Stuttgart runs entanglement-based QKD from a quantum dot over a deployed campus fiber

A Stuttgart-led team generated 14.2 Mbit of secure key over a 1.59 km campus fiber using entangled photons from a quantum dot, converted to the telecom band.

A team led by the University of Stuttgart has run entanglement-based quantum key distribution (QKD) over an installed fiber link using photons from a semiconductor quantum dot, rather than the attenuated lasers or nonlinear crystals that most field systems rely on. In a preprint titled Deployed entanglement-based BBM92 quantum key distribution using frequency-converted photons emitted by a quantum dot, posted on October 27, the 17 authors report a secure key rate of 206 Hz sustained over 19 hours across the Stuttgart-Vaihingen campus.

What was announced

According to the abstract, the link spanned 1.59 km of field-deployed fiber and produced 14.2 Mbit of secure key in total, which the team used to encrypt and decrypt a colour image. Quantum Computing Report says the collaboration also involved Saarland University and Leibniz IFW Dresden, and describes the image encryption as a one-time pad.

The technical details

The source is a droplet-etched gallium arsenide quantum dot that emits polarization-entangled photon pairs near 780 nm, driven at a 1 GHz excitation rate. Because that wavelength travels poorly in standard telecom fiber, the team used quantum frequency conversion to shift one photon of each pair into the telecom range while keeping the polarization entanglement intact. QCR reports the conversion took photons from 779.8 nm to 1515 nm in a Sagnac-type converter with up to 50% internal efficiency.

Other figures reported by QCR:

a fidelity of 91.4% to the target Bell state, within a 275-picosecond coincidence window;
total loss of 1.09 dB on the fiber link, which connects a lab to a receiver rack elsewhere on campus with motorized polarization tracking;
a raw key rate of 504 Hz and a secure rate of 206 Hz, with an average quantum bit error rate of 4.70%, below the 11% limit for the BBM92 protocol;
error correction with 5,000-bit LDPC blocks and privacy amplification by Toeplitz hashing.

QCR says hourly polarization checks with motorized waveplates kept the link stable through the 19-hour run, with small throughput dips traced to thermal drift in a narrowband filter.

Why it matters

Quantum dots can emit photons on demand, which avoids a basic trade-off of laser- and crystal-based sources between brightness and unwanted multi-photon emissions. Their drawback has been wavelength. The authors say the experiment shows that a bright entangled-photon source running at high rates, an efficient frequency converter and picosecond-level time synchronization can work together on a real deployed network, a step toward hybrid quantum networks.

What to watch

The results are a preprint and not yet peer reviewed. The distance is campus scale. The next questions are whether the approach holds up over metropolitan distances and multi-node networks, and whether secure key rates can rise as source and converter efficiencies improve.