of microwave photons may pose difficulties, since it requires lower temperatures, to reduce the black body background, affecting signal-to-noise ration. Note however that the formula quoted in the Q. is the Nyquist noise, i.e., the classical version of fluctuation-dissipation theorem. It has to be modified in quantum case.

Related post: Quantum description of radio antenna

Answer 2

How is EM radiation generated? Firstly, through the emission of previously excited subatomic particles. Secondly, through nuclear fusion, nuclear fission and annihilation. In all cases, photon currents are emitted, i.e. EM radiation.

And thirdly, through the conversion of kinetic energy of subatomic particles into EM radiation. An extreme case is demonstrated in the free-electron laser, in which electrons with high kinetic energy emit photons - of high brilliance and polarized - in a spatially changing magnetic field. The free-electron laser is a variation of the Lorentz force and therefore one of the cases that should be assigned to EM induction.
And there is another case of electromagnetic induction that is of interest in everyday life as well as for radio astronomers. These are radio waves.

Radio waves are generated for communication purposes by a wave generator that periodically generates an electrical potential difference in antenna rods, which causes the surface electrons to emit photons during their periodic accelerations. Individually for each electron with varying energy content (the potential difference in the antenna rod changes along the rod and especially at the tip, collision process of the electrons vary the kinetic energy of the electrons, ...). However, this photon flow (or - I am a native German speaker - should it be called a photon stream or a current?) is polarized, electrically in the direction of the antenna rod (aka the direction of acceleration), magnetically perpendicular to it and both field components perpendicular to the direction of movement of the photons.

You don't have to look at radio waves in this way - emission of photons from zillions of accelerated electrons - but it is helpful in some cases. And that brings us to your question.

Firstly, it is now clear why the receiving antenna does not have to be half the wavelength of the radio wave. The impact of a stream of photons with a periodically changing intensity on any conductor causes the acceleration of electrons on the conductor. We can count ourselves lucky that photons have two field components and that radio wave generation leads to polarized photons. Only in this case can you get electrons to move synchronously, which can then be filtered and amplified (radio receiver).

Radio astronomers can consider themselves even luckier. Suns induce magnetic fields, and the accelerated electrons in them emit polarized photons of certain frequency spectra. And even at distances of 13 billion light years, individual photons are sufficient to filter them out statistically from the noise of the background radiation. From an EM wave, astronomers are left with individual photons from which we can draw our conclusions. What a wonderful world.