Thursday, 13 July 2017

Earth-to-space quantum teleportation: not yet?

From The Guardian, July 12

The headlines the past few days have been full of stories about an experiment performed by a Chinese group of researchers.  You can read the report of the experiment here, if you like:

The claims about ground-to-space quantum teleportation are, I'm sorry to say, misleading.  and not merely for the usual reason, that what's called "quantum teleportation" is nothing at all like a Star Trek transporter.  In this case, it's because what was done isn't even what's usually called quantum teleportation between the earth lab and the satellite.

The quantum teleportation protocol (see any good book on quantum information theory, or the original paper), involves preparing a pair of particles, say, photons, in a maximally entangled state.  One of the pair is given to Alice, the sender, and the other, to Bob, who is some distance away.   The cool thing is that, once they share this entangled pair, they can use it to prepare Bob's photon in some desired state (which might even be unknown to both Alice and Bob), with less classical communication than would otherwise be required.

Here's how it works.  Alice is given a photon in the desired state.  Call this photon #1, and call the pair shared by Alice and Bob, photons #2 and #3.  Alice performs what is known as a Bell-State Measurement (BSM) on photons 1 and 2.  This is an operation that leaves photons 1 and 2 in one of four maximally entangled states, and projects Bob's photon, #3, into a state that, depending on the outcome of the BSM, is either the desired state or related to it in a simple way.  Alice then communicates the result of the BSM to Bob (this involves distinguishing between four possibilities, and so requires only two classical bits), who then knows what he has to do to put his photon into the desired state.  The possible choices for the desired state are endless.  This means that communicating what the desired state is to Bob, via classical communication, would require an unlimited amount of classical communication.  The shared entanglement drastically reduces the amount of classical communication desired.

The key to this, and what makes it hard to do, is that Alice and Bob must first share entangled particles, and it's not easy to maintain entanglement over a distance.

But that's something that the Ren et al. teleportation experiment did not do.  They produced the entangled pair 2,3 in the earth-based lab (Ngiri), perform the Bell-State Measurement, which disentangles 2 from 3,  on the ground, and then send photon 3 to the satellite (Micius).  In the report, they write,

In the current work, the entangled photon source and the BSM are performed at the same location on the ground. A next step toward real network connections is to realize long-distance entanglement distribution prior to the BSM.
No long-distance entanglement, no long-distance teleportation.

In a separate experiment, reported in a paper uploaded one day after the first, the group exhibited long-distance entanglement distribution.

 But, if I'm reading these papers correctly, they haven't yet combined the two.  So, they're close to earth-to-satellite quantum teleportation. But they're not there yet.

No comments:

Post a Comment