Deployed entanglement-based BBM92 quantum key distribution using frequency-converted photons emitted by a quantum dot

Abstract

Establishing secure communication is an important requirement in existing and future networks. Although current solutions rely on classical approaches, quantum mechanics offers the possibility of moving from computational to information-theoretic security. Quantum key distribution (QKD) has been extensively studied as a means of achieving such security. Recently, epitaxial quantum dots started to be studied and employed in quantum cryptography, because of their rapid development that made them one of the brightest sources of quantum light to date. In particular, they are known to be excellent sources of polarization-entangled photon pairs, especially droplet-etched dots operating at $\sim$780 nm. Here, we employ such a quantum dot to generate entangled photon pairs under 1 GHz excitation rate. Using quantum frequency conversion, we interface the emitted photons with the telecommunication wavelength range while preserving their polarization entanglement. This enables quantum key distribution over a 1.59 km field-deployed fibre link across the University of Stuttgart - campus Vaihingen, using the BBM92 protocol: we achieve a secure key rate of 206 Hz over 19 h of stable operation, resulting in a total secure key length of 14.2 Mbit. This key enables the safe encryption and decryption of a colour image with further implementation of error correction and privacy amplification. These results demonstrate important steps towards hybrid quantum networks by combining high-quality entangled-photon sources operated at high excitation rates, efficient frequency conversion module, and picosecond-level time synchronization in a realistic deployed network.

The paper tests whether a quantum-dot entangled-photon source can support BBM92 QKD over deployed fibre when one photon is frequency-converted to telecom wavelengths. It combines a GaAs quantum dot under $1\,\mathrm{GHz}$ resonant two-photon excitation with a polarization-preserving Sagnac frequency converter, sending the converted biexciton photon from $779.79\,\mathrm{nm}$ to $1515\,\mathrm{nm}$ while measuring the exciton photon locally. The deployed link spans $1.59\,\mathrm{km}$ with $1.09\,\mathrm{dB}$ loss; remote timing synchronization has uncertainty below $25\,\mathrm{ps}$. The converter reaches up to $50\%$ efficiency. A $275\,\mathrm{ps}$ coincidence window optimizes the trade-off between accepted pairs and errors from fine-structure-induced phase evolution and conversion noise; the reported Bell-state fidelity for this window is $91.4\%$. During $19$ hours of operation, the average sifted raw-key rate was $504\,\mathrm{bits/s}$ and QBER was $4.7\%$. LDPC error correction on $5000$-bit blocks and Toeplitz-hash privacy amplification yielded a reported average secure-key rate of $206\,\mathrm{bits/s}$ and $14.2\,\mathrm{Mbit}$ total. The key-rate calculation uses the binary-entropy privacy term and measured error-correction leakage. The resulting key was used for one-time-pad image encryption. Reported limitations include increasing QBER for wider coincidence windows, conversion-generated background, and wavelength drift that gradually reduced transmission through a $25\,\mathrm{GHz}$ spectral filter. Fibre polarization drift was minor during this run, with hourly monitoring and compensation when needed. The demonstration covers a short, low-loss deployed link; higher rates and longer distances are identified as future directions.