If you haven’t been paying attention, enormous progress has been made towards quantum computing and quantum teleportation in recent years. Just recently, researchers in Zurich have demonstrated teleportation inside a solid-state circuit which they estimate can transfer 10,000 quantum bits per second, which is somewhat of a land speed record. Meanwhile, researchers in Tokyo used a hybrid technique to teleport photons with a 79 to 82 percent accuracy, another major advance over previous experiments.
But don’t count on being beamed up to the Enterprise just yet. The type of teleportation we’re talking about isn’t likely to rearrange molecules into the form of a human being, rather it communicates information (such as position, momentum, spin, and polarization) about subatomic particles across time and space. So while we’re likely going to have to wait a while to have our own transporter rooms, a type of quantum teleportation may be the key to unlocking the power of quantum computers. And as we’ll see, there is an outside chance that a similar type of technology could be used to create a mind-boggling type of communication system that is usually only found in science fiction.
Let’s start by getting a handle on quantum computing. Today’s computers encode information into bits — binary digits, either “0” or “1”. These bits are usually stored on your computer’s hard disk by changing the polarity of magnetization on a tiny section of a magnetic disk, or stored in RAM or flash memory represented by two different levels of charge in a capacitor. Strings of bits can be combined to produce data that is readable by humans. For example, 01000001 represents the letter A in the extended ASCII table. Any calculations that need to be performed with the bits are done one at a time.
Quantum computers, on the other hand, use the various states of quantum particles to represent bits. For example, a photon spinning vertically could represent a 1, while a photon spinning horizontally could represent a 0. The odd thing about photons is that while they can spin vertically, horizontally, and diagonally, they can also spin in all those directions at the same time. Huh? That’s right, it’s the equivalent of traveling North, South, East, and West at the same time. How’s that possible? I don’t know, ask a photon. This is the bizarro world of quantum mechanics.
So while a quantum bit or qubit can be 0 or 1, it can also be any combination of 0 or 1. For example, a qubit could be 90 percent “0″ and 10 percent “1.” And because a photon can represent multiple states simultaneously, theoretically millions of calculations could be performed at the same time where traditional computers can only perform one calculation at a time.
Just like with a traditional computer, you need a way to “write” or encode the spin of the photon to represent the qubits. This is done by sending unpolarized photons (those spinning in all directions) through a polarization filter to achieve the desired spin then by some other techniques to read the spin later on. Not only does this allow us to construct a quantum computer, but we could also use the photons to transmit qubits to other people in a communications system.
Now you might ask, how would such a communication system differ from current technology where we send information down fiber optic cables as pulses of light or through microwaves? Doesn’t it all travel at the speed of light? Correct. For one, since we are transferring quantum bits instead of ordinary bits, we can send much more information in the same time it takes to send a single bit. There is a problem, however, it isn’t easy to transport quantum bits from one location to another without knocking them out of position. So far only very short distances have been achieved.
The key to making this work is a process called quantum teleportation. Instead of sending a quantum bit down a cable to it’s location, it can instead be “teleported” there. There is a sense in which this concept is similar to the type of teleportation we’re familiar with from Star Trek. Imagine what it would take to teleport a particle from one location to another. First, we would need to measure the particle (it’s position, momentum, spin, polarization, etc), send this information to another location where a new particle could be manipulated into the exact same state. The process of measurement would knock the original particle out of its current state meaning that the “teleportation” wouldn’t not simply clone the particle. Rather only the second particle would posses the same characteristics of the first particle. So while the first particle isn’t technically disappearing and reappearing somewhere else (what we typically think of with teleportation), the characteristics of the particle are transferred to a completely different particle.
There is a problem with teleportation as I just described it, namely you can’t simply “measure” the first particle. In physics there is something called Heisenberg’s Uncertainty Principle which states that you cannot measure all aspects of a quantum particle. For example, any attempt to measure a particle’s velocity will knock it out of its current location. You have the same problems measuring spin, etc. So while you can measure a couple attributes of a particle, you can’t get a complete measurement that would be needed to “teleport” it.
Scientists have solved this problem by using another bizarre phenomenon called quantum entanglement. Quantum entanglement occurs when two particles get “entangled” in such a way that any change to the state of one of the entangled particles causes a change to the state of the other. For example suppose a photon with a spin of zero were to decay into two separate photons. This pair must have a spin equal to the original photon — zero. So what will happen is that one photon will take on a spin of 1/2 and the other -1/2. Any change to the spin of one photon causes the spin of the other to change in the opposite direction.
Scientists have been able to use this phenomenon to aid in the measurement of a particle they are trying to teleport. Here’s how they do it: If trying to measure and “teleport” photon A, they will also make use of entangled pair B and C. Photons A and B will be measured together, then this information will be sent to the second location where this measurement along with the information from photon C (remember since B and C are entangled, the state of C provides clues to the state of B) is used to deduce the full measurement of A. Having now obtained a full measurement of A they can simply manipulate another photon into this same quantum state, effectively “teleporting” photon A.
Pretty cool huh? Could this process be used to teleport a human a la Star Trek? I really don’t know, but I imagine it would be incredibly difficult to do so. You would need to measure every molecule in a person’s body, send the information to another location, then used the measurement rearrange every single molecule. It’s hard to imagine this being possible, but hey, you never know.
This type of teleportation currently be performed in labs may, however, be the key to constructing viable quantum computers or quantum networks. From a cryptographic standpoint, quantum teleportation would provide a completely secure means of communication. Currently, when you communicate with someone else over the internet, your packets of information are sent down cables and routed to their destination. Anyone along the path can intercept your communications and view them. With quantum teleportation, since the information being transmitted from point A to point B is only a measurement, only the person in possession of the other half of the entangled pair can decipher the quantum bit. This means that eavesdroppers are completely locked out of the communication. Technically, you wouldn’t even need to encrypt your communications as the information is “teleported” from one location to another.
I should mention that the above teleportation does not allow for faster-than-light communication since the measurement is still relayed to the location using traditional communication channels. This can be is a major inconvenience especially if you start to think about interplanetary communication. It takes microwaves anywhere from four to twenty four minutes to reach Mars depending on where the planet is in relation to Earth. Any astronauts on a manned mission to Mars will not be able to communicate in real time with mission control here on Earth.
But maybe we shouldn’t resign ourselves to only light-speed communication just yet. In the bizarro world of quantum mechanics, it seems as if anything is possible. As we mentioned, with quantum entanglement any change to the state of one of the entangled pairs causes an instantaneous change to the other. The mind-blowing part of quantum entanglement is that information can be transmitted from one half of the pair to the other no matter how much distance is between the photons. This phenomenon has been confirmed in experiments. If it were possible to encode one half of the entangled pair with information, you could theoretically transmit that information instantaneously to the other half of the pair. We could have one of the entangled photons here on Earth and the other on Mars and use them to communicate in real time. How is it possible that the information regarding a change to the state of one photon could be transmitted faster than light? I have no idea. Physicists are still working this stuff out.
There are problems, however. All attempts to encode an entangled photon with information will break the entanglement. And there’s also a problem measuring the polarization of the photon on the other end. Experiments require the photon be sent through random filters and the results be communicated (through traditional means) to the sender in order to calculate the state of the photon. So it seems we’re left with the weird conclusion that while information is somehow transferred superluminally but yet we can’t make use of it for communication. Bummer.
Einstein famously thought that something had to be grossly wrong with our understanding of the phenomenon since his theory of general relativity suggests information cannot travel faster than light. He suggested entangled pairs must contain hidden information about their states which would imply there isn’t anything spooky going on with entanglement. However, experiments have proved Einstein wrong and have shown that there cannot be any hidden information. Thus the phenomenon is very real.
This is the most frustrating part, not only are we forced to accept a bizarro world, but we can’t even make use of it for something cool.
Or at least we can’t do so now. Nobody really understands quantum mechanisms on a deep level. Physicists can describe it with equations but nobody really understands it. At this point I would not be willing to place a bet that, at some point in the future when physics do have a deep(er) understanding of quantum mechanics, that they will not be able to make quantum communication work.
Still, it’s fun to imagine what you could do with such technology if/when it becomes reality. In theory, you could communicate with anyone instantly, not just out to Mars, but anywhere in the universe. You could Skype with someone in the Andromeda galaxy!
In one of my all-time favorite videos games, Mass Effect, the characters used quantum entanglement communicators for superluminal interstellar communication. Check out this video of EDI explaining quantum entanglement:
We live in exciting times indeed. I can’t help but think: given the enormous size of our universe and the vast distances we need to travel to reach new worlds, what are the probabilities that the laws of physics would “just so happen” to shake out such that space-time can bent in a way that allows for interstellar travel (check out my tongue-in-cheek post on how to build a warp drive) and instantaneous communication anywhere in the universe? It’s almost as if someone up there intended for us to venture out into the final frontier.