Are photons really particles or just a misconception?

[lightarrow]

...or by using parametric downconversion to have a heralded-single-photon source by measuring (and absorbing) one of the entangled photons. See, e.g., this thesis

https://www2.physics.ox.ac.uk/sites/default/files/2013-11-08/mosley_2007_pdf_12235.pdf

Very interesting, thank you!

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[Pgotthelf]

Excuse my coming late to the party, but from a practical sense, the nature of photons as field manifestations is being studied in relation to Bose-Einstein condensates. Although this is more enthusiastically being pursued with respect to states of matter, the original focus was light and Lene Hau has now stopped a light pulse using a condensate. We can study the nature of light as any of the presumed definitions from wave to particle to field. So, the answer is coming very soon.

 

[carllooper]

As I understand it the wave function of entangled photons should be considered as belonging to both photons (or both photons as belonging to one and the same wave function), in the sense that subsequent detections will conform to the same pure state (rather than a mixed state) - so detection of one photon is, in a sense, detection of the other.

Part of the problem is the whole dichotomy between particles and waves. Obviously, in concept they are different things (and part of the reason for the problem) but when referencing physical particles using the term "particle" we really mean wave (or wavelet), or field (or subfield). The classical "particle" component only really enters into the conceptual framework at the point of detection (and emission). Elsewhere it plays no real part other than in endless mind games trying to imagine the particle (wave/field) as a particle-like thing. Or another way of saying this is that a particle is in name only. Particle detections, on the other hand, are particle-like (or point-like) things.

There is probably a third category necessary - which is on the threshold between physics and information theory - where in addition to particles, and particle detections, would be detection patterns (such as an interference pattern). Indeed, one might say a detection pattern is far more important than any single detection (or indeed any single particle), and indeed that arguably wave functions (or field descriptors) are really just a particularly clear way of describing detection patterns - be they ones that have occurred, or have yet to occur, or can't occur.

And the relationship between individual detections on the one hand, and detection patterns on the other, is clearly statistical. But also between these two it becomes a lot clearer the distinction between classical particle-like concepts on the one hand (a single point in space and time) and wave/field concepts on the other (a signal spread across space and time).

For example in the following is a photo-graph of a cat (in this case, one that looks alive). We can clearly see both point like and signal/pattern like aspects - and at the same time. The cat we see occupies the signal/pattern domain whereas each pixel occupies the classical particle domain. Of interest are these pixels (or "particles"). We can clearly see each pixel is either black or white - there are no in-between grey tones, and yet we can also quite clearly sense the grey tones. The cat's fur is dark in some areas and lighter in other areas - but this is not discernible when looking at an individual pixel. And indeed we can clearly sense (see) a cat. The cat (and the grey tones) are not a function of individual pixels. It is the individual pixels (particle detections) which are a statistical (or stochastic) function of the cat (the signal/pattern component).

http://cgnip.com/sites/default/files/1314730006/cat0-variant.png

C

 

[vanhees71]

Excuse my coming late to the party, but from a practical sense, the nature of photons as field manifestations is being studied in relation to Bose-Einstein condensates. Although this is more enthusiastically being pursued with respect to states of matter, the original focus was light and Lene Hau has now stopped a light pulse using a condensate. We can study the nature of light as any of the presumed definitions from wave to particle to field. So, the answer is coming very soon.

Do you have a link to the corresponding publication? A photon BEC seems to be very exotic, and one must put it into context. Usually there can't be a BEC of photons, because there's no conserved charge related to the photon. Usual black-body radiation determines the photon-number density (although that's also a problematic notion) due to the temperature. Because photons can be destroyed and created without constraints at the walls of the container, there's no chemical potential for photons (that would be difficult anyway, because the photon is a massless particle).

 

[bjbbshaw]

To go back to jorgdv's original question, long wavelength electromagnetic radiation does indeed behave as waves, as described by Maxwell's equations. There is a discontinuity in how e-m radiation is modeled, depending on the scale: long wavelength radiation interacts with matter as described by classical field theory and short wavelength radiation interacts with matter as described by quantum theory. For example, the energy of long wavelength radiation depends on its amplitude, whereas for short wavelength radiation there is no amplitude: the energy is entirely dependent on the frequency. So if long wavelength e-m radiation and short wavelength e-m radiation are the same phenomenon, it is the mode of interaction with matter that determines which model works at a given scale. Do photons "exist"? No, but the interaction of matter with e-m radiation on the quantum scale only happens in chunks of energy or angular momentum that are multiples of Planck's constant.

 

[bhobba]

To go back to jorgdv's original question, long wavelength electromagnetic radiation does indeed behave as waves, as described by Maxwell's equations. There is a discontinuity in how e-m radiation is modeled, depending on the scale: long wavelength radiation interacts with matter as described by classical field theory and short wavelength radiation interacts with matter as described by quantum theory

There is no discontinuity - all is modeled by QED. But the approximation of EM is good enough for long wavelength radiation.

Thanks
Bill

 

[lightarrow]

There is no discontinuity - all is modeled by QED. But the approximation of EM is good enough for long wavelength radiation.

I don't think that "long wavelenght" is enough for appoximating EM field as a classical field: if the intensity is very low, you have one photon at a time even with very long wavelenght, or at least it should be so.

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[bhobba]

I don't think that "long wavelenght" is enough for appoximating EM field as a classical field: if the intensity is very low, you have one photon at a time even with very long wavelenght, or at least it should be so.

Yes - but at our current technology such is undetectable.

The bottom line here is QED models everything - but in some cases EM is good enough. Exactly what those cases are isn't really germane.

Thanks
Bill

 

[H_A_Landman]

Crudely, Quantum Mechanics is about treating classical particles as waves, and Quantum Field Theory is about treating classical waves as particles. That it's incredibly accurate (when we can actually manage to do the integrals!) is evidence that this point of view has some merit.

On the experimental side, both the photoelectric effect and the Compton effect argue strongly that when light interacts with matter, it behaves "as if" it were made of discrete packets of energy and momentum. Einstein's original (German) paper uses this exact wording: "als ob". He doesn't say that light *is* particles, he says it behaves *as* *if* it were made of particles.

The classical wave description of light fails for many reasons. It can't explain entangled photons, or lasers (which depend on photons being bosons), or Bose-Einstein condensates of photons. And yet, if you have a large number of photons and can ignore the quantum effects, it's perfectly fine for designing (e.g.) the radio antennas in your cell phone. Like Newtonian gravity, it's approximately right most of the time, but can't possibly be exactly right.

 

[bhobba]

Crudely, Quantum Mechanics is about treating classical particles as waves, and Quantum Field Theory is about treating classical waves as particles

I know you said crudely but I can't really agree with that even in a crude way. Its a difficult thing though because what people take away from something as its 'essence' varies widely.

My take would be the following. At rock bottom QM has nothing to do with waves - wave-functions yes - waves no (note waves and wave-functions are not the same thing eg waves are not complex valued). The real essence of QM is its the most reasonable generalised probability model that allows continuous transformations between so called pure states:
http://arxiv.org/pdf/quant-ph/0101012.pdf

Ordinary non relativistic QM is what you get when you assume position is an observable and the Galilean transformations - see chapter 3 Ballentine - QM - A Modern Development.

QFT starts from a field, breaks that into small blobs, describes those small blobs by ordinary QM, then let's the blob size go to zero to get a continuum. When you do that something interesting happens - mathematically its nearly the same as the harmonic oscillator from ordinary QM:
https://en.wikipedia.org/wiki/Quantum_harmonic_oscillator

It turns out the creation and annihilation operators is the exact formalism of particles.

Thanks
Bill

 

[vanhees71]

I don't think that "long wavelenght" is enough for appoximating EM field as a classical field: if the intensity is very low, you have one photon at a time even with very long wavelenght, or at least it should be so.

Well, it's a tautology, but you have one photon at a time if and only if the state of the electromagnetic field is prepared to be a one-photon state. Any other state this is not the case. A very dim laser light is still a coherent state, whose superposition in terms of occupation-number (Fock) states consists mostly of the vacuum, but also states of all photon numbers.

That's why bhobba is right in #40 in saying that it is always wrong to claim one can think of the electromagnetic waves as consisting of particles. That's even "wronger" than thinking of massive quanta as classical particles. In the latter case, there is a classical limit of the quantum description in terms of classical particles. For massless quanta with spin $\geq 1$ it's not even possible to define a position observable. So there is no classical limit in terms of classical particles at all.

 

[f95toli]

Yes - but at our current technology such is undetectable.

I think it depends on what you mean by "long wavelength". Several groups around the world are now routinely working with single photons (Fock states) with frequencies of a few GHz, meaning the wavelengths involved are several cm. There are several working designs for emitters but building "click" detectors for these frequencies is still challenging, but we are very nearly there (at the moment most people use linear detectors, meaning you need to average for quite a while if you want to measure correlations functions).

It is an interesting field to work in, on my desk I have books and papers about QM as well as books about microwave engineering and you end up having to shift your "perspective" depending on the problem you are working on

Hence, I agree that there is no such things as a "classical "limit. There is certainly no reason why you couldn't work with photons of frequencies of say a few hundred MHz, i.e. essentially FM radio.

 

[vanhees71]

Wow, that must be challenging. I guess it's mostly single-photon states in the microwave range trapped in a "cavity" (quantum dot). Or do you really mean free single-photon states in the cm-wavelength range? If so, how do you produce single-photon states with so long wavelengths in practice?

 

[f95toli]

Wow, that must be challenging. I guess it's mostly single-photon states in the microwave range trapped in a "cavity" (quantum dot). Or do you really mean free single-photon states in the cm-wavelength range? If so, how do you produce single-photon states with so long wavelengths in practice?

We mainly use superconducting qubits. They can be used both to create states in a cavity (which in our case is simply a microwave resonators) and to "launch" single photons into a microwave transmission line. Hence, most of the equipment we use is just standard microwave components (e.g. a 90 degree hybrid is the microwave analogue of a half-silvered mirror). meaning once the single photons leave our sample they travel in standard microwave coax, using normal SMA connectors etc.
Of course we have to keep our sample and most of the components cold (about 10mK) to avoid being swamped by thermal photons, but otherwise it is a pretty normal microwave setup.

 

[lightarrow]

How are single photons detected? Which kind of detector is it?

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[H_A_Landman]

The Schroedinger Equation is a wave equation. That it contains a square root of -1 is weird, from the viewpoint of classical wave equations, but doesn't really change its nature. Wave functions come out of the SE as its solutions in particular contexts, so it is not entirely unreasonable to view them as derived and not primary. (QM is similar in some ways to Hamilton-Jacobi theory in classical mechanics, where particles are modeled as wavefronts, and the H-J action plays a similar role to QM's phase.) So anyway, QM uses a wave equation to describe the "motion" of classical "particles", which is what I meant by treating classical particles as waves.

I don't think we really disagree about what QFT is. The harmonic oscillator in standard QM describes a particle in a particular, simple potential. It has quantized energy levels that are equally spaced. You can go up or down a level using the raising or lowering operators. In QFT, we just model a field as a bunch of QHOs and interpret the energy levels as representing number of particles (occupation number), and thus the operators get renamed as creation and annihilation operators and are viewed as adding or subtracting a particle. (The mechanical analogy here is phonons.) In this sense, the field (which was classically described by wave equations) is modeled as a set of particles instead.

Getting back to the original question, if the field is electromagnetic, then the particles are photons, and at least in QED photons are as real as any other particle.

 

[vanhees71]

Photons are massless quanta with spin 1. It is dangerous to depict them as "particles". There is no classical particle limit for photons, at least not a very intuitive one, as far as I know.