Shape of the wave of a single photon

[DParlevliet]

Yes they do interfere. You need two lasers, you aim those at the same spot, where you will see an interference pattern. This experiment has been done.

 

That is remarkable. I don't know the details of lasers, so can two lasers be arranged that both send photons at the same time with the same phase?

[Enthalpy]

[...] can two lasers be arranged that both send photons at the same time with the same phase?

Yes.

 

It's done for instance at nuclear fusion experiments, where many lasers add their power to compress the target. In this case; one master oscillator laser provides reference light split among the many amplifier lasers, whose powerful outputs are merged. Which isn't simple, because the phase must be coherent despite the long paths.

 

Other people use fibre lasers as amplifiers and merge the light. Fibre lasers are easier to cool. Some consider it could be the next candidate as a light source for laser nuclear fusion.

 

You can also synchronize several oscillator lasers, "just" by letting them share their light... it needs matched caracteristics, sure.

 

In a more general view, one laser is just several synchronized sources of light like individual molecules or dopants or charge carriers. Merging the light of several lasers means only that these lasing elements belong to several entities, for instance are in different cavities, but with enough coupling.

[DParlevliet]

Why is the output of two lasers coherent and what comes out of two detectors in front of the double slits not coherent?

[Enthalpy]

(1) Why is the output of two lasers coherent and

(2) what comes out of two detectors in front of the double slits not coherent?

(1) The outputs can be made coherent, sometimes, with a significant technological effort to synchronize the oscillations

(2) I didn't understand the question.

[DParlevliet]

(2) I didn't understand the question.

 

Why does two lasers interfere and the double slit with two detectors not? (2) is always explained as being not coherent after the detector. Why not?

[DParlevliet]

And other questions

 

- If the two lasers are set on very low intensity, photon by photon, is it sure that both lasers stil emits each a photon at the same time?

 

You can turn the other laser off too. That just makes that laser's photon wave really weak and non-coherent. The interference pattern is the interference pattern of one strong beam and very many really weak beams in very many different phases. (The turned off laser emits thermal radiation, you see)

 

- And how can the thermal not cohorent radiation interfere with the laser?

[Toffo]

And other questions

 

- If the two lasers are set on very low intensity, photon by photon, is it sure that both lasers stil emits each a photon at the same time?

 

 

- And how can the thermal not cohorent radiation interfere with the laser?

 

 

It is impossible for an object, that is radiating thermal radiation, to remember what kind of radiation it has radiated, and when, and into what direction. The information content of a radiating object is decreasing, not increasing.

 

That is why the radiations coming from two thermal radiation radiating objects interfere.

 

We don't see any interference pattern when non-coherent radiations interfere, because there are many interference patterns on top of each other.

 

 

About low intensity lasers, like for example such lasers whose intensity is one photon at random moment in a day:

Two these kind of lasers produce a good visible interference pattern, unless information about photon emission time is stored somewhere.

 

But if one laser produces only blue photons, while the other laser emits just red photons, then the information about which laser emitted a photon is in the photon, and therefore the red photon waves and blue photon waves do not interfere.

Edited November 5, 2013 by Toffo

[DParlevliet]

About low intensity lasers, like for example such lasers whose intensity is one photon at random moment in a day:

Two these kind of lasers produce a good visible interference pattern, unless information about photon emission time is stored somewhere.

 

If both lasers produce single photons random, then also independent from each other I suppose. Both lasers does not each emit a photon at exactly the same time.

So if one laser produces a single photon, then there is no other photon from the other laser. With what interferes the photon?

Edited November 5, 2013 by DParlevliet

[Toffo]

 

If both lasers produce single photons random, then also independent from each other I suppose. Both lasers does not each emit a photon at exactly the same time.

So if one laser produces a single photon, then there is no other photon from the other laser. With what interferes the photon?

 

It's a somewhat strange thing, that's why I brought it up.

 

So let's see now. A quantum wave from laser1 is hitting a screen for a day. And another quantum wave from laser2 is hitting the screen for a day. Or maybe it's 100 quantum waves which each have 100 days average detection time.

 

Anyway, those quantum waves interfere.

 

About photons: It's hard for me to say anything those things, because I don't know what those things are or what the word means.

[DParlevliet]

A photon (whatever it is) has wave- and particle properties. When the photon is not emitted there is no wave. If the photon is absorbed the wave is collapsed. So if one laser emits a single photon and the other not, then there is nothing to interfere with. So with two lasers with high intensity there would be an interference pattern, which low intensity not. That does not look right.

But I don't know how the experiment with lasers works in detail, so I cannot find arguments against it. I try to find something (if nobody else here has more information).

[Enthalpy]

[...] About low intensity lasers, like for example such lasers whose intensity is one photon at random moment in a day.

Two these kind of lasers produce a good visible interference pattern, unless information about photon emission time is stored somewhere. [...]

I have my doubts about that. The coherence time of a laser is hugely shorter than a day, which means that photons arrive well separated at the detector. A laser emitting photons one at a time must also have a much shorter coherence time than with many photons simultaneously.

 

What's still possible is make interferences from a single laser emitting a single photon at a time.

 

About interferences among non coherent, or just among non synchronized sources: the resulting pattern wobbles quickly. It holds during one coherence time of the source, which is unobservable with thermal radiation, and seriously short with lasers even good. If the observation time clearly exceeds the coherence time, the interferences patterns sum at random, giving a uniform result, undistinguishable from the absence of interference.

 

One gets interferences with thermal radiation, but then each photon must interfere with itself, which requires a single light source, and paths length closely matched so the offset is less than the coherence length (or time).

[Toffo]

A photon (whatever it is) has wave- and particle properties. When the photon is not emitted there is no wave. If the photon is absorbed the wave is collapsed. So if one laser emits a single photon and the other not, then there is nothing to interfere with. So with two lasers with high intensity there would be an interference pattern, which low intensity not. That does not look right.

 

But I don't know how the experiment with lasers works in detail, so I cannot find arguments against it. I try to find something (if nobody else here has more information).

 

Ok. We have wave-photons and we have particle-photons. I'll use those terms.

 

There are at least two ways how a wave-photon can have a low amplitude:

 

(1) wave-photon is very long

(2) wave-photon is very wide

 

 

If a wave-photon is both short and narrow, then that wave-photon does not have a low amplitude.

 

So, when we are doing one particle-photon once a day type of experiments, we have either long wave-photons, or wide wave-photons.

 

So the problem is with the wide and short wave-photons, and the possible wave-photon-less areas between those wave-photons.

The solution to this problem must be that there are no such areas.

Now I just have to think why there are no such areas.

 

 

EDIT:

 

Oh yes now I have thought something:

 

If we make any area void of any photon-waves, by trying to find a particle-photon in said area, and not finding any, that will increase the probability of finding a particle-photon elsewhere. So therefore there was a wave-photon in that area, and said wave-photon moved from that are to other areas.

Edited November 5, 2013 by Toffo

[DParlevliet]

What's still possible is make interferences from a single laser emitting a single photon at a time.

 

Then it interferes with itself, like in the double slit.

 

But the question is, does a photon's wave interfere with another photon's wave. That can only happen when both photons are emitted at the same time (and phase). If that is not possible with two laser, then it is no prove that two photons interfere which each other.

 

 

[Toffo]

I have my doubts about that. The coherence time of a laser is hugely shorter than a day, which means that photons arrive well separated at the detector. A laser emitting photons one at a time must also have a much shorter coherence time than with many photons simultaneously.

 

 

There was a precondition in my one photon-particle on a day -experiment: Nobody remembers the launch time of the photon-particle.

 

If we do a photon-particle detection attempt just about anywhere, that changes the knowledge about the launch time.

 

The rule about unitary evolution of a wave function says: If probabily goes up somewhere, then there is an equal drop of probability somewhere else.

 

Photon-particle detection attempt may cause a collapse of a wave function, in which case there was a wave in the area in question.

 

Photon-particle detection attempt may cause a change of a wave function, in which case there was a wave in the area in question.

 

If wave is exactly the same after a photon particle detection attempt, then it was not a real honest photon-particle detection attempt.

 

A photon-particle detection attempt in an area where there is no photon wave is not a real attempt.

 

If a real detection attempt is possible, then there is a wave.

Edited November 6, 2013 by Toffo

[Enthalpy]

[...] one photon-particle on a day -experiment: Nobody remembers the launch time of the photon-particle. [...]

You miss one point. The emission mechanism of photons already defines a duration. It's called a decay lifetime (for the excited state), a transition time... If the emitting object is undisturbed, this defines the coherence time and the resulting linewidth. External influences can change the lifetime, for instance shocks among gas particles, or a resonant cavity, or neighbour emitting objects that influence an other.

 

This lifetime can be 1ns or far less, and in some occasions more (neutral hydrogen, extremely dilute in space, emitting at 21cm).

 

Even if you don't observe in a day an excited atom with 1ns state lifetime, it will de-excite in 1ns. The photon will be emitted immediately, will have this duration, and after that, coherence is as much lost as light is: obscurity. Only within the 1ns is uncertainty.

 

Yous "photon-wave" and "photon-particle" have no sense, that's why nobody uses them.

[DParlevliet]

Or more simple: as long as the photon is inside the laser, there is no wave outside the laser. If the photon leaves the laser and there is no photon from the other laser outside that laser, then there is no wave to interfere with.

 

What is the measuring setup of two lasers? How is the interference pattern measured?

[Toffo]

Even if you don't observe in a day an excited atom with 1ns state lifetime, it will de-excite in 1ns. The photon will be emitted immediately, will have this duration, and after that, coherence is as much lost as light is: obscurity. Only within the 1ns is uncertainty.

 

 

 

 

So if we want to experiment with long waves, we don't use this atom, that produces a short wave.

 

 

A way to produce a long wave: Use atoms with a long decay lifetime.

 

Another way: A geiger-counter-short-pulse-laser combo device. This device emits a short laser pulse when it detects a radiation particle. Except that it is incapable of emitting a short pulse, although the laser part is built to emit a short pulse. So the device emits a long wave. Except that the firing time is very likely to be remembered by the device, so it emits short waves after all. I almost made an error!

 

Another way: Short photon wave packets are stored in a photon container, which has a small leak, a hole in the container wall. A long photon wave is emitted from the hole.

[Enthalpy]

A way to produce a long wave: Use atoms with a long decay lifetime.

Normal permitted transition use to be short in human scale, from ns to few µs. But forbidden transition can be very long, because the superposition of the initial and final states has no efficient method to radiate light.

 

2s -> 1s is an example: the superposition doesn't move the electron back and forth, so no EM field is created - or equivalently, the difference in orbital numbers doesn't fit a photon spin.

 

Or an isolated hydrogen atom with the electron in 1s state. It can have the proton and electron spins parallel or antiparallel; transition occurs at radiofrequency 1420MHz with a probability of 2.9×10⁻¹⁵ s⁻¹

http://en.wikipedia.org/wiki/21_cm_line

that is, it takes 11 million years.

Short photon wave packets are stored in a photon container, which has a small leak, a hole in the container wall. A long photon wave is emitted from the hole.

That won't work. Get a long photon from time to time. It's linked with the spectral line width.

[DParlevliet]

On the web most agree about the interference between photons, but all evade the situation with low intensity, so that remains a weak point.

 

Now another familiar experiment which gives interference:

 

This can be completely explained with waves, so according QM-Copenhagen a discussion about the particle is not relevant. It does not matter of if (or what) path it goes.

According (minority) Broglie/Bohm the particle is driven by waves through one of two paths. Perhaps that could be explained by uncertainly of particle position at the 50% mirror, causing to be taken by the left of right wave (just a guess)

Still it is remarkable that the wave, which is a probability (only at the detector), seems to be able to be 50% reflected (or transferred) in the mirror surface material

[Enthalpy]

A wave half-reflected is perfectly natural.

 

Instead of "doesn't matter which path" or "uncertain", I prefer "it takes both paths".

 

Maybe you want to save the idea of a local particle that takes one path, or one path or an other... but if you admit that "particle" only means that the wave can become local, for instance at a detector, and stays through its path not more concentrated than needed, then QM gets more natural.

[DParlevliet]

A wave half-reflected is perfectly natural.

 

Of course: it happens, so is natural. But the wave is not defined so natural: a formula to predict propability and is no real physical wave. Then it is remarkable that is reacts with the cassical matter of a mirror.

 

Instead of "doesn't matter which path" or "uncertain", I prefer "it takes both paths".

 

A photon is defined in QM as classical wave and particle properties without knowing if it is a real physical wave or particle. A classical particle cannot go both paths and I think it is never proven that a particle can go more then one way. So I think that should not (yet) be used for explaining results.

 

Maybe you want to save the idea of a local particle that takes one path, or one path or an other... but if you admit that "particle" only means that the wave can become local, for instance at a detector, and stays through its path not more concentrated than needed, then QM gets more natural.

 

With conclusions I keep now closer to the present two explanations which have fully fitting math. According main stream Copenhagen it is a wave or particle, not both at the same time. According minority Broglie/Bohm it is a wave and particle at the same time.

[Toffo]

That won't work. Get a long photon from time to time. It's linked with the spectral line width.

 

 

 

Every photon starts to squeeze itself out from the container immediately when the small hole on the container wall is opened.

 

For every photon in the container, the photon is completely out of the container when the container is completely empty.

 

(We are not disturbing these photons by observations)

[DParlevliet]

When detectors are placed on the slits ( I supposed even one detector) there is no interference pattern anymore. Can someone explain from the working principle of a detector (which I could not find) what it changes to the wave, to make it impossible to interfere?

 

It could be that
- in detector 2 the photon is absorbed (wave collapsed) and a new photon (with new wave) emitted. The new wave cannot reach slit 1 anymore so there is only one wave coming out of slit 2.
- in detector 2 the photon is not absorbed (so goes through slit 1) and then the conclusion should be that a wave cannot pass a detector, for some reason.
But I don't know how a detector works, so if above is reasonable

[Sensei]

Detector = polarization filter (in the simplest version of test).

 

Also this article might interest you

http://en.wikipedia.org/wiki/Quantum_eraser_experiment

Edited November 14, 2013 by Sensei

[Enthalpy]

[...] from the working principle of a detector what it changes to the wave, to make it impossible to interfere?

This detector absorbs the photon, or in other words, it stops the wave at the hole. Since the wave can pass only at 1, the fringes aren't poduced.

 

Note: a hole (1) big enough produces interferences by itself, just as if it were many small holes next to an other. They're rings instead of fringes. With two big holes (1) and (2), you get a mishmash of fringes and rings. Only very small holes make no rings, but they let little light through. Though, the rings reulting from big holes are easily unnoticed.

 

Note: some vicious people use less brutal detectors, where the photon doesn't have to disappear every time. But QM gets complicated then.

Every photon starts to squeeze itself out from the container immediately when the small hole on the container wall is opened.

My mistake. You can get longer photons by varied means. And the relation with the spectral width only gives a minimum duration, no maximum.

 

To make powerful short pulses, people spread the pulse over time with a grating, amplify it, then recompress the pulse with an other grating. The small hole would work; you may get the single photon spread over many short places. A semireflective mirror would do it as well, or a coupling by evanescent waves... A dispersive medium can as well lengthen a light pulse, over a contiguous length.

 

The best "box" feasible presently must hold light for few ms. One day is out of reach. But that's nasty technology details, not hard limits of physics.

 

One day, or many years, needs to "store" the photon in a different form, say the excited state of an atom. If no other de-excitation method is available, molecular electrons can stay on a higher level for hours (singlet oxygen, despite high energy) or millions of years (atomic hydrogen, 21cm line hence very low energy). Excited nuclei use to de-excite in ps but can last for hours.

 

Additionally, if you achieve to put the excited thing in an "anti-resonant" low-loss cavity, it lasts longer.