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Sep 24 2015

NIST Research Team Breaks Record in Quantum Teleportation


Star Trek aficionados all over the world may want to jump at the chance – but sadly, a new world record achieved by Hiroki Takesue, a guest professor at the U.S. National Institute of Standards and Technology (NIST), and his team of fellow researchers won't bring them any closer to being beamed up. The results, however, could have a massive impact on future information and communication technologies.

The concept of quantum computing has fascinated scientists and engineers since at least 1969, the year the original Star Trek series was in its third and final season. That year, Stephen J. Wiesner, then a graduate student at New York's Columbia University, introduced "conjugate coding," a cryptographic tool based on the noisy transmission of two or more complementary messages transferred via single photons. For some reason, Wiesner's research was deemed too exotic and never published until 1983, when it spawned a surge of related studies that resulted in technologies like quantum key distribution (QKD), which is used on networks like the DARPA Quantum Network or Europe's SECOQC project to facilitate extremely secure communications that even the proverbial "state-backed hackers" may not be able to break into. In their simplest form, these networks simply have to distribute keys for a VPN overlay running atop an underlying public or private Internet. In other words, the technology could be used to protect data transfers in optical fiber networks. Unfortunately, implementation still faces some serious obstacles. One of them is that the devices used for key distribution so far only work in 1-to-1 connections, which in turn can only cover limited distances, e.g. those typically found in campus networks.

In 2008, SECOQC was among the first projects to address and overcome these limitations by adding so-called trusted repeaters to the basic design. These repeaters pick up and 'amplify' the key exchange signals, which are usually transmitted via photons. That way, SECOQC was able to set up a network that spanned the city of Vienna and connected to another node located in St. Poelten, about 66 kilometers (41 miles) west, using free-space optics (laser beams) for the 'last mile'. Hiroki Takesue and his colleagues now extended the distance over which such photon-to-photon data transfers can be carried out to a veritable 102 kilometers (63 miles) on a fiber network. To achieve this, they used special "advanced single-photon detectors" designed and built at NIST. The problem with this description is that it makes the challenge look much simpler than it actually is. In the words of the scientists, these are the problems they were facing (emphasis added):

"Since the first experimental demonstrations using photonic qubits and continuous variables over very short distances on optical tables, the distance of quantum teleportation over free-space channels has continued to increase and has reached >100 km. On the other hand, quantum teleportation over optical fiber has been challenging, mainly because the multifold photon detection that inevitably accompanies quantum teleportation experiments has been very inefficient due to the relatively low detection efficiencies of typical telecom-band single-photon detectors. Consequently, there have been relatively few reports of quantum teleportation over optical fiber. In the pioneering experiment (...), the quantum states of time-bin qubits were transferred over 2 km of fiber. A recent experiment, which set the record distance for quantum teleportation over fiber (25 km), implemented a quantum relay configuration with the teleported photon stored in a quantum memory. Recently, superconducting nanowire single-photon detectors (SNSPDs) with >90% detection efficiency in the 1.5 μm band have been realized using superconducting nanowires made of amorphous tungsten silicide (WSi)."

Apparently, these 25 kilometers weren't anywhere near the distance SECOQC covered with its setup. So Takesue and his co-researchers turned to SNSPDs made from "molybdenum silicide (MoSi), to perform photonic quantum teleportation over fiber. The choice of MoSi instead of WSi allowed operation at a higher temperature with less jitter." Using four MoSi SNSPDs with a detection efficiency of 80 to 86%, they eventually could "perform highly efficient multifold coincidence measurements, resulting in the successful quantum teleportation (photon-to-photon data transmission, N.B.) over 100 km of fiber" with an average fidelity of 83.7 ± 2.0%.

In essence, this means that it is possible to conduct long-distance communication based on quantum teleportation via optical fiber networks, provided you have adequate endpoint equipment. Plus, you could achieve that with just a quarter of the repeaters required before. But what's the use of the new technology? According to NIST's accompanying press release, the transmission method developed by Takesue et al. offers some considerable advantages over SECOQC-like implementations:

  • Optical fiber networks are easier to build and offer greater flexibility than their free-space counterparts.
  • As noted above, one practical usage scenario for quantum teleportation could be to facilitate "unbreakable encryption": In conventional encrypted communications, it's relatively easy for an attacker to pose as a man-in-the-middle and grab the encryption key(s) that communication partners agree upon. With a key exchange process based on quantum teleportation and prior authentication of the legitimate participants, that's simply not possible, as every attempt to eavesdrop causes anomalies the communication system in use (and hence both parties) can easily detect and act upon. Should the anomalies stay below a certain level, it is literally safe to assume the attacker won't be able to listen in, so the parties may still generate a secure key. By contrast, if the disturbance exceeds that limit, the system will flat out abort the exchange process because a secure key cannot be generated. In both cases, the attacker is left empty-handed[1].
  • In the long run, the researchers hope to be able to refine their quantum repeaters in a way that might allow for the creation of a "quantum Internet."

For detailed information, please see the paper "Quantum teleportation over 100 km of fiber using highly efficient superconducting nanowire single-photon detectors," published by Takesue at al. on September 23, 2015 and also available in print in Optica, Vol. 2, No. 10. For a more entertaining approach, check out Alex J. Martin's piece at The Register. The illustrated explanation – courtesy of NIST – is in the image below.


Fig. 1


[1] However, the very techniques that might enable such ultra-high levels of protection could also permit potential miscreants to break the keys their opponents want to protect. In addition, classic loopholes such as poor implementation or lack of access restriction to key-generating devices will render the QKD process just as vulnerable as any conventional mechanism.


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