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QUANTUM INFORMATION

Spooky teleportationQuantum teleportation in itself is intriguing. But now the combined states of two photons have been teleported — while preserving their entanglement — and this could bring large-scale quantum communication and computation a step closer.

PHILIP WALTHERis in the Physics Department, Harvard University, Cambridge, Massachusetts 02138, USA.

e-mail: pwalther@fas.harvard.edu

Quantum-state teleportation1 is one of the most striking applications of entanglement-assisted quantum communication and plays an

important role in a number of quantum computation protocols2,3. Quantum teleportation has already been achieved in diff erent quantum systems4–6 and over long distances inside7 and outside8 the laboratory, but the technical challenges are still signifi cant. In fact, quantum teleportation has been practically limited to single bits of quantum information, as every ‘qubit’ to be teleported requires an auxiliary pair of entangled quantum objects. Th erefore, the successful teleportation of a two-qubit composite system that Qiang Zhang and co-workers report on page 678 of this issue9 is remarkable. A unique six-photon interferometer is exploited to transfer the combined polarization state of two photons, with the remaining four photons serving as the ‘teleporter’.

Quantum teleportation entered the scene 13 years ago, when Bennett et al.1 suggested the possibility of transferring the quantum state from one particle to another, without having to know the state that is actually teleported. Th e transfer only requires that both transmitter (Alice) and receiver (Bob) have some classical communication channel at their command, and that they share an entangled (but spatially separated) pair of quantum particles that they can use as a ‘quantum channel’. Entangled particles possess correlations stronger than those allowed by classical physics, and can only be described by their joint behaviour, no matter how far the two particles are located from each other. Imagine two ‘entangled dice’, placed at opposite ends of our galaxy, each of which, when measured individually, yields a random number between one and six. However, owing to entanglement between the dice, the throw of the second die would always lead to an outcome that is fully determined by the result of rolling the fi rst die, independent of their separation or the time ordering of the measurements. Famously, Albert Einstein called this behaviour ‘spooky action at a distance’.

If such an entangled pair of particles is shared between two communication partners, one of them

can teleport the unknown state of a third particle to the other. Take, for example, the polarization state of a photon, which can be in any arbitrary superposition of being horizontally or vertically polarized. To transfer the state of this photon (which she does not have to know), Alice makes a joint measurement on that photon, as well as on her component of the shared entangled photon pair, a so-called ‘Bell-state measurement’ (see Fig. 1). Th e random result of this measurement, which makes her photons indistinguishable, is sent to Bob via a classical communication channel (a telephone line, for example). Th is classical information tells Bob enough to determine how he can transform his component of the previously shared entangled state so that it is identical to the original state of the third photon — as a result, the state is transferred to Bob. Importantly, the input state is destroyed by Alice’s measurement, so that teleportation does not result in cloning of a quantum state (which quantum mechanics strictly forbids).

So much for one-qubit teleportation; if the combined states of two photons are to be teleported, the experiment becomes considerably more complex. Basically, the whole setup has to be doubled. Zhang and colleagues9 had to carefully control six photons, two entangled photon pairs — the teleporter — and the two input photon states. Compared with previous experiments, they increased the rate of producing usable six-photon coincidences by an order of magnitude, to about ten six-photon events per minute, an impressive improvement. With this resource at hand, Alice could use two quantum channels to

Figure 1 The teleportation of quantum states. Alice and Bob share an entangled photon pair (blue, with the wavy line representing entanglement), which Alice wants to use to teleport the polarization state of a third photon (red) to Bob. Alice then performs a measurement on the particles on her side, thus destroying the information of the input state. The result of her measurement outcome is then sent to Bob so that he can perform a transformation on the state on his side to obtain Alice’s initial state.

1 2 3 4Measurement

Alice Bob

Classical communication

Transformation

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teleport the polarization state of her two input qubits to Bob. Th e teleportation process then proceeds in the same way as for the single-qubit case, except that every step happens twice. Remarkably, however, the input qubits could also share some entangled state themselves, and the non-local correlations were preserved aft er the teleportation was accomplished. Th is speaks for the high degree of stability reached in the experiment.

Quantum teleportation is a fascinating application of the fundamental features of quantum entanglement, and is also an important element in quantum networks for sharing entanglement between several communication partners. Th e generalized teleportation experiments demonstrated by Zhang et al.9 are also an important step towards teleportation-based quantum computation with photons. Using single photons as qubits has the advantage that their quantum state remains basically unaltered during the experiment, owing to the typically weak coupling of photons to their environment. Moreover, photons can be conveniently controlled with standard optical components.

However, there is no signifi cant photon–photon interaction, but such interactions are necessary in almost any quantum-information protocol. Remarkably, the measurement process itself can suffi ce to induce an eff ective, strong nonlinearity at the single-photon level3, making optical qubits interesting candidates for measurement-based quantum computation, such as the one-way quantum computer10. Th e results of Zhang and colleagues9 suggest that with future gradual increases to the coincidence rate and the fi delity of entangled photons, the implementation of such schemes in quantum information systems may not be so far out of reach.REFERENCES1. Bennett, C. H. et al. Phys. Rev. Lett. 70, 1895–1899 (1993).2. Gottesman, D. & Chuang, I. L. Nature 402, 390–393 (1999).3. Knill, E., Lafl amme, R. & Milburn, G. J. Nature 409, 46–52 (2001).4. Bouwmeester, D. et al. Nature 390, 575–579 (1997).5. Riebe, M. et al. Nature 429, 734–737 (2004).6. Barrett, M. D. et al. Nature 429, 737–739 (2004).7. Marcikic, I., de Riedmatten, H., Tittel, W., Zbinden, H. & Gisin, N. Nature

421, 509–513 (2003).8. Ursin, R. et al. Nature 430, 849 (2004).9. Zhang, Q. et al. Nature Phys. 2, 678–682 (2006).10. Raussendorf, R. & Briegel, H. J. Phys. Rev. Lett. 86, 5188-5191 (2001).

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