Design study for an efficient semiconductor quantum light source operating in the telecom C-band based on an electrically-driven circular Bragg grating.

The development of efficient sources of single photons and entangled photon pairs emitting in the low-loss wavelength region around 1550 nm is crucial for long-distance quantum communication. Moreover, direct fiber coupling and electrical carrier injection are highly desirable for deployment in compact and user-friendly systems integrated with the existing fiber infrastructure. Here we present a detailed design study of circular Bragg gratings fabricated in InP slabs and operating in the telecom C-band. These devices enable the simultaneous enhancement of the X and XX spectral lines, with collection efficiency in numerical aperture 0.65 close to 90% for the wavelength range 1520 - 1580 nm and Purcell factor up to 15. We also investigate the coupling into a single mode fiber, which exceeds 70% in UHNA4. Finally, we propose a modified device design directly compatible with electrical carrier injection, reporting Purcell factors up to 20 and collection efficiency in numerical aperture 0.65 close to 70% for the whole telecom C-band.

1GHz clocked distribution of electrically generated entangled photon pairs.

Quantum networks are essential for realising distributed quantum computation and quantum communication. Entangled photons are a key resource, with applications such as quantum key distribution, quantum relays, and quantum repeaters. All components integrated in a quantum network must be synchronised and therefore comply with a certain clock frequency. In quantum key distribution, the most mature technology, clock rates have reached and exceeded 1GHz. Here we show the first electrically pulsed sub-Poissonian entangled photon source compatible with existing fiber networks operating at this clock rate. The entangled LED is based on InAs/InP quantum dots emitting in the main telecom window, with a multi-photon probability of less than 10% per emission cycle and a maximum entanglement fidelity of 89%. We use this device to demonstrate GHz clocked distribution of entangled qubits over an installed fiber network between two points 4.6km apart.

Long-term transmission of entangled photons from a single quantum dot over deployed fiber.

Entangled light sources are considered as core technology for multiple quantum network architectures. Of particular interest are sources that are based on a single quantum system as these offer intrinsic security due to the sub-Poissonian nature of the photon emission process. This is important for applications in quantum communication where multi-pair emission generally compromises performance. A large variety of sources has been developed, but the generated photons remained far from being utilized in established standard fiber networks, mainly due to lack of compatibility with telecommunication wavelengths. In this regard, single semiconductor quantum dots are highly promising photon pair sources as they can be engineered for direct emission at telecom wavelengths. In this work we demonstrate the feasibility of this approach. We report a week-long transmission of polarization-entangled photons from a single InAs/GaAs quantum dot over a metropolitan network fiber. The photons are in the telecommunication O-band, favored for fiber optical communication. We employ a polarization stabilization system overcoming changes of birefringence introduced by 18.23 km of installed fiber. Stable transmission of polarization-encoded entanglement with a high fidelity of 91% is achieved, facilitating the operation of sub-Poissonian quantum light sources over existing fiber networks.

Experimental protocol for high-fidelity heralded photon-to-atom quantum state transfer.

A quantum network combines the benefits of quantum systems regarding secure information transmission and calculational speed-up by employing quantum coherence and entanglement to store, transmit and process information. A promising platform for implementing such a network are atom-based quantum memories and processors, interconnected by photonic quantum channels. A crucial building block in this scenario is the conversion of quantum states between single photons and single atoms through controlled emission and absorption. Here we present an experimental protocol for photon-to-atom quantum state conversion, whereby the polarization state of an absorbed photon is mapped onto the spin state of a single absorbing atom with >95% fidelity, while successful conversion is heralded by a single emitted photon. Heralded high-fidelity conversion without affecting the converted state is a main experimental challenge, in order to make the transferred information reliably available for further operations. We record >80 s(-1) successful state transfer events out of 18,000 s(-1) repetitions.