Quantum Teleportation
Should we buckle on our communicators and prepare to get sparkly?
If you’ve glanced at any of the popular science journals or the more aspirational daily rags over the last several years you may have noticed in the summer of 2017 several breathless reports stating that Chinese scientists successfully achieved quantum teleportation between a ground station and one of their satellites.
To the naïve, this inevitably conjured up visions of Star Trek. So are we about to enter a world of matter transporters and instantaneous communications over vast distances, unimpeded by the pesky limitation called lightspeed that General Relativity shows to be the universal speed limit?
Well, actually, no.
The first problem comes from the rather misleading term teleportation. For sci-fi fans this means disassembling the atoms of an object or person and automagically whizzing them across some significant distance and then re-assembling them with perfect fidelity at the destination. Originally invented as a script device because a TV show couldn’t afford to pay for the special effects necessary to show the Enterprise landing on whatever planet Captain Kirk and his merry men were visiting that week, the idea of matter transportation has had an irresistible appeal ever since.
But when scientists talk about teleportation they aren’t talking about moving atoms from Point A to Point B at all.
What they’re talking about is the inducing of a quantum change of state in two conjoined particles instantaneously regardless of the distance separating them. But how is this possible when every measurement ever made proves that nothing can travel faster than light, which moves at approximately 300,000 kilometers per second?
The answer lies in a phenomenon called quantum entanglement. When particles are said to be entangled it means they become part of a single system; so a change in one automatically causes an identical change in the other. This change is instantaneous — a fact that annoyed Einstein who derisively called it “spooky action at a distance.” Ever since quantum entanglement was demonstrated both mathematically and then in the laboratory, some people have been eager to use the phenomenon to create instantaneous methods of communication unbounded by the speed of light.
OK, so now we know that when physicists talk about teleportation, they’re using the word in a very different way than sci-fi enthusiasts. They’re talking about two things being connected across some amount of distance in much the same way as two telephones can be connected by means of a very long wire (or, in today’s world, the global telecommunications network). When Mary talks to Bob on the telephone, the telephone doesn’t disappear out of her hand and instantly reappear in Bob’s hand. Likewise teleportation in the sense used by physicists doesn’t involve something disappearing or being disassembled in one place and reappearing or being reassembled somewhere else. All that’s happening is two things that are connected across some distance can influence each other instantaneously.
Well, if we can’t have sparkly Star Trek effects, can we at least have the equivalent of subspace communications where a Captain on a starship many light-years away can boldly go and order pizza from a friendly neighborhood starbase without having to wait for several years before they ask for his credit card number?
Unfortunately the answer is also no, but the reasons are more subtle than the simple linguistic confusion we’ve just dealt with regarding teleportation.
Let’s dive into the problem in order to understand why instantaneous communication isn’t enabled by quantum entanglement even though on the surface it seems like it should be.
We begin by creating the quantum entanglement, which in the case of the Chinese experiment was done by entangling photons. A photon is a corpuscle of light, both a particle and a wave simultaneously. Photons have no mass (which is how they are able to travel at the speed of light, something matter can never do because of relativistic effects) but they do have momentum, and they propagate as conjoined electric and magnetic waves of the same length and frequency oriented at 90 degrees to each other. This orientation is why polarized lenses work: light oscillating in the same plane as the polarizing filter can pass though, while light oscillating in a contrary plane will be blocked.
The easiest way to create a pair of entangled photons is to take one photon and pass it through a special kind of prism that splits it into two identical photons each of which is half the frequency of the “parent” photon. Now whatever happens to one of the pair will instantaneously influence the other.
In the Chinese experiment they split a photon to create an entangled pair and sent one of the two photons off toward an orbiting satellite while the other remained on the ground in the lab. The scientists then entangled the lab photon with another photon and measured the quantum states (polarizations) of this new “mashup” photon combination. But the word measured is important here. There are four possible quantum states of this “mashup” photon pair: vertical-vertical, vertical-horizontal, horizontal-vertical, and horizontal-horizontal. The researchers didn’t determine what the actual state was; they simply measured whether the states were the same, or were different.
Thanks to entanglement, the photon on the satellite would have to correspond to half of whatever state the mashup pair in the lab were in. But until someone or something on the satellite was told what the ground state was (same or different), they’d be unable to determine this by measuring the entangled photon in their possession — there are simply too many variables (four in all). The best they could do would be to determine two of the four possibilities — but that wouldn’t enable information to be sent.
The essence of quantum entanglement is that knowledge of the whole system is required in order to fully describe it. It’s not enough to measure just a part of the system. So although it’s true that a pair of entangled photons probabilistically “mimic” each other even across vast distances, measuring one on its own doesn’t tell you everything about the entangled system — and measuring the other on its own likewise doesn’t tell you everything you need to know.
Let’s use an analogy: imagine Bob has two small tubs of paint. One is orange and the other is blue. Bob mixes (entangles) them to make brown. He then sends Mary a sample of brown. But she can’t deduce whether the brown was made by blue & orange or by red & green. Unless Bob sends her some information (telling her orange or blue will work equally well) she can’t reconstruct the “message” in Bob’s transmission of brown.
This isn’t an easy concept to understand. Surely the photon transmitted to the satellite has information in it! But transmission isn’t the same as information. Noise isn’t the same as signal.
It helps to think about using a walki-talki. These use radio waves to transmit signal from sender to receiver. Let’s imagine a walki-talki operating normally: the radio waves get a bit distorted as they pass through and bump into air molecules and some are absorbed along the way by trees or buildings or rock, but most make it to the receiver and the sender’s voice can be more or less understood. Now let’s imagine a lot more atmospheric interference: lightning sends out intense bursts of electromagnetic radiation across all frequencies including those being used by the walki-talki. It’s raining, so falling water molecules absorb and distort some of the transmission. The rocks and trees are more dense and further absorb some of the transmission, and radio waves bouncing between reflective obstructions causes the waveforms to distort significantly. Now the receiver can hear a lot of static so they know the sender is sending something, but all the information is lost. Not a single word can be discerned.
This is the difference between transmission and information (or signal). The satellite has received the entangled photon as a transmission but the photon is unable to convey any information. All it can say is “here I am” but whatever message it was supposed to bring with it is inaccessible — until the scientists in the lab communicate by conventional means with the satellite to give it the missing information it needs in order for any measurement of the photon’s polarization to yield meaningful information about the entangled mashup in the lab. And that information from the lab can only travel at the speed of light by conventional means because — as we’ve just seen — there’s no way to use the phenomenon of entanglement to carry that critical information.
Just as with the example of Bob sending Mary some brown, she can travel far away and look at the brown but Bob can only send her a message about blue or orange using conventional means — which is limited to the speed of light.
We’re in the classic chicken-and-egg scenario here. No matter how many entangled photons we send to the satellite, each one has the same problem: the signal can be detected, but no information can be extracted until the lab sends information by conventional means. At the speed of light, but no faster.
This is why subspace faster-than-light communication can’t be achieved by means of quantum entanglement.
And there’s another problem: textbooks and popular science articles often illustrate quantum effects as though they were deterministic — like classical mechanics. But this isn’t the real story. To see why, let’s look at the famous double-slit experiment. Here, individual particles (photons or electrons) are aimed at a detector, but between the emitter and the detector is a barrier that has two small slits. Each photon can pass through one slit or the other on its way to the detector. What happens is that the particle behaves both as a point-element that passes straight through just one of the two slits and as a wave that interferes with itself as it comes out the other side of the slit — as if it passes through both slits simultaneously.
But… if you fire a single particle, this is not what you see. The detector doesn’t detect several semi-particles across its surface in the classic interference wave pattern. It just records a single photon striking a single point.
The interference wave pattern only emerges after hundreds or thousands of particles have been fired through the slits. And the location of any one particle hitting the detector doesn’t tell us anything about where the next particle will be detected. In other words, single quantum particles don’t tell us anything at all. We need a plethora of them in order to build up the picture. Quantum mechanics is probabilistic: given a large enough number of particles we can be sure that most of them will cluster around the median value, but the wave-function (which can be thought of like a Gaussian distribution, the so-called bell curve) means that the position of any individual particle cannot be known.
So why did the Chinese bother? Simply because the one thing quantum entanglement can provide is security. If I send a message to Mary via entangled photons, and then send her some information about my side of the entangled pair by conventional means, she will then be able to measure the photons that carried my message (by using the information I send her by conventional means, which in this scenario acts as a “key” to “decrypt” the entangled photons) and see if they are the same or different from mine. If they are the same then the message wasn’t intercepted so she can be 100% sure no one has been listening in. But if her photons aren’t the same as mine then she can be sure someone intercepted the message — because their measurement will have induced unavoidable change in the photons. This is because in quantum systems any measurement irrevocably changes the thing being measured.
This is also why Mary will need to receive conventional signals from me: measurement is a one-time thing. If she measures at Time A but I haven’t yet tried to induce any change in the photons, she’ll get a result but her measurement will destroy the entanglement. So I can’t send her my message at Time B because the photons are no longer entangled. And Mary can never know when I try to send the message unless I tell her, which I can only do by classical means: sending a conventional signal at the speed of light.
This is why quantum entanglement can never be used to communicate faster than light.
What the Chinese were demonstrating wasn’t Star Trek sci-fi magic but rather showing that entanglement can, under certain very specific circumstances, be used to protect information flows. Anyone intercepting the message irrevocably changes the photons and reveals the fact that the message was intercepted — but only if they can receive the “code” about what the message state should have looked like, by conventional means.
This use of entanglement may turn out to be quite useful. We are not, however, about to step onto transporter pads; nor will we ever communicate via subspace radio. No need to take out those old Trekkie costumes from the closet and practice saying, “make it so.”
REFERENCES:
https://www.technologyreview.com/2017/07/10/150547/first-object-teleported-from-earth-to-orbit/
https://www.space.com/37506-quantum-teleportation-record-shattered.html
