Making teleportation more energy-efficient
January 20, 2013
An international team of researchers has achieved an important theoretical result by finding that quantum teleportation – the process of transporting quantum information at the speed of light, which could in theory be used to teleport macroscopic objects and, one day, even humans – can be achieved in a much more energy-efficient way than was previously thought.
How teleportation works
For the best part of the twentieth century, teleportation was dismissed as purely as a science fiction pipe dream. The problem lay in the approach: the only possible way to achieve it, scientists thought, would be to measure the position and momentum of every single atom of the object to be teleported, send it to its destination using classical (non-quantum) information, and finally rebuild it based on the set of "instructions" received. But science says that the first step – the perfect measurement of a particle – is simply impossible due to Heisenberg's uncertainty principle.
In 1993, however, researchers showed that teleportation was indeed possible in principle, as long as the original object is destroyed in the process. The mechanism circumvents Heisenberg's uncertainty principle by exploiting one of the many quirks of quantum mechanics – a phenomenon called "quantum entanglement".
Entanglement happens when a pair of particles, such as electrons and protons, are intrinsically bound together. Once entanglement is achieved, the two particles will maintain synchronization, whether they are next to each other or on opposite sides of the Universe. As long as the entangled state is maintained, if one particle changes its state, the other will instantaneously do so as well.
As you might expect, the theory is quite hard to get one's head around, but let's give it a shot.
Imagine that we have an object "A" that we want to teleport. We also have "B" and "C", which are entangled with each other, but not with A. Now let's transport object B to the sending station right next to A, and object C to the receiving station.
Back in 1993, scientists found that they could scan A and B together, extracting partial information from A. Scanning scrambles the quantum states of both A and B, and because B and C are entangled, all the remaining information from A is instantly transmitted to C. Using lasers, fiber optics or any other traditional means of communication, the sending station can then send the partial information it had gathered about A to the receiving station. Now all the information about A is at the receiving station, and object C can be reassembled as a perfect copy of the original. Object A is destroyed in the process – hence we have teleportation, and not replication.
One of the prerequisites for teleportation is that B and C must first have interacted closely to create an entangled state, and then must be able to be transported to their final destinations. This means that we can teleport objects to places we've been before but not, say, to a galaxy or planet that we've never visited.
As already mentioned, the system works because B and C are entangled. But there's a problem: over time, as objects are teleported, the entangled state is slowly depleted. It can be renewed by having B and C interact closely again, but this means transporting manually (without teleportation) both objects to the same place, and then back again to the sending and receiving stations. The idea is that one difficult journey can allow for many quick transfers in the future.
Five years ago, physicists came up with an alternative approach to teleportation that is faster because it doesn't require the correction of C, but which is highly impractical because the entangled state is destroyed every single time that information is teleported.
In both cases, entanglement can be effectively thought of as the "fuel" that powers teleportation.
Now, a group of physicists at Cambridge, University College London and the University of Gdansk have worked out how entanglement could be "recycled" to increase the efficiency of these connections. They have developed two protocols that generalize the two known methods of quantum teleportation and provide an optimal solution in which the entangled state holds much longer for the teleportation of multiple objects, while eliminating the need for error correction.
The first of these protocols can be used to teleport quantum states sequentially, while the second makes it possible to teleport several states at the same time, which speeds up the process and is of particular interest for applications in quantum computing.
The result obtained by the researchers is purely theoretical and didn't involve any quantum information actually being teleported from one place to another. But interest in quantum teleportation is quickly surging, and labs around the world are racing to demonstrate the ability to teleport information at longer and longer distances – last year, for instance, scientists reported teleporting photons over a record 143 km (89 miles) – so it might not be long until this theoretical result is actually put into practice.
But wait – didn't we say that distance shouldn't matter at all when two particles are entangled? While it is true that two particles remain entangled regardless of their distance, for the time being, we are only able to store the entangled state for a very short period of time. This means that, in practice, scientists must create an entangled state between particles B and C and then rush them to the sending and receiving stations as quickly as possible, before the entangled state is depleted. During the transmission, photon losses and signal decoherence also increase with distance, which makes things considerably worse – although scientists are actively tackling the problem.
Beam me up, Scotty
So will the teleportation of people ever be feasible? Last November, a group of Chinese scientists have managed to achieve teleportation from one macroscopic object to another – an ensemble of 100 million rubidium atoms – with an accuracy approaching 90 percent. The human body, on the other hand, is comprised of some 1029 matter particles, all of which would have to be teleported with an extreme degree of precision.
There are other obstacles as well. As mentioned before, the object (or, in this case, person) being teleported will be destroyed at the sending station and reassembled at the receiving station. This could be painful for the traveler; however, the surviving copy is made before the original was destroyed, and so, from the point of view of our traveler – assuming that the traveler's conscience is transported with him – one could argue that no pain would ever be felt.
Moreover, a human traveler is not a static system, and so the process of scanning and reconstructing him or her must be nearly instantaneous – lest we end up with a teleported version of our telenaut that is dramatically different from the original.
One last consideration. At first, it would seem that quantum entanglement could hold the potential for travel at superluminal speeds: when two particles are entangled, no matter their distance, when we modify one particle, we also instantaneously modify the other. Unfortunately, all modern interpretations of quantum mechanics agree that this trick can't be used for faster-than-light travel.
Nobody expects to achieve human teleportation in the foreseeable future: it is an extraordinarily tough engineering problem, and even though the process wouldn't violate any fundamental law of physics, we lack the technology to achieve it – or anything even remotely close to it. In a sense, this piece of research could be seen as a small step toward human teleportation, but don't hold your breath for Star Trek-style teleporters just yet.
The study was published on the journal Physical Review Letters. An open-access version can be found here.
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