Quantum key distribution

Quantum cryptography is a technology arising from the broad field of quantum information which is more developed towards real world applications, with a few companies starting to offer commercial products.

We implement a field version of free space quantum cryptography for the urban environment of Singapore. A compact source of entangled photon pairs is used to implement a BB84-type quantum key distribution protocol. The system is able to detect single photons in an outdoor environment and against a strong background  at moderate daylight conditions. A successful key exchange over a test range on  NUS campus has been demonstrated.

Implementation

To explore key distribution systems relying on entangled photons as a primary physical resource for key generation, we follow currently a simple extension of a BB84 protocol, based on polarization measurements on photon pairs at two legitimate communication partners:

Besides the source for entangled photon pairs, we use compact single photon detection modules for polarization measurements, and a coincidence identification concept with records times relative to local clocks on both sides, obliviating any specific hardware communication channel for the protocol besides the quantum link itself. After a few seconds of our photon-pair based clock synchronization scheme, the key generation process runs continuously using an autonomous standard WIFI link between or two sides for all coincidence detection and genuine key generation protocol elements such as base reconciliation, error correction and privacy amplification.

Somewhat different from other QKD implementations, this scheme requires no large bandwith supply for very random numbers in order not to undermine the in-principle security promised by QKD, but only a very moderate bandwith for the classical protocol parts.

Hardware

Our intent is to provide a reference system for investigating the practical feasibility and security of a quantum key distribution system which is not bound to optical fibers, but is reasonabliy mobile and can be deployed under ad-hoc conditions. This required development of quantum optical equipment compatible with enviromental conditions in tropical Singapore.

Software / Protocol aspects

In an attempt to minimize the effort in expensive and sensitive physical hardware, we moved large parts of the key generation tasks into the software of our system. All coincidence identification key sifting and error correction components, which are essential part of every QKD approach were implemented in a way that key material is generated continuously once the quantum link is established between the two sites. Lossless compression schemes for classical communication operate close to the Shannon limit, allowing us to work with consumer-grade networking equipment. Privacy-amplified key accumulates in well-defined standard packets to be consumed by a key management system ot to be used for encryption of data.

Current performance

In an uninterrupted run of establishment of secret key over the span of a night, we arrived at a key rate after error correction and privacy amplification of 630 bits per second on average. Our initial raw key error ratio is limited by the imperfect polarization correlations of our entangled photon source at night (4..5% QBER), and by ambient light contributions at daytime.

Key generation performance in a field trial over a run at night. The blue trace shows the photon pair rate identified as originating form the parametric down conversion source at the two remote locations, subject to alignment drifts and atmospheric transmission quality fluctuations. The green trace indicates our raw key rate after basis reconciliation in a BB84-type protocol, and the red trace the resulting final key rate after error correction and privacy amplification, implementing a modified cascade error correction protocol. The raw key error fraction (QBER, lower panel) is dominated by source correlations at night, and starts to see a sharp increase with the rising sun shining directly in the single photon detectors.

Ongoing development

Daylight: We are exploring the possibilities to extend our systems' performance into daylight operation, both from the physical hardware side and the implemented protocol, in close collaboration with our theory colleagues here in Singapore and elsewhere.

Hacking: Any cryptography systems needs to prove its worthiness by being subjected to attack. We try to find explicit holes in the implementation and explore the consequences of not addressing them. Recently we looked into the timing information exchanged between the communicating parties as a side channel from which Eve can collect a large amount of information about the key. See here for a more detailed description.

Demonstrations: In December 2007 the full crypto kit was sent to Berlin to be demonstrated live at the Chaos Communication Congress. The kit survived the shipping, and the demo was a sucess. As part of the congress we also gave a talk and released the software that drives the QKD system as open source.

Reference

A technically more detailled writeup can be found here.