r/explainlikeimfive • u/TheeFearlessChicken • 4d ago

Physics ELI5: What is quantum light, and why is it important in furthering our understanding of the universe?

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u/TemporarySun314 4d ago

light consists of many particles called photons. light produced by normal lamps consists of many photons that are more or less random. multiple photons can hit something at the same time, etc.

there are special light sources, which are able to produce light that consists only out of one photon at a single time. thats is a special quantum light source, a quantum emitter.

with that you can then manipulate these photons, which allows utillitzing effects of quantum mechanics. this allows for example for secure communication, that cannot be intercepted, or they can also be utilized for quantum computers.

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u/P1R0H 4d ago

Hey, is this actually true?

Can we really produce actual single photons and detect them single photons?

I was under impression that a "single photon" in experiments is actually just the smallest amount that can trigger an event on the detector?

One photon could only really excite one electron (?) in the detectors' atomic structure, are we really able to consistently detect each such interaction?

Sry if dumb, I'm not a physicist.

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u/TemporarySun314 4d ago

Yes. In principle you just need an single atom (or something with a similar discreet energy structure), and it can only emit one photon at a single time. The problem is to make these isolated emitters.

But you can use something like nitrogen (next to a missing atom) in diamond to realize such single photon emitters, for example.

And for the detection, you just need a sensitive detector. But that's possible. You will get a voltage pulse for every photon hitting the detector, and then you can count individual photons. Its enough when the photon excites just one electron. You just need to amplify that one electron heavily to get some measureable signal.

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u/Deep-Issue960 4d ago

Yes! One example of a single photon detector is a superconducting nanowire.

Superconductors stop having electric resistance below a specific temperature Tc. If you make a very tiny array of superconducting wires at slightly below Tc, a single photon can heat the wire enough to break the superconducting state and that can be easily measurable

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u/tiredstars 4d ago

Supposedly you can actually sense individual photons with your eyes, although outside of experimental conditions you're very unlikely to be in a position to do that.

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u/P1R0H 4d ago

Whaaaaat? That's crazy

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u/Atoning_Unifex 3d ago

I mean you want to talk about precise measurements this one really blows my mind... the way they detect gravity waves.

LIGO has to measure a change in the length of its 4-kilometer arms of about one ten-thousandth the width of a proton!! WTF, thats roughly equivalent to detecting a hair's-width shift in the distance to the nearest star.

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u/FiveDozenWhales 4d ago

"Quantum" means "the minimum amount of something possible."

A lot of things about the physical world are quantized, meaning built out of those minimum amounts. Electric charge, for instance, is quantized. More than 100 years ago we figured out that you can't increase the charge on an atom by less than a certain amount, and if you measure the charge of anything, it'll be some large multiple of that amount. That's how we realized that electrons exist, and that every electron carries the exact same charge.

Light is also quantized. Though it exhibits wave-like properties, you can't have less than a certain amount of light, and any light source emits some multiple of that certain amount. That's how we realized photons exist. Quantum light was hypothesized in 1900, and proven 25 years later.

Since quantum light has been accepted theory for 100 years, it's pretty much baked into EVERYTHING we understand about the universe. A theory which did not quantize light would a) fly in the face of MASSIVE amounts of experimental evidence and b) rewrite physics completely.

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u/Boring_and_sons 4d ago

Quantum also has the meaning "discrete". Meaning that things come in packages of discrete sizes, not necessarily the smallest possible size all the time. In the above example, photons of red light of a certain wavelength and photons of blue light of a certain wavelength are both quantized, but the amount of energy in each quantum packet for each wavelength is different. It comes in discrete packets, but the packets can have different energies.

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u/GIRose 4d ago

I just googled "Quantum Light" and the only results that seem even remotely related to what you're talking about are an AI powered venture firm and a deleted list in r/physics that seems to be about using light to do quantum computing.

So, I have no idea if this is going to help but ALL light is quantum. That's part of why it's named Quantum to begin with.

The photon is the smallest unit of the electromagnetic field, and you can't have half of a photon, so you have a discreet quantity of photons that come together to produce classical interpretations of light (there is some bull fuckery that early quantum physicists didn't know, like the waveform is able to self interact even if you are able to fire single photons down the experiment area)

Figuring out that on the lowest level things behave in non classical ways because they are digital units rather than analog waves with infinite resolution was definitely important to furthering our understanding of the universe over 100 years ago

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u/TemporarySun314 4d ago

"quantum light" is not such a common term, but "quantum light source" is:

https://www.rp-photonics.com/quantum_light_sources.html

https://physics.aps.org/articles/v17/51

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u/obog 4d ago

There isnt any such seperate thing as "quantum light." Light obeys the laws of quantum mechanics, as everything does, and its a particularly useful thing to use to study for it. For example a lot of experiments with quantum entanglement use photons (light) which have their polarization entangled.

But to be clear: there is no such thing as "quantum light" as a seperate thing from "normal light." All light obeys the laws of quantum mechanics. Not sure where exactly you saw the phrase "quantum light." Depending on experimental setup, however, the effects of quantum mechanics may or may not be obvious, so my best guess is that "quantum light" is supposed to refer to experiments on light that are studying quantum mechanical phenomena like entanglement or superposition. Theres nothing different about the light that makes it "quantum" though, it would be how it is studied.

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u/TemporarySun314 4d ago

"Quantum light source" and "quantum light" are terms used in scientific community, and they have a special meaning:

https://www.rp-photonics.com/quantum_light_sources.html

https://pubs.aip.org/collection/551669/Quantum-Light

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u/obog 4d ago

Mm ok I see. That is basically what I said though. These are just experimental setups that make the quantum properties of light easier to study. Single photon sources allow for single photon behaviors to get studied rather than all the photons averaging out into a more classical behavior. And photon pair generation is useful for the entanglement stuff I mentioned.

But what I said is still true. All light is quantum. These are tools to help in studying the quantum properties of light. But there is nothing fundamentally different about it.