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[Image: SQO Team, University of Ottawa]

These are heady days for quantum communication and encryption, as a number of groups have recently demonstrated aircraft- and satellite-to-ground, free-space quantum links that have boosted the prospects for practical, global quantum key distribution (QKD) networks for information security. But the free-space QKD schemes demonstrated thus far have involved only 2-D encryption, in which individual photons encode only one quantum bit (qubit). That limitation holds back the data speed and noise resistance of real-world free-space quantum networks.

Now, researchers in Canada and Germany have reportedly encoded higher-dimensional, 4-D quantum keys on single photons, and have transferred the encoded information 0.3 km across free space in a turbulent, realistic city environment—without any active adaptive optics, wavefront correction or vibration control (Optica, doi: 10.1364/OPTICA.4.001006). The research team concludes that the work “paves the road toward high-dimensional intracity quantum cryptography via quantum key distribution.”

The trouble with turbulence

QKD improves upon standard encryption schemes by using the quantum state of individual photons to encode and decrypt data, and by taking advantage of quantum-mechanical properties such as superposition and entanglement to add a level of security. While it’s already been deployed in some short-run systems, building a global QKD network will require robust links from satellites to ground stations, through Earth’s turbulent atmosphere, which can gum up the optical signal.

What’s more, most experiments in QKD thus far have focused on the so-called 2-D BB84 protocol—an approach developed by Charles Bennett and Gilles Brassard in 1984, which generally encodes quantum bits on photons using the “naturally bidimensional” property of photon polarization. A number of small-scale experiments have shown that higher-dimensional encoding beyond single-bit-per-photon, 2-D schemes can both boost data transfer rates and improve noise resistance. But it’s been tough to put those advantages into practice in any realistic environment beset by air turbulence.

Spin-orbital superposition

To see if 4-D QKD could be implemented practically, the research team—led by OSA Fellow Ebrahim Kaimi of the University of Ottawa, Canada—used a 4-D BB84 scheme that relied on “structured photons.” In these photons, quantum information is encoded as a coherent superposition of the light’s spin angular momentum, associated with polarization, and its orbital angular momentum, associated with the wavefront helicity. As a result, each photon can encode one of four possible quantum states, rather than the limitation of two with polarization-only encoding, creating a 4-D encryption scheme.

The team started by creating photon pairs via spontaneous parametric downconversion in a nonlinear crystal pumped by a 405-nm laser. One photon of the pair, the signal photon, then passes through a sequence of polarizing beam splitters, q-plates, wave plates and other optical elements to encode a specific quantum state on the photon. The other photon, the idler, serves to herald the signal photon at the receiving station.

The researchers installed this transmitting system (in covered wooden boxes, to protect it from the elements) on an office tower at a height of 40 m on the University of Ottawa campus, They placed a receiver that included similar sequence of optical elements (to decode the signal) at a similar height on a building around 300 m away.

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[Image: SQO Team, University of Ottawa]

The team then began transmitting encoded signal and idler photon pairs through those 300 m of free space on a beam that, at the last lens of the transmitter, was about 12 mm wide. By the time the beam reached the receiver, atmospheric influences had spread the beam diameter to around 20 mm. Within that broadened beam, each idler photon “acts as a trigger for the arrival of the signal photon,” the quantum state of which is then decoded by the receiving system. The team used no active adaptive optics or vibration isolation systems to steady the setup or correct for turbulence.

Idler photon as weather reporter

One interesting aspect of the system is that, in the absence of any adaptive optics, the idler photon not only heralds the arrival of the signal photon, but can also serve as a sort of weather vane that provides information on turbulence in the free-space transmission. Because the count of idler photons along the optical path will tend to drop in periods of excessive turbulence, the receiver can discard measurements when that count goes below the optimal value that would be experienced in an environment of little or no turbulence. Thus, by sifting out turbulence-affected pairs, the overall error rate is decreased.

Using the setup, the team found that it was able to transfer quantum information between the two separated stations with a quantum bit error rate of 11 percent—well within the threshold value of 18.9 percent required for robust 4-D quantum cryptography. Moreover, after error correction, the system achieved a data rate (specifically, a rate of transfer of secret keys) of 0.65 bits per sifted photon, around 1.6 times the rate for a system using standard 2-D encoding.

“After bringing equipment that would normally be used in a clean, isolated lab environment to a rooftop that is exposed to the elements and has no vibration isolation,” lead author Alicia Sit, an undergrad at the University of Ottawa, noted in a press release, “it was very rewarding to see results showing that we could transmit secure data.” The next step for the team—which also included other scientists from the University of Ottawa, the University of Rochester, USA, and the Max Planck Institute for the Science of Light, Germany—will be to add active adaptive optics to the mix to further tame turbulence, and to expand the network to a setup that includes three links some 5.6 km apart. That would constitute an even more realistic proxy for an urban QKD network.