Quantum Communications: Teleporting to Satellites

One of the main reasons that quantum technologies have received so much attention recently is the promised speed-ups from quantum algorithms. However, there is one other key reason for interest in quantum technologies: quantum communication. Quantum mechanics (QM) can model groups of subatomic particles as a single object called a state vector. The axioms of QM also separate out the idea of a particle’s state from its measurement. As I discussed in a previous article this is at the heart of the weirdness in quantum mechanics and quantum computing.

In classical mechanics, if the spin of an electron is defined as +1, then when you measure its spin it will be +1. So the electron has a spin of +1, and its measurement must therefore be +1. In QM, the state vector of a particle represents all that can be known about it, and it is usually some form of probability distribution. So the particle does not have a spin value until it is measured. State and measurement are different concepts — in a sense the state is “bigger” than the measurement. In fact it can be shown that a quantum state — unlike a measurement — cannot in general be represented by a classical description (e.g. a bit string) no matter how many trillions of bits can be stored or transmitted.

These properties of QM lead to fascinating possibilities in communications. In particular — it is possible to generate a pair of particles that are represented by a single state vector. Then taking these particles and separating them by say 100 miles *without observing their states* creates a quantum link between the person with one of the particles, and the person with the other. This is the basis of the quantum communication algorithms such as teleportation and superdense coding.

Quantum teleportation is needed when we wish to perform quantum operations on the same data in different locations. It is this performing of operations on that data which is a major motivation for teleportation. If the operations performed at the quantum receiving end do not provide a quantum advantage, then there is not a clear case for the quantum teleportation in a computation system. There are 4 areas that are expected to provide a quantum computing speed-up advantage: logical equation solving, code-breaking, linear equation solving, and physics/chemistry simulation.

**Quantum Teleportation**

Quantum Teleportation enables a piece of quantum information (a “qubit” state) to be transmitted any distance provided that the sender — traditionally called Alice — and a receiver — traditionally called Bob — share an entangled pair. Vitally, Alice must also transmit a number between 0 and 3 (i.e. two classical bits) to Bob using more traditional means (e.g. fibre optics or RF). But once Bob has these bits, he can perform an instantaneous transfer from Alice of her qubit, and Alice’s qubit is destroyed at her end. Hence the name teleportation rather than copy or move. Copies cannot be done in quantum computers, by the No Cloning Theorem of quantum mechanics.

Importantly, given that in general a quantum state cannot be described by a string of classical information (bits), quantum teleportation provides a way of using classical bits and entanglement to move/transmit a quantum state precisely. The catch is, of course, that Alice must provide Bob with half of a stable entangled pair or vice versa. This is no mean physical feat, but is possible and researchers are improving their ability to do this over time. For example a photon’s polarization has been teleported 870 miles to another photon.

The fact that the quantum state itself is teleported, and not just an *observation* of the quantum state, is potentially incredibly powerful, as it allows quantum computation and communications to continue at Bob’s end.

Whilst quantum teleportation does not promise a “beam me up Scotty” effect, it has the remarkable property of transmitting that mysterious and ethereal object — quantum information- across space as far as it needs to go.