Teleportation

 Captain Kirk of the starship Enterprise is standing on the surface of a planet that is crumbling beneath his very feet. He crash landed in his shuttle, he's bleeding profusely, and the shuttle is nothing more than a useless pile of mangled circuits and duranium alloy. He needs to get off the planet immediately. Fortunately he still has his communicator-- so he calls the Enterprise and asks to be beamed directly to Sick Bay.

No problem-- even though there's a lot of electrostatic interference from the planet's volcanic disturbances, the Heisenberg Compensator of the Transporter will be able to decompose his body into its fundamental atomic components, translate them into a digital matter wave, and transfer the signal back to the Enterprise where the Transporter will then convert the matter wave back to its constituent atoms, thereby restoring him to the full stature of his vigorous physical self.

A great many Science Fiction stories make use of the concept of teleportation. All of them share the following key properties with the Star Trek Transporter:

  • Immediacy. It only takes a few seconds to transport an object from one point to another. The process of translating the object into a "matter stream" or "energy beam" or "digital signal" or whatever takes very little time.

  • Completeness. The entire object is transported, not just some subset of the object.

  • Preservation of state. The state of the object is not changed. For a living person, the person is expected to remain alive after transport. Even the person's thoughts and memories are preserved.

  • Repeatability. The transporter can be used over and over again with nothing more than occasional routine maintenance.

  • At most one transporter platform is required. Captain Kirk is standing on crumbling rock, not on a Federation-supplied transporter platform. Fortunately the Enterprise Transporter system is able to beam up someone without requiring the person to stand on any sort of platform with a built-in digital converter or Heisenberg Compensator or other piece of engineering wizardry.

In fact teleportation has been achieved-- for a photon. But sending a single photon from one place to another is not what our desperate Captain needs when he's standing on the surface of a planet that is about to explode.

Quantum teleportation is based on the Einstein-Podalsky-Rosen paradox. Einstein and his associates wanted to demonstrate that the principles of quantum mechanics are absurd. They devised a 'thought' experiment based on a quantum principle now known as entanglement. According to the rules of quantum mechanics, two particles can become entangled in such a way that when the state of one particle changes, the state of the other particle also changes-- regardless of how far apart the two particles are. This, Einstein believed, was an obvious impossibility-- he called it "spooky action at a distance"-- which clearly meant that quantum mechanics was fatally flawed.

In the mid-1980s experiments were conducted that actually demonstrated the reality of quantum entanglement. In principle one person-- Alice-- can send a particle to her partner-- Bob-- across any distance instantaneously by employing quantum entanglement. Here's how it would be done. Alice and Bob each have a box that contains one of a pair of entangled particles. Bob departs for the far reaches of the galaxy. Neither Alice nor Bob may observe the particle in the box until it is time to use it. Then when Alice wants to send a particle to Bob, she causes the particle to interact with the entangled particle in her box. At that moment the particle in Bob's box will change state exactly as did Alice's particle. Note that this change would appear to have happened instantly, even across distances far larger than light could have traveled in that time.

To retrieve the particle that Alice sent, Bob need only extract from the particle in his box the state of the particle before its interaction with the new particle. What is left is the particle that Alice sent.

The catch is that Bob must be told when to open the box and extract the information about the teleported particle. The message that Alice must send to Bob to tell him that it is time to observe his particle can only be sent via an electromagnetic signal, meaning that the message can only be sent at the speed of light (or less.)

Alice and Bob could agree that the particle will be sent at a specific time. In this scenario, Alice wouldn't have to send a message to Bob telling him that the time has come to observe his particle. He would simply observe it at the scheduled time without having to wait for a message to arrive at light speed.

From the point of view of the science fiction this is not a satisfactory way to teleport objects from one side of the galaxy to another. There's no evidence that this principle could ever be extended to the teleportation of an entire person.

So let's imagine that we were to construct a system that would be capable-- in principle-- of transmitting and object from one point to another via quantum teleportation. What would be required?

We would need two systems of entangled particles. Alice and Bob would each need a box filled with entangled particles. How many particles would they need? They would need enough entangled particles to ensure that the state of every particle in the object to be transmitted can be captured. These particles would have to be held in a state in which they are not allowed to change state until such time as we want to capture the state of the object to be sent. How would this be done? The particles can't be stored in a common chamber, since they would then be able to interact with each other-- thereby changing state. They couldn't be held in a magnetic bottle, since interaction with the magnetic field would result in a change of state. Somehow each of the entangled particles would have to be stored in a separate chamber that wouldn't have any possibility of interacting with any other particle until we are ready to transmit.

Supposing that this problem can be solved, we would need to have a way for the source object to interact with Alice's entangled particles instantly. Thinking about just one particle in the source object, it would need to interact with just one of Alice's entangled particles and, in the process, the source particle would have to be destroyed. Why destroyed? Because it is only by destroying the particle that Alice is able to capture its entire state. Note that the destruction of the source particle cannot result in the escape of any other particles. When an electron and a positron collide they completely annihilate each other, resulting in the emission of a gamma ray. If Alice's entangled particles were to allow for the collision of an electron in the source object with an entangled positron, Bob would need to be able to completely recover the gamma ray that would result on his end. He would also have to know which entangled positron was the source particle that produced the gamma ray so that he could fully recover the state of the electron that collided with it.

In principle such a system would be capable of transmitting the state of a single particle from one location to another. To satisfy the completeness requirement we would need to ensure that we can accomplish the same thing for EVERY particle in the source object. Alice would need to have a way to ensure that every particle in the source object is able to interact with a single one of her entangled particles. This would result in the destruction of the source object. Bob would then need to capture every resulting particle at his end and would need to subtract out the known state of each entangled particle from each resultant particle to obtain the particles that were sent. This would result in the destruction of Bob's system of entangled particles, meaning that the transporter could only be used once.

Satisfaction of the immediacy requirement would mean that Alice would need to be able to ensure that each particle of the source object could interact with a specific particle in her repository of entangled particles within a short time-- just a few seconds, at most. And for his part, Bob would need to ensure that he is able to subtract out the states of his entangled particles to recover the particles transmitted within just a few seconds-- at most.

None of this seems at all feasible with any known technology. Even if it were in principle possible to solve the many engineering challenges we have discussed, the resulting system wouldn't be satisfactory to Science Fiction in that it wouldn't be repeatable, and it would require a transporter system at each end of the transmission. Bob simply wouldn't be able to send an entangled message to himself; he would need Alice at the source end with her reserve of entangled particles and her mechanism for forcing each particle in the source object to interact with just one of her entangled particles.

So all in all there seems to be little likelihood that the injured Captain Kirk will be able to beam off the planet. The idea of a transporter beam certainly solves a lot of problems for a narrative story, but it has little chance of ever becoming a practical reality.

 

Copyright (c) 2012 by David S. Moore.  All rights reserved.