Ep. 140: Entanglement

Artist's impression of an experiment to test entanglement

Artist's impression of an experiment to test entanglement

One of the most amazing aspects of quantum mechanics is quantum entanglement. This is the strange behavior where particles can become entangled, so they’re somehow connected to one another – no matter the distance between them. Interact with one particle and the other reacts instantly; even if they’re separated by billions of light-years.

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    Fraser Cain: During our last episode you scrambled for shelter [laughter] from an approaching tornado storm, but everything was fine, right?

    Dr. Pamela Gay: Everything was just completely fine although we did get 6 inches of water in the basement.

    Fraser: This happens like a few times every year right?

    Pamela: It’s the middle of the U.S. we get tornadoes. It’s part of living here. [Laughter] I know, it’s very surreal but we don’t have hurricanes and we don’t have well we do have earthquakes. We don’t have forest fires just all other manner of natural disasters.

    Fraser: I sometimes get emails from readers saying: “did you feel that earthquake?” Apparently we get pretty big earthquakes here on the west coast. There’s nowhere to live where you’re not going to have some natural disaster of some type. Once again, the universe is trying to kill us.

    One of the most amazing aspects of quantum mechanics is quantum entanglement. This is the strange behavior where particles can become entangled so they’re somehow connected to one another no matter the distance between them.

    Interact with one particle and the other reacts instantly even if they’re separated by billions of light years. How on Earth did the concept of quantum entanglement even get discovered?

    Pamela: This basically originates from the fact that there are a lot of different processes that we think are conserved. When you have two particles created they’ll generally have matter and antimatter. That one we’re fairly familiar with from talking about in the show.

    There are other properties as well. Things like spin where if you have a positron and an electron flying in opposite directions we’ll say that one has spin up and the other has spin down. One will have positive charge and the other will have negative charge.

    It is in the process of conserving all of these different properties that we end up with these weird spooky often referred to as actions at a distance where we don’t actually know which particle has which property until we start to measure one. Once we measure them we realize that all of these properties line up.

    Fraser: Give me kind of a classic example of entanglement going on. What sort of starts the process and how would you then interact? What kinds of properties would you see? What starts the process of entanglement?

    Pamela: When you create two particles at the same time. This can be for instance two protons that might share the exact same polarization, the exact same spinning of their electromagnetic wave as they propagate through space.

    Fraser: What would actually generate two photons at the same time?

    Pamela: It could be some sort of a nuclear process generating light. All sorts of things generate light. You could have some sort of a light source and this is something you can actually do in a laboratory. You set up a light source where you have all of the light going through some sort of a crystal. Calcium crystals of different types can do this for instance.

    As the light passes through it, only light that is rotating in one particular way will be able to make it all the way through the crystal. If you get things that are rotating in one direction and then you measure how much they have wavelengths going up and down, left and right you can get the light to behave in all sorts of crazy ways where the photons are in lock-step changing their orientation as they pass through these crystals and these filters. It’s just kind of creepy.

    Fraser: You’re taking regular light, shining it through this crystal and you’re forcing it into a very specific pattern?

    Pamela: Yes.

    Fraser: The pattern of the photons that is coming out of it is these photons entangled? Because they’re kind of coming in sequentially right? You’re getting one then another then another. They’re not going in at the exact same moment.

    Pamela: It’s similar physics. This is a very complicated idea and we usually build up how we explain it in class by first freaking people out with polarized light where you show a beam of light going through different polarizers; the way you can block and change light as it goes through first at a 45 degree angle then horizontally then vertically.

    Then we work on photon pair production where we look at so we’ve figured out how to generate two photons at the same time they have the same polarization, let’s measure them when they’re far enough away from each other that there’s no way they’re communicating back and forth. We can see that they have the same polarization.

    Then we start looking at wow we can start doing this with particles as well. This is where it really starts to get creepy. You can have all sorts of different reactions that will generate as part of the reaction an electron and a positron. This is a matter/antimatter pair.

    They have conservation of momentum so they’ll fly off in opposite directions. They have conservation of charge built in so one is negative one is positive. They have conservation of spin so we have spin up and spin down. We don’t know which has what in terms of the spin until we measure it.

    One of the weird things about the Copenhagen interpretation of quantum mechanics – this is only one of the ways of looking at quantum mechanics but it is the one that most people follow right now – is one of the things that it says is everything can be described by wave function. This includes you and me. We’re just waves. We’re really complicated waves that appear to be completely solid entities but we’re waves.

    Part of being a wave is everything about us is probabilistic. Because we’re really big complicated waves we behave in a deterministic manner. You put your hand on the table you can feel the table. Your hand doesn’t pass through the table.

    When you start looking at individual particles you have a stacking of all the different wave functions of what the particle could be. As the particle travels through space it simultaneously spins up and spin down.

    It doesn’t know which one of these two things it is when you collapse that wave function until you actually observe it and that wave function goes: “Oops I have to be in one place right now. I have to behave like a particle right now. I need to decide which spin I am.”

    Fraser: So whether a particle is spin up or spin down has no affect on the universe until it bumps into something, right? Until it somehow interacts with an observer or bonks into some other matter or somehow interacts.

    If it’s just floating through the vacuum of space, it’s not doing anything. If it’s not touching anything there’s no need for it to choose a spin direction.

    Pamela: Right and what’s cool about experiments like this is people have been trying to say no sorry this is just too creepy. There has to be a better explanation.

    Einstein was one of the people who said this is creepy there has to be a better – he used much more complicated and philosophical longer sentences – but basically to paraphrase it’s creepy. This is wrong. We need a better explanation.

    Fraser: I guess what people find creepy is a particle moving through space. It exists it is real and yet one of its characteristics has been un-chosen. It can be either way, one or the other and so far it does not have to decide.

    It’s like waiting in line for your ice cream and thinking you’ll get vanilla, or chocolate [laughter]. I won’t decide until the person at the counter asks me to choose.

    Pamela: So you live in a super positioning of I want chocolate, I want vanilla, I want bubble gum, I want raspberry sorbet you have the supposition of many different possibilities.

    Some of these have a higher probability than others. It may be that while the bubble gum ice cream is tempting you because who doesn’t want to have 24 balls of bubble gum in their mouth at the same time, you also know it’s a pain to eat.

    But the possibility is there, but chocolate is awesome and Rocky Road is even better because then you have the chunks of stuff.

    Fraser: You have an ice cream problem don’t you?

    Pamela: Well, I’m allergic to it so I spend all my time wanting it and not being able to have it.

    Fraser: Oh, no! That sucks. [Laughter]

    Pamela: There are all these different probabilities and in quantum mechanics we say that the probability is related to the square of the amplitude of the wave function.

    This is sort of like saying make a graph of how much you want something. Then take the square root of where your peak is on that graph and that square root is related to how likely it is that it will actually happen.

    Fraser: It’s like there’s a bell curve, like a percentage right? There’s a percentage chance that it’s going to go this and a percentage chance that it’s going to go that.

    Pamela: All the probabilities add up to one because something will happen.

    Fraser: Right.

    Pamela: It could be that you get up and when the friendly person behind the counter says which flavor would you like? If this experiment happens 10,000 times you’re going to end up answering the question in a way that exactly matches the probabilities.

    At any given moment anything could happen. It’s just what’s most likely is guided by the probabilities.

    Fraser: Right. This is where you got this particle that so far doesn’t need to decide, however it will most likely make a decision based on the probabilities.

    Pamela: Imagine that you and a friend have made this pact that anytime they get chocolate ice cream you’re going to get mint chip ice cream because you’re going to steal bits of ice cream from each other. Whichever one you get the other person will get the other thing. If he gets mint chip you get chocolate; if he gets chocolate you get mint chip.

    This is the way you’ve decided your universe will always function. It could be that he goes to New York and you go to Los Angeles and neither of you have cell phones and you can’t yell at each other across the entire United States. Even though you’re not going to be stealing ice cream from each other you still have this pact. He gets chocolate and you get mint chip.

    According to quantum mechanics if you two were entangled particles he could order chocolate chip in New York and you would know you have to order mint chip. This is where it gets weird is how is it that two things that can’t communicate to each other are able to know what they’re supposed to be when they’re separated by such a great distance.

    Fraser: Just to be clear here the communication is going instantaneously regardless of distance. It’s not going at the speed of light. You could have these two particles – these ice cream orderers on opposite sides of the universe separated by billions of light years and one decision gets made and the other decision gets made perfectly with no communication happening in-between.

    Pamela: When we talk about this scientifically we often refer to it as the Einstein – I’m going to mispronounce this and I’m sorry – Podolsky Rosen paradox.

    Based on this paradox of how is it that two things at a distance can communicate instantaneously which shouldn’t happen according to relativity.

    Fraser: Yeah Einstein how is that possible?

    Pamela: Well and he sat there going no quantum mechanics, no. That was Einstein’s kind of up until he died he did not like all of this stuff.

    There is what is called Bell test experiments. These are experiments that work very hard to figure out what the heck is happening. There have been postulations that there are particles that are somehow communicating between the two events. Experiments are devised to try and eliminate the possibility for these particles.

    So far every experiment that people have done trying to either say no our understanding is wrong or no our understanding is right. All the experiments, no matter what their goal was has come out and said no quantum mechanics actually works. We are getting things behaving exactly the way they’re supposed to.

    They’ve done experiments where they’ve sent particles kilometers and kilometers apart, made measurements and in the time that it takes to make the measurement you can’t send things at the speed of light between the two sites of the experiment.

    This is one of the things that people have said is it takes time to conduct the experiment and while the experiment is taking place particles are flying back and forth and communicating so that everything is kept in sync.

    We’re fine with invisible particles we don’t understand. This is how we explain things like mass and gravity. We were able to eliminate the idea of particles flying back and forth at the speed of light communicating what’s going on in the two experiments by conducting the experiments so far away from the place where the two particles were produced that the measurements can be completed before anything traveling at the speed of light can travel between the two locations.

    Fraser: So then the scientists at the speed of light are then communicating afterwards and saying I got this, did you get that? It’s always turning out correct.

    Pamela: Right.

    Fraser: But the experiment is happening where the measurement is happening too fast for the speed of light to be able to communicate – for the particles to be somehow communicating. It really is instantaneous regards to distance.

    Assuming you could extrapolate it would work if it was across the solar system, across the universe. The moment you do one the other one goes off. Wow.

    So, why is this happening? I understand Einstein went to his grave trying [laughter] to puzzle this out but what is sort of the current thinking of about why this works?

    Pamela: One of the things about quantum mechanics is in many, many cases we have understandings, for instance the Copenhagen interpretation of quantum mechanics. These interpretations state things like well everything is a wave function. The description is essentially probabilistic.

    We have uncertainty principles that say we can never know exactly where something is and how fast it is going. When you put all the pieces together you’re kind of left with if it’s a probabilistic wave function moving in a given direction at a given velocity, I can’t tell you what all of those wave functions are going to collapse down to until I measure it.

    It’s all these different pieces of it’s a wave function, it has a bunch of different probabilities, I haven’t observed the sucker yet that lead to this uncomfortable – well it’s all the things at once until you observe it. The math works. You start from the philosophical statement, build the math around it and the math works.

    Fraser: Right but you’ve got this is the way it needs to behave and the math works but that still doesn’t explain why it works.

    Pamela: And quantum mechanics doesn’t always explain why. That’s one of the uncomfortable parts about it.

    Fraser: [Laughter]. Right. Quantum mechanics does not care that you need an explanation that feels right.

    Pamela: [Laughter]. Quantum mechanics does not care how uncomfortable I am in trying to deal with it. It just does things and predicts things and says these things can happen and then they do.

    Fraser: But isn’t sort of a part of the thinking is that when you get a wave function for a particle that you’re essentially getting that particle existing across the entire universe? It’s bunched up probability-wise in the spot where the particles most likely is, and then think of it as like a peak, a really tall mountain.

    Then with the most likely position of the particle at the peak of that mountain. But then a really steep side all the way up and then a gently sloping side forever across the universe so the particle could be across the universe but it’s not in 99.999% of the time.

    Pamela: This is part of the idea of collapsing wave function. With things like photons it’s even more convenient. The neat thing about photons is because they’re traveling at the speed of light they don’t experience time. As far as a photon is concerned, and I love the way James Tab explains this. He explains it as for a photon everything is goes flat.

    They’re created, they go somewhere, and they are destroyed. That’s the entire existence of a photon is “oh, I’m created, oh, I’m destroyed.” There’s no time between those two different activities. Time stops when you’re moving at the speed of light which is what life does.

    For them it’s sort of like they’re created in one place and then the next thing they know they’re destroyed so they don’t have a time to not know that they’re not next to each other. It hurts, it totally hurts.

    Fraser: But isn’t that kind of the thinking that you’ve got the wave function can extend across the entire universe? That’s how they can somehow be interacting is because you’re going to get these overlapping wave functions, right?

    Pamela: Not everyone goes that direction. There’s no evidence to prove that is the thing. There are places where you read people desperately trying to come up with ways to explain this. Like the wave functions fill up the entire universe up until the moment they’re observed.

    We don’t have evidence. What we know is there are all these probabilistic outcomes. When we make the measurements on particle does one thing the other particle knows what it’s supposed to do and does the other. That’s all we know for certain.

    Fraser: A couple of questions. How far can you scale this up? We talk about photons, electrons, how far can you go with this?

    Pamela: Right now when we deal with entangled particles we only worry about the entanglement of things that when they’re created have to have conserved properties. You have to have conserved spin, conserved charge, conserved matter/antimatter things like that.

    Here we’re talking strictly about particle physics. Things like electrons, neutrinos, individual particles not full out atoms, not full out molecules. Quantum mechanics really doesn’t care how big things are if you manage to entangle them.

    One of the really neat thought experiments that was put forward as a way of basically trying to mock quantum mechanics and I love all these times in physics where a scientist just tries to say “oh what you’re doing is so stupid.” Then the way that they articulate it ends up being the way we think about the idea forever.

    Fraser: Right, we should call this the “Big Bang”.

    Pamela: Right and you know what? The name stuck. “We should call this dark matter.” Well, yeah, let’s move forward with that idea. For quantum mechanics it’s Schrödinger’s cat.

    The idea here is you can actually entangle large systems where you can create the super positioning of wave functions simply by coupling a particle activity with a many atoms, many molecules more physical easy to see/touch tangible situations.

    Fraser: Well why don’t you explain the thought experiment?

    Pamela: Schrödinger’s way of running this was to say: look, let’s take a cat, stick the cat in a steel box. You have no way of observing the cat. We simply know there’s a cat in the box.

    Put with the cat a small amount of radioactive material and a Geiger counter. Make it so that the amount material in the box is such that there’s a finite non-zero probability that within an hour a particle will decay and you’ll have a bit of radiation that will trigger the Geiger counter.

    At the same time there’s a similar probability that nothing is going to decay and nothing is going to trigger the Geiger counter. Then attach to that Geiger counter a vial of poison so that if one of these wave function quantum mechanics probabilities plays out and you do have a particle decaying and releasing radiation, the Geiger counter will trigger and the cat will die.

    They way Schrödinger looked at this was that means that until we observe if it decayed or not, until we observe if the cat is alive or not, the cat in the box is in the state of both alive and dead.

    Fraser: Then its fate is entangled with the potential of that particle decaying.

    Pamela: Up until we, the observers, open the box the cat is both alive and dead. It is us in opening the box that collapses the wave function and determines the outcome for the cat.

    Fraser: Now no cats were harmed in the making of this thought experiment?

    Pamela: Schrödinger made it very clear we should never run this thought experiment. This is a bad idea.

    Fraser: I guess then because I know that you can entangle other particles in this kind of a way right? Can’t physicists take one particle and sort of interact two particles together and cause them to be entangled and then do it again?

    Pamela: No not so much because it is primarily in when we created entangled particles it is through decays, through collisions, through systems where you’re creating two things that have to conserve traits.

    Fraser: Okay.

    Pamela: The thing with Schrödinger’s cat was, the cat is its own observer of its own fate. Its wave functions are fully collapsed already and its fate is fully decided by the quantum mechanics of the system.

    The second that particle does or does not decay as observed by the Geiger counter, the cat is very much dead or not if it doesn’t decay.

    Fraser: It’s almost like the Geiger counter is what’s doing the observing.

    Pamela: Another way of looking at this experiment is for us the observer we don’t know what the cat observed. We don’t know if the cat did live or die. In our reality the cat is both alive and dead until we open the box.

    According to another interpretation of quantum mechanics – which I think is going to get left for another show at this point – there’s the idea that every possibility that could happens. Thus that cat that was alive or dead up until we opened the box and look at it in some other parallel reality when we open the box it has the other state.

    So, if I open the box and find a happy living tabby cat there’s another universe where I forget to open the box and the experiment goes on until the cat has to be dead. There’s another universe where I open the box and the cat is dead from a heart attack.

    There’s a reality where the cat is dead due to quantum mechanics. Everything that possibly could or could not happen to that cat to the point of it randomly had kittens in the box, will occur in some universe. This is a manyverse way of looking at probabilities.

    Fraser: I know that the listeners have a burning question in their mind right now and they’re dying for me to ask it.

    Pamela: Okay.

    Fraser: So I will: couldn’t you use entanglement as a way to have instantaneous communication? Couldn’t I across the universe collapse particles and then you would see them collapsing and we’d somehow be able to communicate?

    Pamela: The problem is, how do I create entangled particles where I’m encoding “aha, this particle is going to be spin up; this one is going to be spin down and I’m able to send a set of information.” All I’m really able to do is create two entangled particles.

    When I observe mine that causes yours to do something else but I can’t set what yours is going to do and the send information. I can send you a stream of particles and your stream of particles is going to be the mirror of the stream of particles I send to myself. But I have no way of encoding information in this yet.

    There are people who are using quantum entanglement to try and figure out how to build quantum computers. This is a new field of research where they’re looking at ways to encrypt information where you actually use quantum entanglement to decrypt the way information is stored.

    I have to admit I don’t fully understand quantum computers. I think there are probably two people on the planet who fully understand quantum computers. I’m nowhere near as smart as either of them.

    Fraser: The gist of them is that because the particles collapse one of the great things about the quantum computer is that you can detect if the message has been interrupted. That’s one of the advantages of it.

    As I understand with not being able to use entanglement as a communication device that the problem is that you still have to somehow communicate with one another at regular light speed to say I got this, what did you get?

    Pamela: The thing with quantum entanglement is the particles are still moving at the speed of light or less. You might as well have used a pulsed laser beam.

    Fraser: No, no but you take a box like we generate a bunch of entangled particles and I carry my box off to Andromeda and you hang on to your box here and they’re both entangled together.

    Then I start collapsing them in some wave trying to communicate with you. The problem is that it will always be random, right?

    Pamela: Yeah.

    Fraser: Then all I’m doing is randomly collapsing these particles but until we actually communicate on the back end and say this is what I got, what did you get there’s no way to communicate.

    Pamela: Right and I can’t even know which particles you have collapsed or not because when I make my measurement the only thing I know for certain is I made my measurement so yours has to be the opposite of mine.

    Fraser: That’s all you know.

    Pamela: Yeah, not very useful.

    Fraser: You could have figured that out beforehand. [Laughter]. It can’t be used as a communication system.

    Pamela: Nope.

    Fraser: Nope you still have to communicate at the regular old light speed.

    Pamela: Yeah, it’s one of those sad realities.

    Fraser: That’s great but it’s a really interesting field. There are some good books on it. There’s one called ‘Entanglement’ – I read that a couple of years ago. It’s is amazing the experiments that have been done.

    That’s the part that I think where quantum mechanics really delivers the goods. Some of these experiments as you said where you separate the particles and the do the experiment, get your measurement before the particles can communicate or before you can communicate and see the results.

    There are just some astonishing experiments that are being done and being done right now and being done in the next decades that are just going to keep pushing this wide open.

    Thanks Pamela. I think we’ve pretty much wrapped up quantum mechanics for now. We may come back around again later on but I think we’ve got some Earth science stuff we want to tackle next.

    This transcript is not an exact match to the audio file. It has been edited for clarity. Transcription and editing by Cindy Leonard.