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Quantum Teleportation By Kenneth Chang ABCNEWS.com
Its not Star Trek. No Beam me up, Scotty, no shimmering sparkles as someone materializes in the transporter room on the U.S.S. Enterprise. Instead, the apparatus sits on a table in a California Institute of Technology lab. The signal travels about a yard, along some coaxial cables. And all thats getting beamed from one end of the table to the other is a bunch of photons. But, scientists say its teleportation, circa 1998.
What physicists at Caltech, Aarhus University in Denmark and the University of Wales have accomplished is to take somethinga beam of light, in this caseand create a replica some distance away.
We claim this is the first bona fide teleportation, says Caltech physics professor Jeff Kimble, one of the researchers. The advance wont lead to Star Trek technology, but could help with sophisticated cryptography and possibly ultra-powerful quantum computers.
Kimble and his colleagues report their findings in the Oct. 23 issue of the journal Science. Last December, two research groups, one in Austria and one in Rome, reported successful teleportation experiments. The earlier experiments, however, were limited to teleporting information about whether a photon was polarized in the up or down position. Kimbles group extends the theory and technique to work more broadly. It is the first to verify that what goes into the transporter is the same thing that comes out of it. I think its a pretty important paper, says Charles Bennett, a researcher at IBM in Yorktown Heights, N.Y., and one of the authors of a 1993 article that worked out the theoretical underpinnings of teleportation. I was pretty sure somebody would find one way or another to do it.
Taking Advantage of Uncertainty
The experiment takes advantage of one spooky aspect of quantum mechanics to circumvent one of its constraints. The constraint is the Heisenberg Uncertainty Principle, which states one cant precisely measure where something is and how fast its moving, at least not at the same time. In essence, if you look too closely, you inevitably bump the object youre looking at, and its no longer where you thought it was. That would appear to be a problem if youre trying to teleport something. If you cant precisely measure a photon or an atom (or Capt. Kirk), how can you tell someone else how to make an exact copy? Surprisingly, the way to get around the Heisenberg Uncertainty principle is to mess up the information so badly that its meaningless.
Quantum Psychic Twins
The magic comes from the fact that under certain, carefully constructed circumstances, two particles or beams of light become entangled, perturbations to one instantly affect the other, even if theyre separated far apart. Entanglement means if you tickle one the other one laughs, Kimble says. Think of them as psychic twins or quantum physics equivalent of encoder/decoder rings. Albert Einstein once described entanglement as spooky action at a distance.
Now teleportation becomes simple. The senderwho physicists insist on always naming Alice takes the original item to be transported and combines it with her special entangled encoder, producing what looks like gibberish. Since its gibberish to her, she hasnt disturbed the items underlying quantum mechanical state. Alice then sends her gibberish to the recipientwho is always named Boband he combines that gibberish with his entangled decoder. Bob puts them back together, Kimble says, and out pops this same quantum state. Voila! Transport complete.
The Experimental Set-Up
In the tabletop experiment, the input is a beam of light.
The encoder/decoder rings are two specially prepared entangled light beams. The input light is combined with the entangled light and shined on a photodiode, which generates an electrical current thats sent to the other end of the table. There, the current is transformed back into light and combined with the second entangled light beam. Out comes the original light beam. The researchers can tell its the same beam of light, because it has the same little imperfections as the original. In a sense, what theyve done is precisely reproduce random noise.
Quantum Teleportation in Your Future
Whats noise now could eventually become messages. Scientists hope that quantum computers, which move information about in this way rather than by wires and silicon chips, will be infinitely faster and more powerful than present-day computers.
I believe that quantum information is going to be really important for our society, Kimble says. Not in five years or 10 years, but if we look into the 100-year time frame its hard to imagine that advanced societies dont use quantum information.
And in principle, teleportation could be used to send information to create replicas of objects, not just light beams. Researchers are already looking to teleport atoms.
Could this mean the transporters of Star Trek could one day be a reality? I dont think anybody knows the answer, Kimble said. Lets dont teleport a personlets teleport the smallest bacterium. How much entanglement would we need to teleport such a thing? Would such a teleported bacterium actually be the same bacterium, or just a very good copy? Again, Kimble says, no one knows for sure.
Reuters contributed to this report.
Experimantal diagram of the teleportation scheme (New Scientist)
Experimental quantum teleportation (extract)
Dik Bouwmeester, Jian-Wei Pan, Klaus Mattle, Manfred Elbi, Haraid Weinfurter & Anton Zellinger
Nature 390 11 Dec 1997 575 (see also New Scientist 14 Mar 98)
Quantum teleportation -the transmission and reconstruction over arbitrary distances of the state of a quantum system-is demonstrated experimentally. During teleportation, an initial photon, which carries the polarization that is to be transferred and one of a pair of entangled photons are subjected to a measurement such that the second photon of the entangled pair acquires the polarization of the initial photon. This latter photon can be arbitrarily far away from the Initial one. Quantum teleportation will be a critical ingredient for quantum computation networks.
The dream of teleportation is to be able to travel by simply reappearing at some distant location. An object to be teleported can be fully characterized by its properties, which in classical physics can be determined by measurement. To make a copy of that object at a distant location one does not need the original parts and pieces- all that is needed is to send the scanned information so that it can be used for reconstructing the object. But how precisely can this be a true copy of the original? What if these parts and pieces are electrons, atoms and molecules? What happens to their individual quantum properties, which according to the Heisenberg's uncertainty principle cannot be measured with arbitrary precision? Bennett et al.' have suggested that it is possible to transfer the quantum state of a particle onto another particle-the process of quantum teleportation-provided one does not get any information about the state in the course of this transformation. This requirement can be fulfilled by using entanglement, the essential feature of quantum mechanics'. It describes correlations between quantum systems much stronger than any classical correlation could be. The possibility of transferring quantum information is one of the cornerstones of the emerging field of quantum communication and quantum computation'. Although there is fast progress in the theoretical description of quantum information processing, the difficulties in handling quantum systems, have not allowed an equal advance in the experimental realization of the new proposals. Besides the promising developments of quantum cryptography' (the first provably secure way to send secret messages), we have only recently succeeded in demonstrating the possibility of quantum dense coding', a way to quantum mechanically enhance data compression. The main reason for this slow experimental progress is that, although there exist methods to produce pairs of entangled photons, entanglement has been demonstrated for atoms only very recently and it has not been possible thus far to produce entangled states of more than two quanta. Here we report the first experimental verification of quantum teleportation. By producing pairs of entangled photons by the process of parametric down-conversion and using two-photon interferometry for analysing entanglement, we could transfer a quantum property (in our case the polarization state) from one photon to another. The methods developed for this experiment will be of great importance both for exploring the field of quantum communication and for future experiments on the foundations of quantum mechanics.
The problem To make the problem of transferring quantum information clearer, suppose that Alice has some particle in a certain quantum state |phi> and she wants Bob, at a distant location, to have a particle in that state. There is certainly the possibility of sending Bob the particle directly. But suppose that the communication channel between Alice and Bob is not good enough to preserve the necessary quantum coherence or suppose that this would take too much time, which could easily be the case if |phi> is the state of a more complicated or massive object. Then, what strategy can Alice and Bob pursue? As mentioned above, no measurement that Alice can perform on |phi> will be sufficient for Bob to reconstruct the state because the state of a quantum system cannot be fully deterniined by measurements. Quantum systems are so evasive because they can be in a superposition of several states at the same time. A measurement on the quantum system will force it into only one of these states-this is often referred to as the projection postulate. We can illustrate this important quantum feature by taking a single photon, which can be horizontally or vertically polarized, indicated by the states |h> and |v>. It can even be polarized in the general superposition of these two states: |phi> = a|h> + b|v>.
where (a and b are two complex numbers satisfying |a|^2+|b|^2=1. To place this example in a more general setting we can replace the states |h> and |v> in equation |0> and |1>, which refer to the states of any two-state quantum system. Superpositions of |0> and |1> are called qubits to signify the new possibilities introduced by quantum physics into information science. If a photon in state |phi> passes through a polarizing beam-splitter - a device that reflects (transmits) horizontally (vertically) polarized photons - it will be found in the reflected (transmitted) beam with probability |a|^2 (|b|^2). Then the general state |phi> has been projected either onto |h> or or onto |v> by the action of the measurement. We conclude that the rules of quantum mechanics, in particular the projection postulate, make it impossible for Alice to perform a measurement on |phi> by which she would obtain all the information necessary to reconstruct the state.
The concept of quantum teleportation
Although the projection postulate in quantum mechanics seems to bring Alice's attempts to provide Bob with the state |phi> to a halt, it was realised by Bennett et al. that precisely this projection postulate enables teleportation of |phi> from Alice to Bob. During teleportation Alice will destroy the quantum state at hand while Bob receives the quantum state, with neither Alice nor Bob obtaining inforniation about the state |phi>. A key role in the teleportation scheme is played by an entangled ancillary pair of particles which will be initially shared by Alice and Bob.
Suppose particle 1 which Alice wants to teleport is in the initial state |phi>1 = a|h> + b|v>, (Fig. la), and the entangled pair of particles 2 and 3 shared by Alice atid Bob is in the state:
|phi->23 = [1/sqrt(2)](|h>2|v>3 - |v>2|h>3)
That entangled pair is a single quantum system in an equal superposition of the states |h>2|v>3 and |v>2|h>3. The entangled state contains no information on the individual particles; it only indicates that the two particles will be in opposite states. The important property of an entangled pair is that as soon as a measurement on one of the particles projects it, say, onto |h> the state of the other one is determimed to be |v>, and vice versa. How could a measurement on one of the particles instantaneously influence the state of the other particle, which can be arbitrarily far away?
Figure 1 Scheme showing principles involved in quantum teleportation (a) and the experimental set-up (b). a, Alice has a quantum system, particle 1, in an initial state which she wants to teleport to Bob. Alice and Bob also share an ancillary entangled pairof particles 2 and 3 emitted by an Einstein-Podolsky-Rosen (EPR) source. Alice then performs a joint Bell-state measurement (BSM) on the initial particle and one of the ancillaries, projecting them also onto an entangled state. After she has sent the result of her measurement as classical information to Bob, he can perform a unitary transforrnation (U) on the other ancillary particle resulting in it being in the state of the original particle. b, A pulse of ultraviolet radiation passing through a non-linear crystal creates the ancillary pair of photons 2 and 3. After retroflection during its second passage through the crystal the ultraviolet pulse creates another pair of photons, one of which will be prepared in the initial state of photon I to be teleported, the other one serving as a trigger indicating that a photon to be teleported is under way. Alice then looks for coincidences after a beam splitter BS where the initial photon and one of the ancillaries are superposed. Bob, after receiving the classical information that Alice obtained a coincidence count in detectors f1 and f2 identifying the |phi->12 Bell state, knows that his photon 3 is in the initial state of photon 1 which he then can check using polarization analysis with the polarizing beam splitter PBS and the detectors dl and d2. The detector p provides the information that photon 1 is under way.
Einstein, among many other distinguished physicists, could simply not accept this "spooky action at a distance". But this property of entangled states has now been demonstrated by numerous experiments. The teleportation scheme works as follows. Alice has the particle 1 in the initial state |phi>1, and particle 2. Particle 2 is entangled with particle 3 in the hands of Bob. The essential point is to perform a specific mesaurement on particles 1 and 2 which projects theni onto the entangled state:
|phi->12 = [1/sqrt(2)](|h>1|v>2 - |v>1|h>2)
This is only one of four possible maximally entangled states into which any state of two particles can be decomposed. The projection of an arbitrary state of two particles onto the basis of the four states is called a Bell-state measurement. The state given in equation (3) distinguishes itself from the three other maximally entangled states by the fact that it changes sign upon interchanging particle 1 and particle 2. This unique anti-symmetric feature of |phi->12 will play an important role in the experimental identification, that is, in measurements of this state. Quantum physics predicts that once particles 1 and 2 are projected into |phi->12, particle 3 is instantaneously projected into the initial state of particle 1. The reason for this is as follows. Because we observe particles 1 and 2 in the state |phi->12 we know that whatever the state of particle 1 is, particle 2 must be in the opposite state, that is, in the state orthogonal to the state of particle 1. But we had initially prepared particle 2 and 3 in the state |phi->23, which means that particle 2 is also orthogonal to particle 3. This is only possible if particle 3 is in the same state as particle 1 was initially. The final state of particle 3 is therefore:
|phi>3 = a|h>3 + b|v>3
We note tilat during the Bell-state measurement particle 1 loses its identity because it becomes entangled with particle 2. Therefore the state |phi>1, is destroyed on Alice's side during teleportation. This result (equation (4)) deserves some further comments. The transfer of quantum information from particle 1 to particle 3 can happen over arbitrary distances, hence the name teleportation. Experimentally, quantum eiitanglement has been shown to survive over distances of the order of 10 km. We note that in the teleportation scheme it is not necessary for Alice to know where Bob is. Furthermore, the initial state of particle 1 can be completely unknown not only to Alice but to anyone. It could even be quantum mechanically completely undefined at the time the Bell-state measurement takes place. This is the case when, as already remarked by Bennett et al., particle 1 itself is a member of an entangled pair and therefore has no well-defined properties on its own. This ultimately leads to emtanglement swapping...... It is also important to notice that the Bell-state measurement does not reveal any information on the properties of any of the particles. This is the very reason why quantum teleportation using coherent two-particle superpositions works, while any measurement on one-particle superpositions would fail. The fact that no information whatsoever is gained on either particle is also the reason why quantum teleportation escapes the verdict of the no-cloning theorem. After successful teleportation particle 1 is not available in its original state any more, and therefore particle 3 is not a clone but is really the result of teleportation. A complete Bell-state measurement can not only give the result that the two particles 1 and 2 are in the antisymmetric state, but with equal probabilities of 25% we could find them in any one of the three other entangled states. When this happens, particle 3 is left in one of three different states. It can then be brought by Bob into the original state of particle 1 by an accordingly chosen transformation, independent of the state of particle 1, after receiving via a classical communication channel theinformation on which of the Bell-state results was obtained by Alice. Yet we note, with emphasis, that even if we chose to identify only one of tile four Bell states as discussed above, teleportation is successfully achieved, albeit only in a quarter of the cases.
Photons emerging from a down-conversion crystal are polarized in pairs as shown. In the region of overlap theyare entangled and have complementary but unspecified polarizations. Theroetical and experimental results coincide for -45 deg (d1f1f2) a dip to zero in the three-fold coincidence rate at zero delay and a constant value in the+45 deg (d2f1f2). The shaded area denotes the region of teleportation.
Experimental realization Teleportation necessitates both production aild measurement of entangled states; these are the two most challenging tasks for any experimental realization. Thus far there are only a few experimental techniques by which one can prepare entangled states, and there exist no experimentally realized procedures to identify all four Bell states for any kind of quantum system. However, entangled pairs of photons can readily be generated and they can be projected onto at least two of the four Bell states. We produced the entangled photons 2 and 3 by parametric down-conversion. In this technique, inside a nonlinear crystal, an incoming pump photon can decay spontaneously into two photons which, in the case of type 11 parametric down-conversion, are in the state given by eqtiation(2) (Fig. 2)'. To achieve projection of photons I and 2 into a Bell state we have to make them indistinguishable.
The next steps In our experiment, we used pairs of polarization entangled photons as produced by pulsed down-conversion and two-photon interferometric methods to transfer the polarization state of one photon onto another one. But teleportation is by no means restricted to this system. ln addition to pairs of entangled photons or entangled atoms one could imagine entangling photons with atoms, or photons with ions, and so on. Then teleportation would allow us to transfer the state of, for example, fast-decohering, short-lived particles, onto some more stable systems. This opens the possibility of quantum memories, where the information of incoming photons is stored on trapped ions, carefully shielded from the environment. Furthermore, by using entanglement purification -a scheme of improving the quality of entanglement if it was degraded by decoherence during storage or transmission of the particles over noisy channels-it beconies possible to teleport the quantum state of a particle to some place, even if the available quantum channels are of very poor quality and thus sending the particle itself would very probably destroy the fragile quantum state.
The feasibility of preserving quantum states in a hostile environment will have great advantages in the realm of quantum computation. The teleportation scheme could also be used to provide links between quantum computers. It also enables us to perform a test of Bell's theorem on particles which do not share any common past, a new step in the investigation of the features of quantum mechanics. Last but not least, the discussion about the local realistic character of nature could be settled firmly if one used features of the experiment presented here to generate entanglement between more than two spatially separated particles.