Quantum Cryptography

Quantum Key Distribution

In the past, quantum mechanics has helped us deepen our understanding of the microscopic world and has made a multitude of new and exciting progress possible. Examples for these developments are the laser, superconductivity, and ferro-magnetism just to name a few.

Part of the photon pair source with illuminated fiber of the fiber amplifier
Part of the photon pair source with illuminated fiber of the fiber amplifier

Secure communication with light particles

Researchers at TU Darmstadt are developing an anti-eavesdropping quantum network

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Crystal for generation of second harmonic laser light
Crystal for generation of second harmonic laser light

Recently the ability to prepare and control quantum systems has rapidly evolved. For the first time, true applications of quantum mechanics are feasible, i.e. applications based on specific quantum mechanical features such as entanglement and superposition. The overall field of quantum information processing has emerged with topics such as quantum teleportation, quantum computing and quantum cryptography. A part of the field of quantum cryptography is quantum key distribution (QKD), a method of generating and distributing a key which subsequently can be used for classical symmetrical cryptography protocols. QKD uses properties of quantum mechanics to distribute a tap-proof key to the involved parties. Well-chosen combinations of QKD with symmetrical cryptography protocols allow a secure communication based on the laws of quantum mechanics

Free space setup with various filters for photon filtering
Free space setup with various filters for photon filtering

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].


  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)
Crystal for generation of second harmonic laser light
Crystal for generation of second harmonic laser light

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 an 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
Free space setup with waveguide for SPDC photon pair generation
Free space setup with waveguide for SPDC photon pair generation

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.

During our work on our QKD projects we have put together a software package available under an open source licence.

Purpose of the software:

  1. Read quantum bits from a detector emplyong an FPGA
  2. Extract the sifted key from the raw key
  3. Employ (classical) error correction on the sifted key
  4. Employ (classical) privacy amplification

Package (1) is based on public domain software by Polyakov. (opens in new tab)

Package (3) is based on the LDPC codes (opens in new tab) (Low Density Parity Check codes) by Neal and (4) uses Toeplitz matrices.

The Technical Report (available here) (opens in new tab) describes the parts (3) and (4) in more detail. The software was put together within the framework of a Bachelor Thesis by Pascal Notz.

The whole package is under an open source license and can be downloaded on gitHub (see Documentation for Link).

CROSSING Collaborative Research Center 1119

P4 Quantum Key Hubs (opens in new tab)

The security of Quantum Key Distribution (QKD) is based on laws of nature. The overall goal of this project is to develop a photon-based QKD structure in a star like topology and to comprehensively explore its fundamental quantum features. The individual goals are to implement wavelength-division multiplexing in a QKD structure including a field test in a real telecom network to determine the multipartite statistical correlations of the realized QKD structure and to investigate the scalability of the QKD structure beyond four parties.