Novel nanophotonic chips for encrypted data transfer: Quantum communication

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Three scientists involved in the QuPad project: Martin Wolff and Jonas Schütte, Three scientists involved in the QuPad project: Martin Wolff and Jonas Schütte, both members of the research group led by Carsten Schuck, and Wolfram Pernice (from left) © WWU - Peter Leßmann
A giant cylindrical refrigerator, an electron-beam pattern generator, a cleanroom, etching equipment. Sometimes it takes a lot of big things to make something very small. The nanoscientists at the University of Münster headed by Prof. Wolfram Pernice and Prof. Carsten Schuck know this only too well: they use these and other devices to produce nanophotonic chips the size of a one-cent piece. These small chips, which also contain much smaller functional structures, can do a great deal - for example, contribute to the tap-proof transmission of data.

Researchers at the University of Münster are currently working on developing a so-called quantum cryptographic process using these chips - a process that uses quantum information in the form of light particles to transmit data in encrypted form. Such transmission systems will become relevant at the latest when it is possible to develop a quantum computer that can attack today’s encryption methods. The Federal Ministry of Education and Research (BMBF) is funding the project, “Ultrafast quantum key distribution by parallelization of detection channels”, or QuPad for short, with 2.2 million euros, with the project, which began last November, running for at least two years. Cooperation partners in the project are the technology companies PicoQuant from Berlin and Entropy from Munich.

“The aim of the project is to look for new possibilities for secure data transmission, i.e. those that offer security based on the laws of quantum mechanics - and not by algorithms, as is the case traditionally”, says Wolfram Pernice, group leader at the Institute of Physics and the Center for Nanotechnology (CeNTech) at Münster University. This is what the scientists wish to achieve with novel detectors located on the chips. These detectors consist of tiny nanowires, only a few nanometres thin. The researchers place the nanowires on optical waveguides that are many times thinner than glass fibres and can be produced in large numbers on a chip. Thus, the detectors can ultimately register and transmit light quanta. The aim of the researchers is to work on as many detection channels as possible in parallel, with 64 channels being planned. “If this parallelization works, then we can achieve both a higher encryption rate and a higher speed with regard to data transmission”, says Carsten Schuck, junior research group leader at the CeNTech. The production of these types of chips is unique in Germany.

New refrigerator for the SoN

The nanowires can only fulfil their function as detectors when they become superconducting, i.e. when they are exposed to extreme cold. This requires a special refrigerator, called a cryostat, in which the temperature is below minus 271 degrees Celsius. Cables inside the cryostat are used to communicate with the detectors electronically. Whoever imagines that the refrigerator looks like a conventional kitchen appliance will be disappointed: it is a kind of barrel that has around it some additional appliances - including a compressor that pumps helium through the cryostat, a vacuum system, and a second barrel that collects the excess helium. The company Entropy is currently in the process of developing the cryostat, and the impressive device is expected to be installed in the Center for Soft Nanoscience (SoN) at the University of Münster in May.

Carsten Schuck’s research group is working in parallel on growing the incredibly thin layers for the nanowires, with the researchers producing these layers in a cleanroom - if even tiny particles of dirt were to come into contact with the fine structures, then the wires would no longer function. The task of Wolfram Pernice’s research group is to connect the nanowires to modern fibre optic networks. This requires nanophotonic methods to bring the light from the glass fibres to the detectors on the chip. The work of the two research groups is closely interlinked. The company PicoQuant is developing the appropriate electronics so that the nanowires can function properly.

Once the chips are ready for an initial test phase, they are sent in a case to the Fraunhofer Institute for Telecommunications in Berlin, where there is a transmission link for such encryption methods. The scientists there then test between two buildings whether secure data transmission works, with a weak laser being directed at an antenna that collects the light.

The researchers led by Wolfram Pernice and Carsten Schuck hope that the nanochips will later be used in quantum communication - primarily in encryption systems, but in the long term also in the development of a quantum computer itself. These types of chips could also be relevant for many other applications, such as biomedical imaging, where very small structures need to be made visible under the microscope.