Applied physics

Quantum signals could soon span the globe

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Quantum communication offers many advantages over classical methods, but it has been limited to sending signals across a few hundred kilometres. Two studies overcome this limitation. See Article p.43 & Letter p.70

The quantum properties of light can be used for communicating in ways that are not possible in classical physics. For instance, they allow encrypted messages to be exchanged with absolute security and enable information about an object to be transferred without needing to send the object itself. In typical quantum communication channels — such as optical fibres — more and more photons are lost as the transmission distance increases, which severely limits how far an information transmitter and receiver can be from one another. On pages 43 and 70, respectively, Liao et al.1 and Ren et al.2 demonstrate quantum communication between an orbiting satellite and ground stations on Earth in which photon loss is minimized because the particles mostly travel through empty space. The authors have extended the communication distance achieved in previous studies3,4 by hundreds of kilometres — a milestone towards bringing quantum communication to a global scale.

Quantum communication relies on encoding information in the properties of quantum particles and aims at transferring this information to a distant location and manipulating it to perform specific tasks5. Two such fundamental tasks, which cannot be achieved using classical information, are quantum key distribution (QKD) and quantum teleportation. In QKD, two parties share a string of bits called a key, which is known only to them and which they can use to encrypt and decrypt a secret message with absolute security. In quantum teleportation, the quantum state of a particle is transferred without sending the particle itself. This task requires two entangled particles — in which measuring the state of one particle determines the state of the other, even when the particles are separated by a large distance.

Although, in principle, any quantum particle could be used for these tasks, photons are the preferred carriers of information in quantum communication because they are less affected than other particles by changes to their environment. Indeed, QKD and quantum teleportation have been successfully demonstrated in photonic systems using both optical-fibre and free-space communication channels over distances of up to a few hundred kilometres4,6. However, any effort to extend the communication range in these channels will inevitably hit the fundamental barrier of losses suffered by light. This limitation exists for classical optical communication as well, but in these systems, signals can be amplified and regenerated — this is not possible for their quantum counterparts.

In space, photons can travel great distances almost undisturbed, and there is advanced satellite technology and infrastructure currently in place for global telecommunications and scientific missions. Consequently, the prospect of satellite-based quantum communication has been thoroughly studied for more than a decade, in particular by an Austrian-led consortium that envisages the use of the International Space Station7. However, the associated challenges are immense. In only a handful of cases8 has it been possible to go beyond experiments on Earth that simulate space-based communication — although such experiments remain extremely useful as test beds9. Satellites in low10 or geostationary11 orbits around Earth and microsatellites equipped with basic quantum hardware12 have been used to implement some quantum-communication functionalities. But fully demonstrating satellite-based QKD or quantum teleportation requires a technological leap.

This is what Liao et al. and Ren et al. have achieved. Both experiments use a satellite in a low Earth orbit (at an altitude of about 500 km). The satellite is specifically designed for quantum communication and is equipped with several modules that are required for various tasks. The authors encode quantum information in the polarization of photons that have a near-infrared wavelength.

In Liao and colleagues' QKD experiment, the transmitter is situated on the satellite and the receiver on a ground station, with a separation distance of up to 1,200 km (Fig. 1a). This configuration, known as a downlink scenario, has the advantage that light goes through Earth's atmosphere only in the last part of its path, therefore limiting the effects of diffraction — the bending of the light as it passes around the edge of an object, such as a cloud.

Figure 1: Long-distance quantum communication.

a, Quantum key distribution (QKD) allows two communicating parties to share a string of bits called a key that is known only to them. The key can be used to encrypt and decrypt secret messages with absolute security. Liao et al.1 have demonstrated QKD between a transmitter on a satellite and a receiver on Earth, in which the key was encoded in the polarization states of photons. b, In quantum teleportation, the quantum state of a particle is transferred without needing to send the particle itself. Ren et al.2 report the teleportation of a photon's polarization state from a transmitter on Earth to a receiver on a satellite. To achieve this, the authors produced a pair of photons (X and Y) that were entangled — a condition whereby measuring the state of one particle determines the state of the other — and sent photon X to the satellite (orange arrow). They then performed a joint measurement of photon Y and a third photon (Z), whose polarization state was to be teleported. Finally, they used the outcome of this measurement to determine whether the polarization of photon X had been transformed into the polarization of photon Z (green arrow).

By contrast, Ren and colleagues' quantum-teleportation experiment uses an uplink scenario, in which the teleported photon is sent from a ground station to a receiver on the satellite, across a distance of up to 1,400 km (Fig. 1b). This setting is more technically challenging than the downlink scenario, and was chosen so that the complex entanglement machinery is accessible on Earth — although in an experiment earlier this year, the same research group showed that it is also possible to use a source of entangled photon pairs on a satellite13. To achieve quantum teleportation, Ren et al. produced a pair of entangled photons, one of which was sent to the satellite. They then performed a joint measurement of the photon on Earth and a third photon, whose polarization state was to be teleported. Finally, they used the outcome of this measurement to determine whether the polarization of the photon on the satellite had been transformed into that of the third photon.

From a scientific point of view, the authors' results provide proof that quantum communication can be performed over distances exceeding 1,000 km, which would be impossible using terrestrial links. But above all, they constitute an engineering feat made possible by an extraordinary investment in technical resources. The authors have developed cutting-edge technologies, including a sophisticated system for tracking light that provides a remarkably high level of precision, synchronization techniques, and compact, lightweight, space-certified photon sources and detectors. These advances have led to performances comparable to quantum optical-fibre communication and open the way to a new era of space-based quantum-communication experiments that transfer signals across more than 1,000 km.

A next step for QKD experiments will be to demonstrate secure message exchange between two distant locations on Earth, with a satellite acting as a trusted key relay. It will also be necessary to consider possible security breaches that are specific to satellite systems. Increasing the range of applications of QKD experiments will probably require a satellite in a higher orbit than that used by the authors, and photons that have a longer wavelength.

To demonstrate quantum teleportation in a more practical setting14, the entangled photon source will need to be operated on board a satellite. Achieving this, together with the possibility of using groups of artificial satellites called satellite constellations15, could eventually lead to global-scale quantum-communication networks that include both terrestrial and satellite links. The reported advances might also be useful in efforts to study fundamental physics in space16, potentially enriching our understanding of the core principles that underpin quantum-communication applications.Footnote 1


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Correspondence to Eleni Diamanti.

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Diamanti, E. Quantum signals could soon span the globe. Nature 549, 41–42 (2017) doi:10.1038/549041a

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