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 1 GHz around a middle frequency of 60 GHz (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'm 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 a simple system like that can detect single photons? I heard about single microwave photon detection experiments, but they are extremely sophisticated and are based on other principles:

https://www.sciencedirect.com/science/article/pii/S209592732100726X

https://www.sciencedirect.com/science/article/pii/S0030399219316500/pdfft?md5=d0533449e4b2d2e3bae779bc604a8070&pid=1-s2.0-S0030399219316500-main.pdf

Answer 1

No, microwave antennas cannot "detect" (and I'm using "detect" as a very loose euphemism... It's really "conversion"...) microwave photons because their energies are too low and get lost amid thermal noise. However, specialized quantum devices, like a superconducting qubit, could do the trick. But I'm waaaay too underqualified to talk about quantum information theory and further questions on that might be outside the realm of focus for this branch of Stackexchange.

Detecting single microwave photons is a significant challenge due to their extremely low energy compared to photons at optical frequencies. A conventional microwave antenna operates on classical electromagnetic principles and interacts with microwave radiation by inducing oscillating currents, which are then amplified and processed. This process involves a large number of photons and is inherently insensitive to individual photon events.

Answer 2

No, the system cannot distinguish individual photons.

A single photon at, say, 10 GHz has an energy of about 6.62607015×10⁻²⁴ J. A barely-detectable radio signal has a power of about 1 femtowatt, or 10^-15 J/sec, which represents (10^-15 J/sec)/(6.62607015×10⁻²⁴ J/photon) = about 1.8×10¹⁸ photons/sec.

While the antenna can be thought of as interacting with individual photons, there's simply no way that you can detect the effects of a single photon buried in the midst of all the other photons.

Your instructor is not doing you any favors by describing the process in this way. The electrical signal you get from an antenna arises from the statistical properties of vast numbers of photons, only some of which represent the desired signal. It is far more useful to think of it in terms of relative field strengths in fields and voltage/current levels in wires. The purpose of the antenna is to convert from one to the other.

Answer 3

An antenna doesn't detect anything: it delivers electromagnetic energy to a receiver. So your question really should be "can a microwave receiver detect single microwave photons".

The answer is yes! Using a very exotic "receiver", physicists have "counted" 173 MHz photons.