The Signal
Stuttgart Team Runs Quantum-Dot QKD for 19 Hours on Campus Fibre
A University of Stuttgart team reports entanglement-based quantum key distribution over 1.59 km of deployed campus fibre. The 19-hour test produced 14.2 megabits of secure key using photons frequency-converted from a quantum dot.
Quantum.world Team · Written with AI · fact-checked ·
A quantum-dot source helped generate 14.2 megabits of secure key during a 19-hour quantum key distribution test on installed fibre at the University of Stuttgart. The team’s preprint, submitted to arXiv on October 27, describes a 1.59-kilometre link across the university’s Vaihingen campus using entangled photons and the BBM92 protocol.
The result combines a semiconductor source emitting entangled photon pairs with a frequency converter that shifts one photon into the telecom band for transmission over standard fibre. The researchers report an average secure key rate of 206 bits per second and used the processed key to encrypt and decrypt a colour image with a one-time pad.
From a 780-nanometre source to telecom fibre
The experiment starts with a droplet-etched gallium arsenide quantum dot that emits polarization-entangled photon pairs at wavelengths near 780 nanometres. The dot runs at a 1-gigahertz excitation rate. A frequency-conversion module shifts one photon from each pair to 1,515 nanometres, while preserving polarization entanglement; the paper reports conversion efficiency of up to 50 percent.
The converted photon travels over the deployed link to a second campus location. The two stations are about 700 metres apart in a straight line, while the fibre route spans 1.59 kilometres and has a reported total loss of 1.09 decibels. Alice’s equipment remained in a laboratory; Bob’s measurement apparatus and detectors sat in a standard 19-inch rack in an office.
“The QBER shows good stability throughout the entire measurement period, demonstrating the high stability of the deployed setup,” write Michal Vyvlecka et al. of the University of Stuttgart and collaborating institutions. QBER, or quantum bit error rate, measures the fraction of mismatched bits before error correction.
Stability relied on active control and post-processing
Fibre polarization can drift as environmental conditions change, so the system monitored the link and used motorized wave plates to compensate when needed. The researchers found that the drift was minor during the trial and checked it once per hour. They also distributed an optical frequency reference between the sites, achieving timing uncertainty below 25 picoseconds, and used the university’s local network for classical communication.
The team reports an average QBER of 4.7 percent across the 19-hour run. In a separate optimization, a 275-picosecond coincidence window yielded a maximum calculated secure key rate of 204 bits per second, with a 4.71 percent QBER. That setting-specific figure differs from the 206-bit-per-second average reported for the full run.
Researchers converted more than 34.5 megabits of raw key into 14.2 megabits of secure key through error correction and privacy amplification, an overall retention efficiency of 40.87 percent. The paper describes the secure-key calculation and processing, including error-correction blocks of 5,000 bits. Its image demonstration shows the practical outcome of that processing: decryption with the secure key removed protocol errors visible when using the raw key.
A campus trial, not a long-distance network
The demonstration tests an important engineering link between quantum-dot sources and existing telecom fibre, but its scope is specific: a short campus route, with a source in a controlled laboratory and a receiving station in an office. The work does not establish operation over intercity distances or show a production-ready service. Its contribution is sustained operation of an entanglement-based setup using frequency-converted quantum-dot photons on installed fibre.
The paper is a preprint, rather than a peer-reviewed journal publication. The authors point to higher-brightness entangled-photon sources as a possible route to higher key rates and longer deployed links. In this experiment, the measured outcome is narrower and concrete: 19 hours of operation, 1.59 kilometres of fibre, and 14.2 megabits of processed secure key.