Researchers from all across Europe have set up a quantum key distribution (QKD) system that demonstrates the growing capabilities of this technology at the network level. Forty-one research and industrial organizations from the European Union, Switzerland and Russia recently demonstrated quantum encrypted information transfer over an eight-node meshed optical network. The demonstration was an outcome of the SECOQC QKD conference held October 2008 in the Austrian capital, the results of which were recently published. SECOQC is the acronym for Development of a Global Network for SEcure COmmunication based on Quantum Cryptography, a project funded by the European Union within the Sixth Framework program. Cryptography, i.e., protecting data, is an important issue when it comes to today’s computer and communications networks, not only for bank transactions and Internet payments, but also for keeping e-mail and other information private. In contrast to traditional public key cryptography, which relies on the computational difficulty of certain mathematical functions for its security, quantum cryptography relies on the foundations of quantum mechanics, which comes with a fundamental benefit: The process of measuring a quantum system generally disturbs it, so a third party trying to eavesdrop will perform measurements that will inevitably introduce detectable anomalies. The two communicating users can discover the presence of any third party trying to gain knowledge of the secret key. The photon’s role In today’s quantum cryptography approaches, not only are photons used to transport information but also their quantum nature is used to facilitate the encryption. The sender – commonly labeled “Alice” by the cryptography community – transmits a string of polarized single photons to the receiver “Bob.” By carrying out a series of quantum measurements and public communications, they establish a shared key while testing whether an eavesdropper (“Eve”) has intercepted any bits of this key en route. This key is then used to encrypt and decrypt the message data, which can be transmitted (at higher speeds) over a standard communications channel. Alice and Bob are the commonly used names for the transmitter and receiver, respectively, of information in quantum cryptography. Devices such as those shown have been used to perform a network-level demonstration of the technology. Photo courtesy of id Quantique SA. In practice, however, generating the single-photon pulses required for BB84, the original quantum cryptography protocol, is a big challenge. Single-photon technology is progressing, and ways have been found to overcome the difficulties. However, QKD is still limited by a number of constraints, in particular the limited distance over which key distribution is possible, as well as its comparably low rate, which decreases exponentially in relation to distance. Last but not least, QKD communication is inherently point-to-point, which could be a significant obstacle in the majority of relevant application scenarios. The SECOQC approach to QKD networks – and core of the network demonstration in Vienna – has therefore focused on the so-called trusted repeater paradigm. Unlike the alternative, the quantum channel-switching paradigm, which creates an end-to-end quantum channel between Alice and Bob, the trusted repeater paradigm transports the keys over many intermediate reliable locations (nodes). During the demonstration, different types of QKD technology and protocols were used between the nodes – with certain interoperability requirements, however. Beyond those essentials and performance requirements, including a 25-km minimum distance (over standard telecom fiber) and 1-kb/s key generation at that range, a variety of tactics were taken for the eight QKD links. The approaches belonged to six types of systems and give a good overview of the various avenues being pursued in QKD today. • The Swiss company id Quantique SA, which produces commercially available quantum cryptography devices, used a technique closely related to the BB84 protocol. However, it shifted the phase rather than the polarization of the photons, and it used a clever self-compensation method to stabilize the phase between both ends. • Toshiba Research Europe Ltd. of the UK contributed a so-called one-way weak coherent pulse system with decoy states. This is also a phase-encoding QKD system with two interferometers that are stabilized by pulses that are time-multiplexed with the quantum signals. • GAP, the applied physics team at the University of Geneva, provided a coherent one-way system belonging to the novel class of devices utilizing distributed phase reference protocols. • Anton Zeilinger’s group at the University of Vienna and the Austrian Institute of Technology used another aspect of quantum mechanics, entangled photons, as a key approach. Using polarization entanglement, the investigators achieved long-term automatic operation based on concurrent active stabilization of optical elements. • A consortium of CNRS, Thales Group and Université Libre de Bruxelles developed a so-called continuous variables system with Gaussian modulation, reverse reconciliation and homodyne detection of the coherent light pulses. • An access-free space link set up by Ludwig Maximilians University of Munich, Germany, demonstrated a QKD for a typical “last mile” application. More detail, including a list of all other partners, is available in a recently published online article that can be found at http://stacks.iop.org/NJP/11/075001. A follow-up test bed called SwissQuantum was installed recently in Geneva, with more information available at www.swissquantum.com.