# What is entanglement really about? [closed]

I am a beginner in this field, I am trying to understand the basics of Quantum Mechanics, I want straightforward answers to few questions on entangled photon/electron:

1- What entangled photons really are? What happens to first photon happens to the other, does that only include polarization?

2- As I understood entangled photons does not mean sending FTL information, but as I understood first photon knows about what happens to the other photon, isn't that by itself FTL information?

3- What are the conditions to create entangled photons?

4- Does making measurement on first photon collapses the wave function of the other photon?

5- in Double slit experiement, it is said that the photon goes in both slits at same time, why not it is another entangled photon going into the other slit? after all I doubt that they really fire single photons through the slits, they must be firing small shots of photons.

Sorry for my novice questions as I said I am trying to understand the basics.

## closed as too broad by Norbert Schuch, Alfred Centauri, Gert, user36790, John RennieMay 27 '16 at 4:53

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

• As a beginner you should focus on important aspects of quantum mechanics, like its use in atomic physics, and you should be learning how simple quantum systems like the free particle, the particle in a well and an a hydrogen atom behave. None of the above qualifies as "basic" questions and you won't be able to develop a reasonable intuition into these questions unless you learn the aforementioned topics. – CuriousOne May 26 '16 at 21:32
• but sir I already know about the other "more" basic stuff.. I am asking about specific topic here.. my questions can be wrong, no problem in that you can explain to me. – DeepBlue May 26 '16 at 21:44
• @CuriousOne: I've never studied QFT, but have studied (and used) photons in many classes, and in my work as a laser physicist, and even just today I am using single photon detectors. I am quite puzzled by your theories of education, and your characterization of useful, everyday language. – Peter Diehr May 26 '16 at 22:20
• @PeterDiehr: I am simply of the opinion that 80 years of poor teaching of this subject are enough. Have we all seen 'clicks' in a PMT channel? Yes. That's the outcome of a quantum measurement on an em field and nothing more and nothing less. Does it mean that the PMT collapsed the entire universe? God no, it didn't. It merely gave us one possible outcome among many for that measurement. – CuriousOne May 26 '16 at 23:21
• @CuriousOne only you and a few others on the site seem to push this photon-less theory. It is you who seems to always be holding on to and evolving these 80-year-old ideas. It is you who is always telling everyone else they are wrong and you are right but you have no way of proving your ideas are any better than the other. You still have never said why the idea of a wave is better than the idea of a particle. – Bill Alsept May 27 '16 at 2:13

Engtangled photons: first you must understand that the photon is the particle obtained when the modes of the electromagnetic field are quantized, and that they are created and destroyed as discrete quanta of energy, in agreement with Planck's relation, $E=hf$, where $f$ is the frequency of the electromagnetic field corresponding to the quantized mode; that is, f is the frequency of the photon.

Now suppose $|A\rangle$ is the complete quantum description of photon $A$, and $|B\rangle$ is the complete quantum description of photon $B$. The two particle system can be written as $|A\rangle|B\rangle$, the direct tensor product of the two states. This describes two independent photons, both parts of a larger quantum system.

However, just as the state space for photon $A$ contains all of the possible linear combinations of the eigenstates of that space, the tensor product of the independent state spaces of photon $A$ and photon $B$ contains all of the linear combinations of the elements of the direct product space: this includes many two-particle states which cannot be represented as direct products.

If the two-particle state cannot be factored, that is, if it cannot be written as a direct product, then that two particle state is said to be entangled; it is clear that you cannot assign definite properties to the two photons in such a state, though you may know a great deal about the pair. We cannot even label them in any unique way.

There are many ways to create such states, and in fact, most quantum objects are partially entangled with other quantum objects; they need not even be of the same type: it is possible to entangle an atom with a photon, or even to transfer the entanglement from one quantum object to another. Quantum information, quantum cryptography, quantum computing, and quantum teleportation all depend upon quantum entanglement for to accomplish certain tasks.

I'm currently creating entangled photons using a non-linear optical technique known as spontaneous down-conversion; UV photons from an intense laser beam, 404 nm, are focused on a specially prepared BBO crystal; a very small percentage of the incoming photons are absorbed and then reemitted as a pair of photons with the same total energy, which in this case corresponds to 808 nm, in the near infrared. There are fixed relationships that must be satisfied. Phase matching conditions for spontaneous parametric down-conversion.

This addresses your questions (1) and (3). The answer to (4) is no, but if there is a conserved quantity associated with the two particle state, knowledge of the first photon can be used to predict that portion of the state of the other; this is the basis for the Einstein EPR Paradox. The consensus is that Einstein was wrong; the Bell Inequalities are routinely violated; they are next on my list of things to do in the lab, as a way to validate the setup under construction, and measure its quantum efficiency. With an understanding of (4) you are ready to read the no-go theorems for FTL communication via entangled particles.

I'll leave (5) for you to reconsider.

"What entangled photons really are?"

Photons are measurements of the state (changes) of a quantum field. Let's take your knowledge about the hydrogen atom as a starting point. Let's say your atom is in a p-state. The atom then changes into an s-state. Its angular momentum and energy change. Angular momentum and energy conservation demand that both quantities have to be carried away in some way. The electromagnetic field can do this. The atom de-excites, giving off "a photon", which is the change of the total angular momentum and energy of the em field. Later the em field can transfer this quantum to another atom or it can just be lost into the darkness of space, never to be seen, again.

That is what photons are: accounting units for the state changes of the em field. Just like all accounting units, photons are indistinguishable. We can't write "Kilroy was here!" on a photon and then watch it percolate trough the vacuum like a classical bowling ball. We can throw a photon into the em field and at some point we can get a photon back somewhere else, but it's not "the same" photon. It's really just "a photon". It's not "Sue the photon" or "George the photon", but one anonymous quantum of energy and angular momentum.

"What happens to first photon happens to the other, does that only include polarization?"

Can you see how that question makes no sense, whatsoever? You are implicitly assuming that there is a first photon (called "Sue") and a second photon (called "George"), but there isn't. There is an em field that is excited with two quanta of electromagnetic radiation. This means we can make two independent measurements in two different places which are linked by a physical constraint and they have to show a correlation. What we are not doing is picking up "Sue", who then somehow phones "George" that she's just been collapsed by a photomultiplier tube.

"2- As I understood entangled photons does not mean sending FTL information, but as I understood first photon knows about what happens to the other photon, isn't that by itself FTL information?"

"Information" is such a poorly defined term for the purposes of physics. Have you seen it being used in your classical mechanics classes? Did we talk about information there? No. We talked about work and energy. If we want to change anything in a physical system, we need energy to do so. Is there an energy transfer here? No, certainly none that happens faster than the speed of light.

So what if we make up a definition for "information"? How about we demand that it satisfies that something about the statistical properties of one of the measurements on the "George" photon side (Cough! - see my previous remark) changes? Would there be a change in the outcomes depending on what we do on the "Sue" photon side (Cough, cough!)? Nope. Not a thing would happen. Only the correlation between "George" and "Sue" will ever change.

"4- Does making measurement on first photon collapses the wave function of the other photon?"

Nobody has ever seen a wave function collapse. That was merely an all wet expression for the Born Rule when your great-grandpa was first talking about this stuff. Since then the Big Cheeses have been beating their gums about what looked like the Bee's Knees back then. But don't take any wooden nickels because everything is Jake if you stop using your great-grandpa's slang. Did you notice how many other expressions from the 1920s we are not using without the world falling to pieces? Put "collapse of the wave function" among the collection I just gave you.

"5- in Double slit experiement, it is said that the photon goes in both slits at same time, why not it is another entangled photon going into the other slit? after all I doubt that they really fire single photons through the slits, they must be firing small shots of photons."

Attaboy! (That means "Well done!") Now you are catching on. Absolutely nothing gets fired trough anything. A wave phenomenon that happens to be restricted by the symmetries of spacetime to quantized exchanges of angular momentum is being observed.

• This answer has value, but it's way too preachy. You can explain the concept of identical particles without resorting to quantum fields. – Javier May 27 '16 at 2:54
• @Javier: I agree. I didn't want to write an answer to begin with, since I was worried it would come out like this, but after criticizing the OP I felt compelled to write something after all. I am not sure that one does not need quantum fields, after all, the problems with entanglement stem from a relativistic system... even though it's almost never treated for what it is. I also find that the interpretation of quanta in QFT is much more natural and correct than in single particle theory, which, if we are honest, is a toy theory that was abandoned in the 1930s. – CuriousOne May 27 '16 at 3:06
• But we can also talk about particles behaving in same way, like an electron which behaves as a particle when measured, but then atoms also can be entangled, which makes me feel the entanglement explanation is incomplete! Another question: if what you are describing is analogous to pilot-waves theory (excuse my ignorance if I am wrong) then why that theory was abandoned? – DeepBlue May 27 '16 at 7:49
• @DeepBlue: You can talk about particles all day long, but you will notice that sooner or later you will be talking about particles taking a certain path trough a slit and then you will be setting up ever more complex Rube Goldberg optical benches where you make photons run forth and back and you will make them run trough delayed choices and erasers and you will find yourself coming up with ever more complex interpretations of QM, absolutely none of which will tell you anything physical about the world. There is an entire cottage industry of man behind the mirror seekers of that kind. :-) – CuriousOne May 27 '16 at 8:04
• @CuriousOne I know QM works very well .. it is almost a hack .. but I also dont want to seek a blind faith I just want to understand.. there must be an explanation be it classical or not I don't mind. We have to stop talking about particles .. I agree.. but what we should be talking about when describing -what we call- particles? Waves? energy quantities wrapped in spacetime? or else? – DeepBlue May 27 '16 at 8:40