Very few scientific concepts enjoy the popularity of teleportation: the idea is equally awe-inspiring among scientists and laymen. To the most inspired, so to speak, what is fascinating is not how it’s accomplished as much as the possibility of “leaving” one space and “arriving” at another, but traversing the interim distance instantaneously. The implications of such travel are significant even at first sight. For example, imagine being able to teleport an object from earth into interstellar space, hundreds of thousands of kilometers away, without having to bother with rockets that might take years to span the same distance.The sole field of physics seemingly capable of tackling the problems associated with such an esoteric and fragile system—quantum mechanics—still has leaps and bounds to go, however, before it can realize the teleportation of objects. For starters, it hasn’t figured out what really happens during the teleportation of a few photons even though it has accomplished just that with an 80-per-cent accuracy over a distance of 97 km.
In a paper submitted to arXiv on May 9, 2012, Jian-Wei Pan, et al, demonstrate how they used an exotic phenomenon called quantum entanglement to achieve teleportation across Qinghai Lake in western China. Using an ultraviolet laser aimed at a barium crystal, Pan’s team generated pairs of quantum-entangled photons. Then, each photon of a pair was transmitted using a telescope to two parties on either sides of the lake, A and B.
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The experimental setup[/caption]Making a measurement of the photons yields a good description of the “state” the photons are collectively in. Therefore, A’s and B’s goals are to see if a third party interacting with these photons ends up in a state similar to the control group even when separated by 97 km of free-space. Here, the state of the system refers to the values of a few fixed variables: if the variables hold a particular set of values, then the system is said to be in a particular state (states are usually independent of extrinsic properties such as mass, etc).
To measure this change, the researchers at Site A let photons generated at locally to interact with the incoming modified photons. Simply put, the foreigners would leave an imprint on the locals upon interaction. This imprinted state is then measured and compared with the state of the photons at Site B. In Pan’s experiment, the between the two agreement was 8 on 10.
The aspect that makes such an outcome wonderful is that the particles didn’t have to end up with the same state. Further, 80 per cent is a value large enough to rule out any coincidence (but small enough to rule out a complete success). If this long-distance communication between nanoscopic particles is further investigated, it becomes evident that their pre-travel entanglement provided for a form of durability and predictability of state that let the particles behave similarly in two very different measurement experiments. At the same time, it is the nature of this entanglement that baffles most scientists.The formal definition of entanglement is very fundamental in the sense that quantum mechanics deals with it in terms of probabilities. When two groups of photons are said to be quantum-entangled, it means that the states that the groups are in are related to each other by means of a variable. If the variable changes, then the properties of the photons change, too. However, the groups’ relationship with each other does not, like siblings who remain siblings despite how old they get or when they each die.
The existence of this variable is not as much disputed as it is hoped into existence (Little wonder then that it’s handled as a product of probabilities?). Because it remains outside the realm of human control, experiments with teleportation tend to leave the hidden variable alone and instead focus on how much the measurement sites can be separated by, how efficiently large molecules can be entangled, etc., i.e., testing the limits of its practicability.
In order to do so, the photons are subjected to a simplified treatment—one conceptualised so as to make the fewest assumptions as well as not introduce new sources of error. Instead of groups of photons, two are addressed, and each is “allowed” to exist in one of two states. Schrodinger’s cat takes off here and asserts that the particles may exist in this state, that state, or the counter-intuitive superposition of both, and that revelation can come only with observation.Let’s say the two particles are ‘a’ and ‘b’, the states ‘0’ and ‘1’. The four possible combinations of states, then, are:
{0, 0}
{0, 1}
{1, 0}
{1, 1}
Entanglement is said to have occurred when b is in a particular state when a is in a particular state. That is, if b is 1 every time a is 0, then a and b are entangled. Since this property is commutative, a will be 0 every time b is 1. Further, the change occurs instantaneously irrespective of the distance between the two particles (giving the impression that they're "communicating" at a speed faster than light's). The presence of such an order coupled with the four possible outcomes makes each outcome a particular state of the system, called a Bell state.
To find out what the Bell state is, a Bell measurement is made. Because of Heisenberg’s uncertainty principle, however, the act of making the measurement changes the state of the system. Even so, this alteration doesn’t matter as long as the pre-measurement state is discovered. In Pan’s experiment, with six initial states, the Bell measurement was made using the imprinting mechanism to reveal that the entangled photons had interacted with other particles to yield a final state that resembled the initial.
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xkcd #824[/caption]Earlier, another experiment had been conducted that demonstrated the teleportation of quantum information across 16 km. The principal shortcoming of that experiment was that the photons to be teleported—the “locals” —had been specially generated within the lab under careful conditions. Practically, this is a highly ideal condition that can seldom be met: if this blogger is to be teleported, he cannot be carefully “prepared” in a lab. Pan and his colleagues eliminated this necessity by generating local photons with random quantum states.
At the same time, they have failed to discount a possible source of error: the entangled photons and the to-be-teleported photons were generated by the same source. Even though this limitation doesn’t interfere significantly, it is a limitation nonetheless. (What is the confidence with which it may be asserted that the “local” photons are created in a mixed state? If it’s being assumed that this is not a source of error, what were the considerations made? Et cetera.)
I must concede that looking behind the teleportation curtain kills a lot of the fantasy. Even if entanglement continues to elude understanding, simplifying something so enigmatic to probabilistic proportions and then to linear algebra can be a bit of a buzzkill—disregarding that that is the purpose of scientific endeavour, of course. With their paper, Jian-Wei Pan and his team currently sit pretty at the forefront of quantum mechanical teleportation. Even if we still have a long way go, the knowledge of Pan’s experiment gives us the best shot at ultimately achieving the teleportation of complex objects.
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