Quantum Teleportation: the ultimate form of transport. Disappear in one place and simultaneously appear in another place, no delays, no popping ears, no in-flight entertainment required. Is this actually possible? Will long distance never be an issue again? We have already touched on the crazy implications of quantum theory in our article “What is a Quantum Computer”, but here we look at crazier implications still. How does quantum teleportation work? And could we be beaming ourselves around the world soon?
Has quantum teleportation actually been done yet?
In 1993 six scientists from all over the world, including IBM Fellow Charles H. Bennett, confirmed the intuitions of the majority of science fiction writers by showing that perfect teleportation is indeed possible in principle, but only if the original is destroyed. Since then, quantum teleportation was first realized with single photons and later demonstrated with various material systems such as atoms, ions, electrons and superconducting circuits.
The record distance for quantum teleportation is 143 km (89 mi), set in open air experiments done between two of the Canary Islands.
Cool concept, but how does it work?
To understand this, we must first understand quantum mechanics which is the science that allowed Bennett and his team to teleport those particles. Let’s take this slowly.
According to modern Quantum Mechanics (the so-called Copenhagen Interpretation), the state of any particle is fully described by the particle’s wavefunction. This wavefunction contains all the available information about the particle and its characteristics. However, it does not tell you these things with absolute certainty, like in our classical everyday world. Instead, it tells you the probability of finding the particle in a particular place.
An important feature of this wavefunction is its collapse when it is measured. Before you perform a measurement on the wave function, you only know the probability of finding the particle at each point. But when you measure, you actually find the particle – so at that moment you know its location with certainty – and its wavefunction has collapsed to this state of certainty.
Linking two particles through their entanglement
The basics of teleportation is the phenomenon of quantum entanglement. If two particles are entangled, this means that their combined wavefunction cannot be separated into two independent wavefunctions. The behaviour of the particles is completely linked together, and they can no longer be thought of as independent particles. These particles have hooked up in a rather unhealthy way where they don’t do any of their own stuff.
Consider just two particles, one held by Alice and one by Bob…
Initially Alice and Bob meet up and entangle their particles (no, not in that way…), with a specific overall entangled wavefunction called a Bell state.
After they’ve done this they can move far apart. Entangled particles don’t care about distance or time (how romantic) and unless they interact with an environment, the particle pair will remain entangled in the same way for better, for worse, through sickness, through health etc etc.
This entanglement is a special connection that allows a certain kind of “quantum channel” between two different locations to be established first, before the teleportation can occur.
Ok… I think I get it. So how does this become useful for teleportation?
Now we can begin to see the relevance of all this theory to teleportation. Bob owns some unknown state which he wants to give to Alice. Let’s call this state banana, just for fun. So Bob wants to give Alice his banana. This is the state describing the object that will be teleported from Bob to Alice. After all, any object can be described by a wavefunction.
Alice is there holding on to her own particle until Bob measures his part of the entangled particle pair. Since the particle pair is intrinsically linked by their special entanglement wavefunction, this measurement will also affect Alice’s particle – even though she didn’t do anything.
In fact, it will change the state of her particle into an encrypted version of the state banana. Importantly, this change is instantaneous, which is what leads many people to believe that teleportation is faster than the speed of light.
We must now be careful: Alice is now in possession of the encrypted state banana, but she first needs to decrypt it. She cannot access the information in banana without decryption.
The correct ‘decryption key’ is indicated by the outcome of Bob’s earlier measurement on his particle. Remember, in order to send the information banana to Alice in the first place, Bob had to make a measurement with an uncertain outcome. Now that he has made the measurement, he can send the outcome to Alice via classical communication – for example, a Morse signal or bizarrely, even good old WhatsApp or Facebook message.
Of course these channels of classical communication are never faster than the speed of light. So the information itself is not transmitted faster than the speed of light.
Does this mean that people who believe it will allow FTL (faster than light travel) are wrong?
Well simply put: yes, they’re wrong. The decryption ultimately depends on classical communication, which can proceed no faster than the speed of light. However, FTL travel is being researched in other forms such as through a warp-drive – read more about the Star Wars-inspired technology here.
How do Alice, Bob and their banana translate into teleportation?
Once Alice has decrypted her state, she is now in possession of the state banana – we have teleported it from Bob to Alice.
Imagine we have a teleporter that can transport large wavefunctions like that of a banana. Alice just has a little pile of Carbon, Hydrogen, Oxygen atoms and whatever else is in that banana. Bob on the other hand, has the banana. He adds his banana to his part of the entangled particle pair and then performs the measurement.
Now Alice’s pile of atoms magically transforms into some cryptical quasi-banana. Bob whips out his iPhone and texts Alice the required decryption key. Alice makes the decryption and proudly finds herself one banana better off.
So can we transport humans like in Star Trek ?
Definitely not for humans, so far only single atoms, electrons, ions, photons and superconducting circuits can be teleported. Also, if you did get into a quantum teleporter – remember, theoretically, it would not literally bring you from one place to the other, but rather you would be split up into your constituent atoms at Bob’s place, then relied on him to text Alice the right information, trusted Alice to actually decrypt the quasi-you and then you’d find yourself recreated from atoms that have never before been a part of your body.
Although your consciousness, memory and everything else would be perfectly recreated, it’s still putting a lot of trust in Bob and his texting skills – fingers crossed he’s not been out on a big night and is starting to send incoherent drivel (we’ve all been there).
That was complicated, how can I sum it up briefly?
- Quantum teleportation is a process by which quantum information about the exact state of an atom or photon can be transported to a different location.
- This is done with the help of both classical communication and quantum entanglement
- There must be previously shared quantum entanglement between the sending and receiving location.
The most tricky part is that the original object is destroyed in the process. So perhaps for the time being, the fundamental idea of quantum teleportation isn’t that appealing.
Despite this, it’s seriously cool to know that scientists worldwide are looking into making teleportation of some kind a reality and hey, even if it’s not faster than light, it’s probably still a damn sight quicker than anything else we’ve got.
Great, got it! But how does it affect me?
So nobody has actually managed to send a physical object yet, and teleporting humans is a long distant dream, why should we care about quantum teleportation at all?
Even though no matter was teleported, this type of quantum teleportation could help to greatly improve the security and strength of Internet connections.
What does all this quantum teleportation malarkey mean for life as we know it?
Devices called quantum repeaters will replace repeaters we currently use in our networks (using quantum information instead), which receive a signal and then retransmit it at a higher level, to give our information the legs travel around the whole world. Quantum repeaters could potentially extend their reach far enough to build an entire “Quantum Internet”, which would be faster, more efficient, and more secure than the networks we rely on today.
Oh, the troubles we go through to stream Game of Thrones faster on our laptops…
While we’re still a long way off building a whole network made up of entangled light particles, it’s a pretty great demonstration of the potential.
For the Quantum Physicist: Decryption, the last step of the teleportation process, is in fact a unitary transformation on the quantum state. The decryption key is then just the inverse transformation matrix that will recreate the state.
This article was mainly based on the Quantum Information lecture course at the Cavendish Laboratory, University of Cambridge. Further resources include:
- C.H. Bennett, G. Brassard, C. Crepeau, R. Jozsa, A. Peres, and W. Wootters, “Teleporting an Unknown Quantum State via Dual Classical and EPR Channels”, Phys. Rev. Lett. vol. 70, pp 1895-1899, 1993. (The original 6-author research article.)
- Tony Sudbury, “Instant Teleportation”, Nature 362, 586-587, 1993. (A semipopular account).
- Ivars Peterson, Science News, April 10, 1993, p. 229. (Another semipopular account.)
- Q&A on Teleportation by BBC News Online science editor David Whitehouse
- Ma, X. S.; Herbst, T.; Scheidl, T.; Wang, D.; Kropatschek, S.; Naylor, W.; Wittmann, B.; Mech, A.; et al. (2012). “Quantum teleportation over 143 kilometres using active feed-forward”. Nature489 (7415): 269–273. Bibcode:2012Natur.489..269M.