Entagled Photons

This QKD system was developed by an Austrian-Swedish consortium consisting of the University of Vienna , the Austrian Research Centers GmbH and the Royal Institute of Technology from Kista.

The principal idea of this setup is to use the unique quantum mechanical property of “entanglement” in order to transfer the correlated measurements into a secret key (Fig. 1). A passive system performing measurements was implemented, where all photons find their way towards their detectors without the need to control any of their properties actively. As a result, correlated measurements are generated at Alice and Bob without any input for choice of basis or bit value for individual qubits.



Figure 1. QKD-system using entangled photons measured at Alice and Bob. The correlated measurements from single-photon detectors are further processed and transferred to a symmetric, secure key by the QKD software.

Both pair photons are generated at different wavelengths in order to use the Si-SPADs with their nearly perfect properties for the 810nm-photon at Alice's side, but also to send the telecom 1550nm-photon to Bob with low transmission losses. The latter photon is detected by InGaAs-APDs that need to be gated as usual. Therefore an optical trigger pulse co-propagates with each signal photon to open the detector for few nanoseconds.

For long-distance fiber-communication systems it is essential to have a high flux of photon pairs that we realized using spontaneous parametric downconversion (SPDC) in the orthogonally oriented two crystal geometry (see Fig. 2). Our compact source (40×40 cm) is pumped by a 532-nm-laser and its polarization is rotated to 45° for equal crystal excitation. The two nonlinear periodically-poled KTiOPO4 (ppKTP) crystals are quasi-phase matched for all three wavelength.



Figure 2. Schematic of the source of entangled photons. Within the nonlinear ppKTP crystals, single pump photons are converted to two photons at 810nm and 1550nm. If two indistinguishable conversion processes are possible within the V- and H-oriented crystals, polarization entanglement is generated.

Besides the optical part, the system integrates the electronics (see Fig. 3). Control circuits are used to stabilize the QKD-link against ambient temperature drifts of the alignment from the source as well as polarization changes of the quantum channel. An embedded system contains the time-tagging unit to time-stamp the detection events in order to establish correlations. As soon as Alice and Bob measure photons, the QKD stack running on an embedded processor (or PC) transfers the measurements to a secret key by the classical steps of error correction and privacy amplification. In order to communicate with the node, the protocol Q3P was implemented to overtake further functionalities and key management.





Figure 3. The goal within the SECOQC project, to integrate the whole system in telecom racks was reached as well as to implement all necessary interfaces to connect to the QKD-network