Contrary to many other QKD experiments, we have chosen an approach which allows to connect a large number of users (key exchange parties) simultaneously to our system, making it a promising solution for real-world environments. Thus, our system can be seen as a quantum key hub.

The immediate goal of our experiments is the setup of a city-wide QKD system based on the time-bin entanglement protocol. We are using entangled photon pairs from a source based on spontaneous parametric down-conversion in PPLN waveguides. We have successfully demonstrated the operability of our setup in a field test together with Deutsche Telekom [1]. As a next step, we plan to miniaturize our setup employing a new generation of photon pair sources based on microring-resonators on integrated photonic chips (PIC).

For a thorough characterization of our setup it is important to characterize the detectors as well. We demonstrated a new method using detector tomography [2]. These results can be used in upcoming experiments to evaluate our results.

A QKD setup does also involve some software. To generate a secret key from the measurements we need to employ post-processing and are working on a variety of methods to improve this processing. For instance, we have analyzed the software for cache side-channels in collaboration with SFB CROSSING project E3 [3, 4].

References

1. Erik Fitzke et al. Scalable Network for Simultaneous Pairwise Quantum Key Distribution via Entanglement-Based Time-Bin Coding; PRX Quantum 3 (2 May 2022), p. 020341 (opens in new tab)
2. Fitzke, E., et al. Time-dependent POVM reconstruction for single-photon avalanche photo diodes using adaptive regularization; New Journal of Physics 24.2 (2022): 023025 (opens in new tab)
3. Weber, Alexandra, et. al. (2021): Cache-Side-Channel Quantification and Mitigation for Quantum Cryptography. 26th European Symposium on Research in Computer Security (ESORICS), virtual Conference, 04.-08.2021
4. Collaborative Research Center CROSSING (opens in new tab)

A future quantum computer will be able to solve certain complex problems such as the factorization of large numbers much faster than a regular computer. This leaves many current cryptographic schemes insecure. Specifically, the RSA algorithm which is used in today's everyday internet-communication will be rendered unsafe.

Fortunately, quantum mechanics provides a solution to this problem as well: Quantum Cryptography. An encrypted communication even sent over a classical channel can be proven to be absolutely safe if a random cryptographic key is used by both parties. This is true once the key is as long as the message and it is only used once (one-time-pad). The task at hand is to devise a means of distributing random keys in a secure manner over a quantum channel. Thus, quantum cryptography really means quantum key distribution (QKD). Many protocols exist that enable such a QKD. An example is the BB84 protocol.

The problem is that powerful sources of entangled photons or single photon sources are needed to produce single photons on demand. An understanding of the setup/implementation of a specific QKD-protocol is also necessary to avoid employing loopholes when transferring a theoretical model to an actual QKD-setup. Furthermore, every experiment induces a portion of errors for each measurement. Fortunately, these errors can be dealt with by introducing the so called post-processing which enables to generate the required secret key from the measurement data.

Our interests

• Generation of entangled photons in parametric downconversion and related schemes
• Single photon generation in doped photonic crystals
• Photon pair generation employing new methods e.g. integrated photonic chips
• Simulation of photon propagation in our setup
• Development of post-processing and management software for our setup and to generate secure keys in cooperation with the computer science department

As our standard approach we are working on photon pair sources based on PPLN waveguides.

Short-wavelength photons are converted by spontaneous parametric down conversion (SPDC) in a periodically poled lithium niobate waveguide into near-infrared photon pairs at telecom-compatible wavelengths. We are employing type-II and type-0 processes which produce the two photons in different polarization modes. They can be separated easily by a polarizing beam splitter or wavelength-division multiplexing. By splitting, both photons can be redirected to the key exchange parties involved, namely „Alice“ and „Bob“. From the detection time and the detectors involved, we can learn about the statistics involved to exchange a secure key in a standard phase-time coding protocol.

As a next step, we are working on photon pair sources via spontaneous four wave mixing on integrated photonic chips. Contrary to SPDC, we can start at near-infrared wavelengths right away, hence bypassing the need for short-wavelength photons. This technology allows the integration of the photon pair setup including filtering and separation of the photons, consequently leading to a much more compact and rigid design.

In the last couple of years, photonic chips (photonic integrated circuits) became a fast growing field of research. Much like electronic microchips, they offer the possibility of miniaturizing photonic components and setups. Building upon the same principle as glass fibers (internal total reflection) photonic chips utilize small waveguides (width about 1 μm) to guide the light in specific ways. Chips made up of certain materials possess different capabilities and may be used for different purposes, e.g., the waveguides can also be used to form geometric microstructures to realize small resonators and interferometers. Chips with silicon nitride as waveguide material are suitable for optical nonlinear processes that can be used to generate entangled photon pairs through spontaneous four wave mixing. Other chips, based on the semiconductor material indium-phosphide, allow the realization of lasers, amplifiers and fast modulators. With this, photonic chips offer a promising, scalable platform for implementing completely integrated photon sources for a wide range of applications. int the Laser and Quantum Optics group, we are conducting research on the design and characterization of such photonic chips. Ultimately, our goal is to replace the entangled photon source of our already existing QKD system with such photonic integrated circuits.