Entanglement-based quantum key distribution (QKD) promises enhanced robustness against eavesdropping and compatibility with future quantum networks. Among other sources, semiconductor quantum dots (QDs) can generate polarization-entangled photon pairs with near-unity entanglement fidelity and a multiphoton emission probability close to zero even at maximum brightness. These properties have been demonstrated under resonant two-photon excitation (TPE) and at operation temperatures below 10 K. However, source blinking is often reported under TPE conditions, limiting the maximum achievable photon rate. In addition, operation temperatures reachable with compact cryocoolers could facilitate the widespread deployment of QDs, e.g., in satellite-based quantum communication. We demonstrate blinking-free emission of highly entangled photon pairs from GaAs QDs embedded in a p-i-n diode. High fidelity entanglement persists at temperatures of at least 20 K, which we use to implement fiber-based QKD between two buildings with an average raw key rate of 55 bits / s and a qubit error rate of 8.4%. We are confident that by combining electrical control with already demonstrated photonic and strain engineering, QDs will keep approaching the ideal source of entangled photons for real world applications.
Quantum key distribution (QKD) relying on single photons is regarded as one of the most mature quantum technologies.1,2 However, the impossibility of amplifying single photons sets restrictions on the transmission distance. Entanglement-based QKD schemes are able to overcome these range limitations when embedded in quantum networks,3,4 while also exhibiting a lower vulnerability to eavesdropping attacks.1,5–8 For both fiber-based9 and satellite-based10 quantum cryptography, the most prominent sources of entangled photon pairs to date are based on the spontaneous parametric downconversion (SPDC) process. These sources are commercially available and can be operated in a large temperature range.11 As a drawback, SPDC sources exhibit approximately Poissonian photon pair emission characteristics,12 which severely limits their brightness when a high degree of entanglement—and thus a low qubit error rate (QBER)—is demanded. The biexciton–exciton (XX-X) spontaneous decay cascade in epitaxially grown semiconductor quantum dots (QDs) has been demonstrated to be a viable alternative to SPDC sources due to the sub-Poissonian entangled photon pair emission statistics.13 In particular, GaAs QDs obtained by the Al droplet etching technique14 are capable of emitting polarization-entangled photon pairs with a fidelity to the Bell state beyond 0.98,15,16 owing to an intrinsically low exciton fine structure splitting (FSS),17 a low exciton lifetime of about 230 ps, and a near-zero multiphoton emission probability even at maximum brightness.18 This allowed the demonstration of entanglement-based QKD with a QBER as low as 1.9%.16,19