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Comment by thatcherc | Hacker News Reader

Weird article. This section stuck out:

> Hajimiri, who led the Caltech that developed MAPLE, explained how the wireless transmission of energy through space is based on a quantum phenomenon called “interference.”

And then they go on to describe classical wave interference. I think the point they were trying to make is that the satellite uses a digital phased array antenna to direct its energy downward, but there's nothing quantum about that - it's the same technology used in Starlink terminals and new-ish wifi routers (among many other applications).

They also don't list how much power was received! That's the one figure that matters here I think. I'm curious whether it was watts, milliwatts, or kilowatts.

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To be fair, technically optical interference is a quantum phenomenon, it's just we've known about it for a long while since photons in the radio and visible light end of the spectrum is toward the wave-like side of the wave/particle duality and we've been modelling it as entirely a wave as a result.

Although describing it as a "quantum phenomenon" does admittedly dress it up to be far more exotic than it is, since it's an effect that appears in all waves.

There is a very specific metric that is used to distinguish whether a beam of light/EM wave is "classical" or "quantum". The metric is called the second order correlation function, labelled g^(2).

If a EM wave has g^(2)>1 then experiments that demonstrate quantum phenomena cannot be successfully performed with this wave. Hence, the EM wave is called classical. Only if g^(2) < 1 can you start doing quantum stuff with it.

In this case, almost certainly the waves have g^(2) > 1.

Coherence is a function of time/length though. At a sufficiently short time interval, all waves are coherent. It's effectively a measure of the time (or equivalently distance) between phase shifts.

Since they're claiming to use interference, which only exists in coherent waves, I think it's safe to say in g(2)<1 for the scale they're utilizing interference at. Coherence is so inexorably linked to interference, it's measured using an interferometer.

Simple water waves or simple sinusoidal EM wave (such as AM radio waves) display interference. Both are classical objects because their g^(2) > 1. You can look at, for instance, the g^(1) function to determine how nicely a particular wave interferes with itself.

Second order correlations directly test for the bosonic nature of photons. In quantum light, photons clump together because they are bosons, while in classical light photons are independent of each other and any clumping that happens is purely probabilistic. The g^(2) function tests this degree of clumping.

I'm guessing a report stuck the word quantum in there, just because it seemed like a nice modifier. The CalTech video referenced indeed describes just classical wave interference controlled by a phased array transmitter, as you suggest.

As for how much power - the team only claims to have "detected" the beam, not to have captured and utilized it, when transmitting from orbit to earth. Given that even with less than a meter between the transmitter and receiver, they only lit up a couple of LEDs, I think discussions of power are a bit premature.

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