Optical quantum-communication networks require the development of practical technology, particularly sources of entangled photon pairs, to mitigate the impact of photon loss on error rates in long-distance transmission. Sources generally do not operate at telecom wavelengths, are incompatible with existing fiber networks, or suffer from classical photon statistics---a potential threat to security. By using a semiconductor quantum dot, the authors demonstrate a quantum relay that is compatible with standard telecom infrastructure and at the same time intrinsically secure.
Optical quantum-communication networks require the development of practical technology, particularly sources of entangled photon pairs, to mitigate the impact of photon loss on error rates in long-distance transmission. Sources generally do not operate at telecom wavelengths, are incompatible with existing fiber networks, or suffer from classical photon statistics---a potential threat to security. By using a semiconductor quantum dot, the authors demonstrate a quantum relay that is compatible with standard telecom infrastructure and at the same time intrinsically secure.
One promising technology expected to enable long-haul quantum communication networks with untrusted nodes are quantum relays. Their most practical implementation requires an entanglement source with operation at telecom wavelength and intrinsic single photon character. Here, we use a semiconductor quantum dot emitting in the O-band to demonstrate for the first time a system fulfilling both of these criteria. For implementation of a standard 4-state QKD-protocol with weak coherent input states, the system achieves mean fidelities above 88%. Further characterization of the relay with process tomography reveals teleportation for arbitrary input states. The results represent a significant advance in demonstrating feasibility of semiconductor light sources for the development of infrastructure-compatible quantum-communication technology for multi-node networks.
For quantum light sources in everyday telecommunication networks, quantum science needs to be fully transformed into quantum technology. The first necessary step to move outside a well-controlled lab environment requires the use of quantum light sources operating in the technologically relevant telecom O- and C-band. This can be provided using epitaxial quantum dots as deterministic sources of quantum light. Particularly intriguing is that emitters operating at telecom wavelengths are rapidly catching up with their short wavelength counterparts in terms of performances. Here, we make a decisive step forward in the development of quantum communication networks: a state-of-the-art source of quantum light, a semiconductor quantum dot (QD), with record coincidence rate for entangled photon emission in the telecom C-band, is operated inside an optimized rack-based setup. This setup includes a tunable pulsed laser for the QD excitation (from quasi- to fully resonant excitation), all optics for the excitation filtering, and QD signal coupling into single-mode fibers. Overall, the setup allows for above 50% transmission for both exciton and biexciton photons. These results show that quantum dots-based telecom light sources can now be transported and integrated into existing fiber infrastructures, an important step to demonstrate the feasibility of the upcoming quantum internet.
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.
Semiconductor quantum dots (QDs) are well established as sub-Poissonian sources of entangled photon pairs. To improve the utility of a QD light source, it would be advantageous to extend their emission further into the near infrared, into the low absorption wavelength windows utilised in long-haul optical telecommunication. Initial experiments succeeded in interfering O-band (1260—1360 nm) photons from an InAs/GaAs QD with dissimilar photons from a laser, an important mechanism for quantum teleportation. Interference visibilities as high as 60 ± 6 % were recorded, surpassing the 50 % threshold imposed by classical electrodynamics. Later, polarisation-entanglement of a similar QD was observed, with pairs of telecom-wavelength photons from the radiative cascade of the biexciton state exhibiting fidelities of 92.0 ± 0.2 % to the Bell state. Subsequently, an O-band telecom-wavelength quantum relay was realised. Again using an InAs/GaAs QD device, this represents the first implementation of a sub-Poissonian telecom-wavelength quantum relay, to the best knowledge of the author. The relay proved capable of implementing the famous four-state BB84 protocol, with a mean teleportation fidelity as high as 94.5 ± 2.2 %, which would contribute 0.385 secure bits per teleported qubit. After characterisation by way of quantum process tomography, the performance of the relay was also evaluated to be capable of implementing a six-state QKD protocol. In an effort to further extend the emitted light from a QD into the telecom C-band (1530—1565 nm), alternative material systems were investigated. InAs QDs on a substrate of InP were shown to emit much more readily in the fibre-telecom O- and C-bands than their InAs/GaAs counterparts, largely due to the reduced lattice mismatch between the QD and substrate for InAs/InP (~3 %) compared to InAs/GaAs (~7 %). Additionally, to minimize the fine structure splitting (FSS) of the exciton level, which deteriorates the observed polarisation-entanglement, a new mode of dot growth was investigated. Known as droplet epitaxy (D-E), QDs grown in this mode showed a fourfold reduction in the FSS compared to dots grown in the Stranski-Krastanow mode. This improvement would allow observation of polarisation-entanglement in the telecom C-band. In subsequent work performed by colleagues at the Toshiba Cambridge Research Labs, these D-E QDs were embedded in a p-i-n doped optical cavity, processed with electrical contacts, and found to emit entangled pairs of photons under electrical excitation. The work of this thesis provides considerable technological advances to the field of entangled-light sources, that in the near future may allow for deterministic quantum repeaters operating at megahertz rates, and in the further future could facilitate the distribution of coherent multipartite states across a distributed quantum network.