Quantum cryptography is not picky, physicists show

In the times of mass data exchange, the confidentiality of transmitted information is of paramount importance. Quantum cryptography may ensure total privacy of transmission, guaranteed by the fundamental properties of quantum particles. Nowadays, it is sources of particles in which certain properties of particles are tightly and ideally correlated -- maximally entangled -- that are used in quantum encryption. A group of Polish physicists working within the framework of the National Laboratory for Quantum Technologies has experimentally proven for the first time that even seemingly useless sources in which the entanglement of particles is substantially noisy can be used for secure transmission of a cryptographic key.

A cryptographic key is a random sequence of numbers used by a sender to encrypt and by a receiver to decrypt information. In order for both parties to exchange data confidentially, they need to have the same key, known only to each other, at their disposal. Quantum cryptography is currently applied for this very purpose: the secure transmission of a key between a sender and a receiver.

In 1991 Polish physicist Artur Ekert introduced E91 quantum key distribution protocol, which makes use of entangled quantum particles. Entanglement means that certain properties of particles are correlated. For example, it is possible to create photon pairs with entangled polarizations in a nonlinear crystal. It follows that a sender who has observed vertical polarization of his photon is safe in the knowledge that the second photon on the receiver's side has been horizontally polarized. An analogous phenomenon holds true for any given pair of perpendicular directions. While for the sender and the receiver the results of their own measurements appear to be completely random, they will immediately notice correlations between them -- emerging from the entanglement -- if they both compare their results. Quantum cryptography makes use of this very mechanism. Any attempt at eavesdropping the transmission would destroy the entanglement and shatter the perfect correlations between the results of the sender and the receiver -- a spy would be immediately detected.

The situation described above pertains to an ideal case of maximal entanglement between two objects. In reality, non-maximal entanglement is not infrequent, correlations between results are imperfect and it is becoming increasingly hard to establish whether a transmission was being eavesdropped on. The standard procedure in such cases is distillation of entanglement: a procedure which allows to extract a certain number of maximally entangled states from noisy states. Nevertheless, there are numerous states from which distillation of entanglement proves impossible or highly inefficient. Such states had long been considered to be of little use in quantum cryptography. In 2005 in Gdańsk, however, Polish physicists from the Horodecki family together with Jonathan Oppenheim theoretically demonstrated that it is possible, in certain situations, to effectively transmit a cryptographic key in spite of the difficulties with distillation of entanglement.

The scientists working within the framework of the National Laboratory for Quantum Technologies tested the assumption of the Gdańsk physicists in a carefully designed experiment. The experiment was conducted by a team coordinated by Prof. Konrad Banaszek from the Faculty of Physics, University of Warsaw (FUW) and Prof. Paweł Horodecki from the Faculty of Applied Physics and Mathematics, Technical University of Gdańsk (PG). Krzysztof Dobek, PhD, on academic internship at the National Laboratory for Atomic, Molecular and Optical Physics within the Nicolaus Copernicus University in Toruń, was responsible for the experimental side.

The experiment involved a laser emitting short pulses of light at a high frequency through a nonlinear crystal. Every now and then entangled particles flew out of the crystal. Most often these were pairs of photons (up to 6000 per second), less frequently four entangled photons (merely 2 per second). The electronic equipment was configured such that it only registered the polarization state of four entangled photons. Several hundred thousands of such occurrences were registered in the course of the four-day-long experiment.

Rafał Demkowicz-Dobrzański, PhD, and Michał Karpiński, MSc, both from the Faculty of Physics, University of Warsaw, were responsible for the data analysis and the theoretical reconstruction of the registered quantum states. "In this case, the precise analysis of the data from the experiment was of particular importance. We needed statistical confidence that a generated quantum state was indeed the state we had had in mind," explains Rafał Demkowicz-Dobrzański. It was proven that, despite the noisy entanglement, it was possible to securely transmit an average of 0.7 bits of a cryptographic key for each four entangled photons.

The experiment may be of vital importance to practical quantum cryptography. Currently, these are sources of pure, maximally entangled states that are used for quantum key distribution. The experiment conducted by the Polish physicists reveals that it will be possible to use future sources of entangled particles for transmitting a quantum cryptographic key even in situations in which generated entanglement is noisy and difficult to distill. "We have experimentally proven that sources of entanglement need not be perfect in order to be useful in cryptography. Even if a new source generates noisy entanglement, it will be possible to put it to efficient use if it proves to be more productive or less expensive than the current ones," concludes Prof. Banaszek.

An article describing the experiment and the data analysis appeared in the latest edition of the scientific journal Physics Review Letters. The research was supported by CORNER and Q-ESSENCE projects financed under the European Union's Seventh Framework Programme with the support of the TEAM Programme of the Foundation for Polish Science and the Polish Ministry of Science and Higher Education.