Although Captain Kirk of the U.S.S. Enterprise never actually spoke the phrase, “Beam me up, Scotty”, it is known by many as a hallmark of the Star Trek series. In the show Scotty wielded the power of the transporter, a fictional machine capable of instantly moving Captain Kirk and his crew from the Enterprise onto alien planets, ships or what have you. It was Werner Heisenberg who single handedly dashed the hopes of many science fiction buffs imagining a future of teleporation with his famous uncertainty principle (Zeilinger, 2010). The Heisenberg uncertainty principle states that one cannot measure with accuracy all the aspects of a system (Zeilinger, 2010). That is, if one were to determine the position of an electron one would need to shine light on it. According to the principle of the photoelectric effect, the electron would gain kinetic energy from the light, consequently changing its momentum and raising the uncertainty of its speed (Zeilinger, 2010). Heisenberg’s uncertainty principle is applicable to all aspects of reality, but the magnitude of uncertainty for macroscopic objects is inconsequential enough to be ignored in day-to-day life. What this means in the case of teleportation, however, is that all information about a system, such as the state of every atom in a human being, can never be measured, and thus never be transmitted in entirety (Zeilinger, 2010).
There remains a loophole, however. Making the assumption that measurement is necessary immediately restricts teleportation. If the state of a system were to remain unmeasured and intact, then theoretically all that is needed to achieve teleportation is an information channel. This information channel is provided through what Einstein called a “spooky action at a distance”, or what Schrödinger more eloquently termed entanglement (Zeilinger, 2010). Quantum entanglement is an intimate connection between two systems, a connection much stronger than any physical link. The systems are connected in such a way that if the state of one is measured, the other instantly assumes the same state [note a] (Zeilinger, 2010). The interesting part, and the part Einstein did not like in the least, is that these two entangled systems, for example particles, can be any distance apart, and still once one is measured, the other instantly inherits the exact same properties (Zeilinger, 2010). This statement implies that the communication of particle A’s state to particle B can potentially occur faster that the speed of light, a phenomenon known to be impossible. What exactly is going on at the quantum level is still under hot debate, but regardless of the cause entanglement has many interesting applications, including teleportation.
The quantum teleportation schematic is fairly straightforward. Using the familiar names of Alice and Bob from information science, the scheme works as follows. Alice and Bob are some distance apart connected by a classical information channel (i.e. a telephone line), as well as a quantum channel, provided by two particles A and B pairwise entangled with each other – that is they will turn out to be identical when measured (Zeilinger, 2010). Alice then takes a particle X that she wants to teleport to Bob and performs a Bell-state measurement on particles X and A. A Bell-state measurement entangles particles X and A, while removing any unique properties X may have had (Zeilinger, 2010). With a bit of luck, X and A will be pairwise entangled [note b], meaning that the state of X equals A, and since A and B are entangled the original state of X is transferred to B and teleportation is achieved (see Figure 1). This phenomenon has many useful applications in computing and data transfer.
Figure 1: A quantum teleportation setup. The EPR-source [note c] produces a pair of entangled particles 2 and 3, in this case photons, to act as the quantum channel. Alice then performs a Bell-state measurement (BSM) on photon 1 which she wishes to teleport and photon 2. Instantly, Bob receives photon 3 in the exact state photon 1 was originally in (Bouwmeester et al., 1997).
Unfortunately, teleportation of the kind seen on Star Trek still remains impossible (even despite the producer’s attempt at a workaround in the form of the Heisenberg compensator). Quantum entanglement, however, opens a whole realm of possibilities waiting to be explored that may lead to far more exciting things than teleportation.
[note a] Please note that this relationship describes only one type of entanglement. It is possible for two systems to be entangled in such a way that measurement of one will result in the other assuming a state orthogonal to the first, as well as other possibilities (Zeilinger, 2010).
[note b] In the event the entanglement between X and A is not pairwise, then Alice can communicate to Bob through the classical channel the outcome of her Bell-state measurement, and Bob can adjust his particle accordingly (Zeilinger, 2010).
[note c] An Einstein-Podolsky-Rosen (EPR) source is capable of emitting two entangled photons. The source is named such for its creators Albert Einstein, Boris Podolsky and Nathan Rosen, three physicists responsible for writing the famous ‘EPR paper’ entitled “Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?”
References:
Bouwmeester, D., Pan, J.W., Eibl, M., Weinfurter, H., Zeilinger, A., 1997. Experimental Quantum Teleportation. Nature, 390, 575-579.
Zeilinger, A., 2010. Dance of the Photons: From Einstein to Quantum Teleportation. Farras, Straus and Giroux: New York.