Sunday, October 24, 2021

Quantum Teleportation in a nutshell with experiment


Quantum Teleportation - 3 dimensional image

Quantum Teleportation: There and Back Again 


When one hears about the weirdness of the quantum world, one gets the impression that it is mostly about fuzziness and uncertainty. It seems that, as soon as we want to know anything specific about quantum particles, we hit a brick wall. Want to know which of the two slits the electron traveled through? No can do (©1). Want to know position and momentum of an electron at the same time? Bad luck (©2). Is the cat in that box dead or alive? Probably both (©3). We don’t seem to have a good grasp of the quantum world. 

The Mysterious Quantum World - Quantum teleportation 2021

The Quantum World: Mysterious … 

But that impression is wrong! We know precisely how the quantum world works – it’s just that it works differently from the world we experience every day. There are certain questions we cannot expect to get answers to, because they are nonsensical questions. A quantum particle does not have a well-defined position and momentum at the same time. Asking where and how fast a particle is precisely is like asking how the color green tastes. 

We do not only have a good idea of how the strange world of quantum mechanics works, we can also use its properties to our advantage, to accomplish amazing things! One of the most interesting examples is quantum teleportation. One can use the fact that particles can be entangled (©4), together with the fact that every measurement of a system changes the system, to teleport a qubit (©5) from one place to the other.

… But We can bend it to Our will! 

Now, before you get your hopes up for teleporters like in Star Trek – that is not quite what we are talking about. What we mean by quantum teleportation, is that a spin state of an electron – a qubit – can be transferred from one electron to another, without moving the electrons themselves. 

Emmy’s Got an Electron 

Imagine that, in her lab, Emmy has an electron with a certain qubit. That means, its spin could be up, down, or any quantum superposition in-between. Actually, Emmy will most likely not know which state the electron is in precisely. She doesn’t need to, for the teleportation to work. Actually, if the qubit is unknown, there is no way of finding out what it is precisely, either. One could try to measure it, but the only results one can get are “up” and “down”. That in itself would not tell us much about which qubit the electron was in, precisely – and after that measurement we would have changed the spin anyway, so there would be no way of finding out! 

But this is the marvel of quantum teleportation: the qubit can be transported without knowing it! You never measure the qubit – you actually measure something different. 

Maxwell Awaits a Message from Emmy … 

Now, there are going to be more electrons involved in the whole teleportation process, so we’ll start giving them names. 

Maxwell sits in another lab. Before the entire experiment has been set up, Emmy and Maxwell have prepared something: they have created a pair of entangled electrons (©4). Let’s call them A and B. Emmy has taken electron A with her, and Maxwell has taken electron B with him. They have to handle them carefully, so as to not destroy the entanglement between them. Emmy is in her lab, and has her original electron – let’s call that one C – which has some unknown qubit, which she wants to teleport to Maxwell. She also has electron A, which is entangled with the one Maxwell has in his lab. Now the trick comes: Emmy performs a measurement on A and C at the same time. She measures how much the two are entangled.

Quantum Teleportation circuit - Quantum Teleportation experiment - Quantum Teleportation multiple qubits

Wait a minute – weren’t A and B entangled? C wasn’t entangled with either of them! Well, that is right, but remember that every measurement process changes the state of the system. After Emmy has measured the entanglement of A and C, they will be entangled in some way after wards! And her measurement will tell her precisely in what way.

This measurement process by Emmy has some influence on B as well, though. As it turns out, because A and B were entangled, the measurement of A and C changes the state of B, too. In fact, after Emmy determined the entanglement between A and C, Maxwell’s electron B will not be entangled anymore with either of them. Rather, it will be in the original state that A was in from the start! So simply by per forming a measurement, Emmy has switched the role of B and C! Then teleportation has happened. 

… to tell Him which way to Rotate His Electron. 

Actually, that is not quite the whole story yet. Emmy needs to write down the result of her measurement, and send it to Maxwell. 

Depending on the precise way in which A and C turn out to be entangled, B might not be in quite the same state as C before, but rotated slightly. The result from Emmy’s measurement tells Maxwell which way B has to be rotated in order to have precisely the original qubit of C. 

Okay, we need to comment on what just happened: the quantum teleportation did not actually move the electrons from one place to the other. It is just that we have transferred all information of one electron to another one far away (from C to B). The teleportation process begins when Emmy makes the measurement of how A and C are entangled. It ends when she has told Maxwell the result of her measurement. She needs to do this with conventional methods, which is why teleportation does not hap pen faster than light! 

There is no Cloning with Teleportation! 

It is also important to note that teleportation is not copying. Because Emmy performs a measurement on the electrons A and C, she changes their state. Before the measurement, C has some qubit, but after the measurement, C is in some strange entangled state with A, not in that qubit state anymore. It is B, which carries the qubit after the teleportation process has finished. So in order to teleport a state, you necessarily need to destroy the original information. That might actually be a relief – if some day in the future we manage to actually teleport whole people with this method instead of just electrons, we should not have the problem of accidentally copying them, having several versions of the same person around. 

Quantum Teleportation lab in 2021 - Quantum Teleportation experiment - Scientific Cartoon

By the way: although quantum teleportation sounds like science fiction, this kind of teleportation has actually been done. A successful teleportation of the spin of a single electron has been performed over several kilometers! Sounds like good news. But consider: in a human body there are about 1028 electrons, not to speak of all the protons and neutrons. So it is still a long way to go until we don’t have to commute by car anymore…

B. Bahr, B. Lemmer, R. Piccolo, Quirky Quarks

Buy this amazing book: 

Quirky Quarks: A Cartoon Guide to the Fascinating Realm of Physics

 

Tags: Quantum Teleportation 2021, Quantum Teleportation experiment, Quantum Teleportation circuit, Quantum Teleportation humans, Quantum Teleportation applications, Quantum Teleportation speed, Quantum Teleportation superpower

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