Stable transmission of polarization-entangled photons from a quantum dot over metropolitan network

Quantum light sources based on single-photon emitters are a core technology, which is expected to enable a multitude of emerging quantum-photonic applications with superior performance. As the development of sources is... [ view full abstract ]

Quantum light sources based on single-photon emitters are a core technology, which is expected to enable a multitude of emerging quantum-photonic applications with superior performance. As the development of sources is approaching a certain level of maturity, the important next step for quantum-network related applications is to distribute the generated photons in the most practical way. However, so far these remained within research facilities or were only transmitted over short specially installed fiber. In this regard, operation of the sources at standard telecom wavelengths such that existing classical communication infrastructure can be used, seems the most desirable solution.

Here, we make use of a single InAs/GaAs quantum dot, emitting entangled photon pairs in the telecom O-band. One photon of a pair is sent over 18.23km of standard telecom fiber across the city of Cambridge. The entanglement fidelity with the partner photon is measured after detection of the photon back in the laboratory. We employ an independent feedback system to cancel changes in birefringence over the deployed fiber, enabling the stable transmission of polarization qubits with high duty cycle.

We demonstrate continuous operation of the system over 7 days with stable entanglement fidelity, resulting in a mean value of 91% over the entire measurement run (see Figure 1). Over this time, the fiber was naturally exposed to changing environmental conditions resulting in a change in the time-of-flight of photons by more than 1.8 ns.

The results show that quantum dot entangled photon pair emitters provide a long-term stable source of entanglement which is directly compatible with existing communication networks and therefore highly competitive in terms of simplicity. The use of polarization qubits might prove advantageous for certain network architectures as the detection of polarization states can be done with cheap passive components compared to the higher complexity involved when working with time-bin qubits.