Quantum Crypto Part 4

This is the last in the Quantum Cryptography series of posts. This post covers the limitations of current methods and contemplates possible avenues of evolution for the technology.



Most QKD systems today rely on optical transmission of photons, either via telecommunications fiber or free space. Both media pose technical challenges in terms of attenuation and preserving polarization that must be overcome if global QKD is to become a reality. The primary form of noise in photonic transmissions is loss over distances [1]. Currently, the best systems known span distances of between 100 and 140km (missing reference). Classical communications work around the problem of attenuation by using repeaters, something that is difficult to emulate in the quantum regime due to the no-cloning theorem. A working quantum repeater would require matter quantum memory (missing reference), and might even be more difficult than the task of developing a universal quantum computer, due to the constraints involved (in particular, the quantum memories in question must satisfy DiVincenzo’s five criteria for universal quantum computers, as well as his additional (harder) criterion. [2]). However, the forefront of research today suggests alternate formulations of quantum repeaters that do away with the requirement for quantum memories and may provide a feasible way to extend the range of QKD [3].


Current QKD systems are limited in key transfer rate by a multitude of factors; the sifting process, transmission errors, detector inefficiencies, attenuation, additionally key bits sacrificed to privacy amplification, as well as other influences all combine to reduce the number of secure viable key bits transmitted in a QKD link. The transmission rate is intricately tied to the range of the system; increased range decreases the probability that a qubit reaches the destination receiver, and attenuation along the way may mean that detection probability is lowered even if the destination is reached. The rate also depends on the details of the system used; certain approaches offer higher rates than others. The experimental SECOQC project [4] recommended a baseline key transmission rate of 1kbit/s over a 25km link, which was exceeded handily by some systems used in the project, with even higher rates at shorter distances, as high as 50 kbits/s over an 80m free space link [4]. In fact, rates as high as 1.02 Mbit/s have been seen at distances of 20km using fiber links [5]. In comparison to traditional communications infrastructure, such rates are almost laughably small, which means that QKD is likely to be used only for the highest security applications in the near future. However, advances in high-speed single photon detectors [6] promise, among others, promise far higher rates in years to come.


A significant obstacle to the widespread adoption of QKD systems is the high cost of setting up and maintaining the equipment required for their use. Here, the use of telecommunications fiber, as well as the reliance on classical communications technologies for certain parts of the protocols mitigate some of the investment, as existing infrastructure can be reused. On the other hand, specialized equipment for the generation of entangled pairs, for instance, or the modulation of weak light pulses, require capital investments that are many orders of magnitude higher than traditional cryptographic systems. The emergence of a number of companies in the QKD space (ID Quantique, MagiQ Technologies, Swiss Quantum, Battelle, etc.) are a promising indicator that demand for such systems is healthy in environments where security is of paramount importance, and the prevalent mood is that mass production and economies of scale, coupled with technological advances in the manufacture of the components required (detectors, especially), will eventually bring the cost of QKD systems down to a manageable amount. Furthermore, QKD systems are often much cheaper than the alternative in many high security environments, where the prevalent method of key distribution is the use of couriers to physically exchange random key material, with a much lower security guarantee.


Possible adoption trajectories

The types of environments best suited for the balance of security and cost offered by QKD networks are controlled, high-value organizations that require extremely strong security guarantees over long periods of time. QKD, combined with one-time pad encryption, offers unconditional security, as defined by Diffie and Hellman [7], which protects secrets from any form of cryptanalysis, no manner how computationally or mathematically advanced. Compare this with the vulnerability of traditional encryption schemes to expanding computational power over time; leaps in the amount of processing power possessed by novel systems reduce the complexity of a brute force attack on, say, 256-bit AES to the point where ciphertexts from today, stored by an adversary until such time in the future as computational power has reached the point where it is feasible to perform cryptanalysis on such ciphertexts, maybe decrypted in the future. Only the one-time pad offers any guarantee against such attacks, and only QKD has the potential to distribute keys for use in the one-time pad in a relatively efficient, scalable manner. Therefore, sensible predictions about the adoption of QKD must begin with environments such as government organizations, financial institutions, global military organizations, among others. It is possible that QKD technology may follow a trajectory not too different from that of the internet; beginning as the sole province of research institutions and government-funded projects, and then gradually expanding to encompass businesses and industry, eventually reaching a sort of threshold where widespread mass adoption becomes possible, and the dream of provably secure communication for all becomes true.

Satellite-based QKD

One of the most attractive proposals for a global QKD system leverages free space transmissions between ground and satellite systems for key exchange. Development in this area is well underway. A research effort from 2002 demonstrated secure key exchange over a free space link at 23.4km [8], with the attenuation results obtained indicating that near earth key exchange (at a range of 500-1,000km) should be possible in the near future. It has been almost 13 years since this result, and the proposal made by that paper seems achievable at present. A collaboration between industry partners and the Canadian Space agency over a mission called QEYSSat (Quantum Encryption and Science Satellite) is underway, proposing the use of a microsatellite located in low earth orbit at about 600km carrying an optical receiver with 40cm aperture as the main optical instrument. The group has conducted studies of transmission losses in such regimes, successfully operating a system at upto 60dB losses [9], even in the face of strong turbulence, countered with careful post-processing [10]. Ensuring the robust and reliable operation of such a service remains a goal for the future, paving the way for optical links to low earth satellites that may eventually form the basis for a global system of QKD. Further applications of satellite QKD could be in the secure distribution of keys for satellite remote access and for secure inter-satellite links [11].

[12] An artist’s rendering of QKD via a trusted satellite node

The Quantum Internet [13]

One of the key results of research into quantum cryptography is the maturation of technologies that allow for the creation of purely quantum links between systems, transporting quantum states between geographically separated sites with high fidelity, maintaining important quantum properties like entanglement. Carrying out this idea to its logical conclusion leads to the idea of a quantum internet, whereby a network of computational nodes linked by quantum channels would be empowered to perform computational tasks beyond classical physics. For instance, Preskill’s notion of quantum software [14], used to describe difficult-to-create quantum states that perform useful quantum computations, would find a distribution mechanism in such a quantum internet. Perhaps most dazzling is the exponential increase in state space that would be produced by full quantum connectivity between nodes of such a network; a fully quantum network of $n$ nodes each with $k$ quantum bits would have a state space on the order of $2^{kn}$, whereas such a network that utilized only classical connections would have a state space of significantly lower dimension, on the order of $n2^k$. However, fully realizing such a system would require overcoming rather large technical difficulties in local quantum processing, quantum repeaters, and error-corrected quantum teleportation, as well as advances in quantum memory. Theoretically, the developments required would signal a shift in focus from concentrating on highly specialized components (say, systems comprising of single electrons trapped in crystals) to more complex dynamical quantum systems composed of many such building blocks.


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