Quantum teleportation has made a leap up in complexity. Physicists have teleported more information at once than has ever been possible before, paving the way for a global quantum internet that would be extraordinarily secure from hacking.
This is not teleportation as you might imagine it from science fiction. Rather than transporting matter through space, it involves moving information about the quantum state of a particle.
So far, scientists have only been able to teleport quantum bits, or qubits, the simplest unit of quantum information in which a particle can be in two states at once. For instance, a photon that is simultaneously vertically and horizontally polarised would be a qubit.
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Now physicists from the University of Science and Technology of China and the University of Vienna in Austria have managed to teleport a more complicated unit of quantum information called a qutrit for the first time. If a qubit is considered two-dimensional, a qutrit is three-dimensional: the photon is polarised in three perpendicular directions.
Higher dimensions
鈥淭he higher the dimensions of your quantum system, the more secure you can ensure your communication is and the more information you can encode,鈥� says Ciar谩n Lee at University College London. 鈥淏ut going from a qubit to a qutrit is especially difficult 鈥� the tricks you use for qubits have to do with a nice symmetry that qutrits don鈥檛 have.鈥�
To teleport a qubit, you begin with three particles. One is the qubit whose information you want to teleport. The other two are a pair of particles that have been entangled in such a way that making a measurement on one will affect the result of a measurement made on the other.
Now imagine two people, who we鈥檒l call Alice and Bob. Alice has the qubit and one of the pair of entangled particles, and Bob has the other particle in the pair. If Alice wants to send the qubit鈥檚 information to Bob, she performs a special kind of measurement on both her particle and the qubit particle. The process of doing this measurement means that Alice鈥檚 particle is now entangled with the qubit, as well as Bob鈥檚 particle.
Because of all this entanglement, Alice鈥檚 measurement forces Bob鈥檚 particle into one of four possible states. The results of her measurement, which she can send to Bob using non-quantum methods 鈥� an email, for example 鈥� lets him determine how the state that he got is related to the original qubit. Once he knows that, he can extract the information contained in the original qubit. The information has been teleported.
Qutrits are a level up in difficulty because it鈥檚 much harder for Alice to perform her measurement and entangle her particle with the qutrit.
The researchers got around this by introducing an extra particle to the system so that she is measuring three particles instead of two. Then the results of her measurement contain more information, which she sends to Bob, allowing him to reconstruct the qutrit.
Using this method, the researchers were able to teleport qutrits with 75 per cent fidelity, meaning Bob鈥檚 final reconstructed qutrit was 75 per cent similar to Alice鈥檚 original qutrit. That may not seem particularly high, but the highest fidelity possible without using quantum entanglement is 50 per cent.
鈥�75 per cent is probably not good enough to start communicating in this way with much accuracy, but this is early days,鈥� says Lee. The researchers claim that their method could be used to teleport even larger packets of quantum information with higher fidelity.
This is important for quantum communication because we do not know how to maintain entanglement between particles that are very far away from one another. Passing information using quantum entanglement would be far more secure than the methods of encryption we have now.
Quantum teleportation of information with more than two dimensions, like qutrits, could enable that information to be passed over larger distances by secure quantum networks, Lee says. 鈥淭he ability to teleport a high-dimensional system is going to be one of the bedrocks on which a future quantum internet is built.鈥�
Physical Review Letters
