Skip to comments.Quantum Teleportation and Computation
Posted on 12/20/2001 5:17:16 AM PST by Father Wu
Teleportation is a name given by science fiction writers to a procedure in which an object disappears in one place and reappears in another instantaneously (this is classic teleportation; some authors explore the possibility that the original object doesn't disappear, resulting in there being two sets of the same thing). A good analogy of how a teleporter works is that it works like a 3-D fax machine.
For a long time scientists thought that teleportation was impossible because it violated one of the basic laws of quantum mechanics (Quantum mechanics is a discipline that describes the structure of the atom and how the particles in and around an atom move and react with each other. It also explains how atoms absorb and give off electromagnetic energy. It explains that when an atom releases light energy it doesn't release it in a steady flow. Instead it releases it in bundles of energy called quanta.), called the Heisenburg Uncertainty Principle (I'll talk about this later), which says that you can never exactly copy something. Then, in 1998, an international group, made up of six scientists and centered at the University of Innsbruck, proved that classical teleportation was possible, but at the moment only possible for photons and electrons. We won't be able to teleport ourselves in the near future, but it is not impossible that one day we might be able to.
Werner Heisenburg was a great German physicist who is best remembered for his contributions to quantum theory. He was born on December 5, 1901 in Wuzburg, Germany. He studied under Arnold Sommerfeld and earned his doctorate in 1923. For three after this he worked with Niels Bohr in Copenhagen. During most of this time he was working on the problem of how to describe the path of an electron using a matrix, which is a set of numbers use to plot the path of something. He was awarded the Nobel Physics Prize for his work in 1932.
He discovered the Uncertainty Principle in 1927, one of his most important pieces of work. The U.P. (Uncertainty Principle), summarized, states that one cannot know the exact position of something and its velocity (all this would tell you exactly where the object would be any given time) at the same time. You can find out one or the other, but you can never know both. This rule holds true for the most accurate measurements that we can take. The principle works because with each measurement that you take you disrupt the particle's path and the path of the particle that you used to measure the object. So, you can never accurately get both the position and velocity of an object due to the disruption caused by the measurement.
Another part of the U.P. states that the more accurately an object is scanned the more it is disrupted (this relates to the first part of the theory). This eventually causes the object to become completely disrupted before the scan is complete.
This has always been a stumbling block for scientists who are trying to find a solution to teleportation, because to teleport an object you first have to completely scan the object before teleporting it; but the Innsbruck team found a way of getting around this by using another aspect of quantum theory called the Einstein-Poldosky-Rosen Effect, or entanglement. Albert Einstein, Boris Podolsky, and Nathan Rosen discussed this effect in a paper. When two particles are entangled (say a pair of photons), the share the same properties at all times. If you entangled a pair of die, the dice would always turn up on the same number, no matter how far away they were from each other. And the number would still be completely random. Einstein called entanglement a "spooky action at a distance".
For many years it was thought that entanglement had no use, other than to prove the quantum theory, because quantum mechanics was the only field that could explain the bizarre behavior.
The Innsbruck team used the EPR Effect to bypass HUP by entangling the object to be teleported. That way all the unscanned information in the object would be passed to the teleported object through EPR.
The form of quantum teleportation that the scientists at Innsbruck came up with works like this. Alice wants to teleport an object A to her friend Bob. To do this she firsts entangles objects B and C. The n she sends object C to Bob. Once she knows Bob has object C she scans objects A and B together. This disrupts both of them and causes B's state to become equal to A's state (this part is difficult to comprehend). Now since A=B and B=C, A=C. Once this is done the scanned information is sent to Bob by conventional means (radio, ex.) and Bob processes object A, formerly object C, accordingly. In the scanning process the original object A is destroyed, ending in only one copy of object A, a classical teleportation.
This differs from a classical fax in that the original copy is destroyed in the process. Another major difference between the two is that teleportation takes three objects instead of just two.
The first action in the teleportation experiments done by the Innsbruck group is to create two entangled particles. This is done by sending a pulse of ultraviolet light through a type of crystal called a calcite crystal. This type of crystal is called a "non-linear crystal", probably because it splits photons (I wasn't able to find the definition). Inside of the crystal the UV photon is split into two photons whose polarization is entangled (polarization is the electrical charge of the photon. The polarization constantly changes). These first two photons are photons (objects) B and C. After the photons exit the crystal the UV pulse is reflected back through the crystal, while B and C are reflected to different stations. Photon C goes on to the receiving station where the teleported object will end up. Photon B is directed to the sending station. The pair of entangled photons are detected and the experiment starts. When the UV pulse is reflected back through the crystal photon A is created. A is sent to the sending station where a Bell-State measurement is performed on it and on photon B at the same time. A Bell-State measurement is the type of measurement the changes the state of C into the state of A. During the measurement A is scanned and the information is sent to the receiving station. There is a 25% chance that photon C will turn out exactly like A. So if the polarization is determined to be not the same polarization as A was it is sent through a crystal that will rotate its polarization until it matches A's (A's polarization could have been up, down, right, or left). The process has not been perfected yet and has a success rate of 75%.
The future of quantum computing is a promising one. Unfortunately, we won't be able to teleport humans in the foreseeable future. This is for a variety of reasons, all of them engineering. One of the problems is that the object to be teleported has to be completely isolated. That would be hard to do with a living organism. Another problem would be entangling the objects, although it could be done with large objects. Entanglement has already been demonstrated with Buckyballs, molecules made up of 60 atoms of carbon.
One of the most promising aspects of quantum teleportation would be in the field of quantum computing. Quantum computing is an experimental field of computing that uses atoms and molecules as bits. It is ultra-fast, about 1x10^9 times faster than today's super computers (the most powerful computer in the world could download the entire Internet in 2 seconds). This means that it would take a quantum computer 1 year for something that would take a conventional computer 1,000,000,000 years. Quantum computers have another advantage over conventional machines. Conventional computers will eventually hit physical limits or the facilities used to manufacture them will become too expensive to build.
Nobody thought much about the theory of quantum computing until 1994. A scientist named Peter Shor at AT&T discovered that how you could factor the prime factors of a number using a quantum computer much faster than with a conventional computer. The discovery fascinated scientists and horrified the security industry. It started off a wave of research in the field.
The great speed of quantum computers comes from the way they use atoms for qubits, or quantum bits. Unlike conventional computers a single qubit can represent more than one conventional bit. This is called superposition, or one thing representing more objects or ideas than just it. Qubits can do this because the atom or molecule that it is made up of can be made up of usually have more than one characteristic (ex. Electrical charge, spin axis, etc.) that fluctuate. Scientists control and measure the effects of these characteristics. They then are able to transform them into an extremely powerful computer.
In 1996 Neil Gershenfeld set out to build a quantum computer with a group at the University of California. Their first problem was to find a material that could be completely isolated and could have information entered, calculated, and measured with out decoherence occurring (decoherence occurs when an object or substance that is totally isolated interacts with outside forces or objects. This would cause calculation to become impossible in a quantum computer. It's like you were reading a book and then somebody started changing the script, ripping out some pages, added in new ones, and scribbled over other pages). The group then realized that liquids would be perfect, instead of isolating a single atom or molecule (this is for a very low powered quantum computer). Since all the molecules or atoms in the liquid would be the exact same, it wouldn't matter if the molecules interacted during the computations.
An atom's nucleus is constantly spinning like a gyroscope. The direction of the spin of the nucleus of an atom depends on the outside magnetic forces that are influencing it (like a magnet). The spin can either be parallel with the magnetic field (this would be like a gyroscope spinning on top of your finger, right side up) or anti-parallel (this is like a gyroscope spinning on your finger upside down). Now, when you apply an outside magnetic field, the spin axis of the nucleus will spin (like a gyroscope starting to wobble on your finger). If you turn a magnetic field on and off very fast it will cause the spin axis to completely rotate (you could rotate the spin axis 90 degrees or 180 degrees; it just depends on how long and how fast you turn the magnet off and on). Then, when you turn the magnet off the spins go out of alignment, until the magnet is turned on again. When the spins go out of alignment the atoms lose energy, which they emit in the form of radio waves. So if you rotated a spin 90 degrees it would give off a different amount of energy than if it had been rotated 180 degrees. The radio signals are picked up and translated by the same device that sent out the magnetic field. This process of manipulating and reading the energy emitted from the atoms is called NMR or Nuclear Magnetic Resonance. It works exactly like a MRI does. Different frequencies of NMR affect atoms of different elements in different ways. Like a hydrogen atom might remain the same while a carbon atom is rotated.
In QC (quantum computing) the spin of an atom (parallel, 90 degrees, anti-parallel, and anti-parallel 90 degrees) stands for a qubit. Parallel equals 0,0, ninety degrees equals 0,1, anti-parallel equals 1,1, and anti-parallel 90 degrees equals 1,0. Scientists measure the energy levels emitted by the atoms and are able to tell what qubit an atom represents.
Another thing the spins of an atom are affected by is the spin of its neighboring atom. In molecules atoms of different atoms are often side by side. In the molecule of chlorophyll (CHCl3) the spin of the carbon atom is dictated by the spin of the hydrogen atom next to it. This could have been a liability to deal with while designing a qc (quantum computer) but instead it forms the basic unit of computing, called the logic gate. In a computer a logic gate data is processed. Microchips are made up of logic gates. The interactions of the carbon and hydrogen atom forms a type of logic gate, the exclusive-OR logic gate. This is sometimes called the controlled-NOT gate. A NOT logic gate is the simplest type of logic gate. All it does is inverts the input. On a controlled-NOT gate the output depends on the state of the inverter (the output will be different depending on the spin of the hydrogen atom). Once the spin of the carbon atom has been inverted it sends out a radio signal which the operator of translates into the output.
Using an array of these devices that are all coordinated together it would be possible to create a super supercomputer, billion times faster than today's super computers.
Quantum teleportation might eventually be used for transferring information between logic gates. It will be a while before we will be able to build a quantum computer that is fast enough to compete with today's fastest computers, but it will definitely be worth the wait. One huge advantage to qc is that they are much easier and cheaper to manufacture than conventional computers.
Not to esoteric for this crowd, but to esoteric for this early in the morning
Not really. I don't claim to fully understand all of the concepts discussed, but I minored in Physics in school so I have a general idea of what he is talking about. Before you get your feathers ruffled my comment wasn't a slight against anyone here. Most people are here to discuss politics. Few will take the time to read something that doesn't interest them.
Can we be sure of that?
If you knew the answer, you wouldn't know where the light bulb is.<--Highlight text to see answer.
One fun part of the high factoring ability that a QC would have is the fact that it could work for high-level encryption, as well as against it (which the article seems to suggest). It would be fun to get ahold of one of these QC's if they ever become reality (in a mass production sense) and work out some new, really snazzy encryption algorithm on one, or at least try to implement a hard core version of an existing one on it.
Thanks for the fun read for the day!
OK so I was wrong. Pardon my mistake. Please.