The objective is to increase the efficiency of the micropillar SPS design beyond 98% extraction efficiency by exploiting suppression of the background emission. 

Expected results include (i) control of the background emission in a micropillar using a photonic bandgap effect resulting from concentric rings, (ii) a novel micropillar SPS design with ridges with extraction efficiency to a fiber above 98 % and (iii) a novel nanopost design at 780 nm with extraction efficiency above 70 %.

Experimental demonstration of highly efficient SPSs

The objective of IRP2 is to fabricate and characterize highly efficient micropillar-based SPSs exploiting suppression of the background emission based on designs from IRP1. The devices will be optically characterized in terms of purity, efficiency and photon indistinguishability using resonant and dichromatic pumping schemes. 

Expected results include (i) Stark tuning of QD line and charge stabilization using electrical contacts, (ii) full QD population inversion >0.98 and (iii) pure single photon emission g(2)(0) < 0.01="" using="" dichromatic="" pumping,="" />(iv) demonstration of improved efficiency thanks to the background emission engineering and (v) demonstration of >0.99 indistinguishable photon emission.

The objective is an experimental study of single spin in a GaAs QD in a cavity with manipulation, control and spin-photon entanglement. 

Expected results include (i) a deterministically fabricated SPS based on a droplet GaAs QD in a cavity, (ii) an experimental study of the spin coherence time using phonon-assisted excitation, and (iii) demonstration of spin-photon entanglement (with DC14).

The objective is the development of a tuneable source of indistinguishable photons operating at 780 nm. Wavelength tuneability will be achieved by leveraging the strain generated by a planar piezoelectric actuator integrated below the nanopost. The tasks include the design, fabrication and optical characterization. 

Expected results include (i) design of a broadband AlGaAs nanopost cavity operating at 780 nm (in collaboration with DTU), (ii) fabrication of an AlGaAs nanopost cavity embedding a GaAs QD, (iii) demonstration of strain-tuning of the QD emission and (iv) demonstration of the efficient >70% emission of indistinguishable >90% photons at 780 nm.

The objectives are to model, design, grow, and assess charge-tuneable QD heterostructures through optical and electrical methods to establish quantitative thresholds for low charge noise and to develop QD growth techniques suitable for various photonic structures. QDs will be embedded in an n-i-p or n-i-n diode structures, enabling charge control and Stark tuning of the emission to photonic cavity resonance. Ensemble and single optical properties will guide optimization at RUB across high-electron-mobility heterostructures, quantum wells and tuneable QD devices. 

Expected results include (i) ultra-low-noise and low-density QD heterostructures deterministically coupled to thin electron reservoirs that are located in the standing wave antinodes of the anticipated photonic cavities, (ii) adaption of the QD emission wavelength to Rb memory cell, (iii) a methodology to calculate and determine an optimal tunnel coupling to a charge reservoir and (iv) a “sample compare” standard and clear roadmap on how to mitigate charge noise in MBE grown SPSs.

The objectives are design, fabrication and calibration of vacuum-sealed integrated photonic circuits directly written in glass by fs laser pulses. This achievement will be key to scale the number of phase shifters that can be integrated on a single chip without having to dissipate a large amount of heat. In addition, this result will reduce the thermal crosstalk, thus increasing the accuracy of the implemented quantum transformation. In parallel, we will also reduce the size of the integrated photonic circuits by reducing the waveguide pitch, hence aiming at a largely improved chip transmission. 

Expected results include (i) implementation of a vacuum-sealed packaging for integrated photonic chips, (ii) proof of a reduction of dissipated power, for a 2π phase shift, below 10 mW per thermal shifter and (iii) demonstration of a programmable photonic chip with at least 10 optical modes fully packaged with optical fibers and electrical connections with a total transmission >85%.

The objectives are to design, fabricate and test superconducting nanowire single-photon detectors (SNSPDs) with photon number resolving capability. It has been proposed that the risetime of the detection pulse of SNSPDs is photon number dependent. However, no readout schemes are currently available that are suitable for implementation in a quantum computer. The DC will work on developing readout schemes and electronics to measure directly the rise time of the detection pulse. Furthermore, the fidelity is highly dependent on the quantum efficiency for single photons, for example if the efficiency of 90%, the fidelity for a two-photon event is only 81%. Therefore, the DC will work on improving the quantum efficiency and bring it close to unity. For fault tolerant photonic quantum computers thousands, if not millions, single-photon detectors will be needed. Therefore, the DC will also work on establishing recipes for improving the yield of these high efficiency detectors. 

Expected results include (i) development of single-photon detectors with >95% efficiency at 900 and 1550 nm, (ii) development of wafer-scale fabrication techniques and recipes for detectors with >10% yield, (iii) demonstration of single-click photon number resolving measurements and (iv) new scientific insights through implementation in experiments. Experiments will involve quantum optical, quantum computational and correlative imaging experiments.

The objective is to define device performance requirements for applications using arbitrary photonic graph states. From this, we define device requirements as a function of network size, postselection probability, and graph-state structure.

Expected results include (i) use of stabilizer formalism and imperfection modelling to link component performance with success rate and fidelity of obtained graph states of different topologies, (ii) evaluation of performance of different computational tasks using photonic graph states and (iii) study of alternative physical architectures for fusion-based quantum computation and the corresponding required device performance.

The objective is development of protocols for secure delegated quantum computing adapted to discrete-variable photonic platforms. The possibility to generate spin-photon entanglement at the source offers new avenues to delegate computations from a client to a server. The project will require adapting the protocol to the characteristics of the hardware to make it as resource-efficient and secure as possible. 

Expected results include (i) improvement of a protocol for blind delegated quantum computing where the client manipulates an attenuated laser pulse, (ii) design of a protocol for secure delegated variational quantum algorithms (VQAs) and (iii) implementation of the protocol (in cooperation with DC14).

The objectives are exploration of existing and new optimized methods for design of secure photonic quantum circuits and certification of their functioning under noise and imperfections. We will adapt existing methods for the characterization of post-selected photonic circuits, and also for photonic graph (stabilizer) states. 

Expected results include (i) adapted zero-trust framework for the purpose 
of masking of photonic quantum computation protocols, (ii) improved generation of arbitrary graph states of a few qubits in a linear-optical platform, certifying their fidelity with an exploration of computational applications and (iii) adaptation of trap-based methods to account for experimental imperfections: limited photonic indistinguishability, photon loss, imperfections in circuit components.

The objectives are to develop efficient tomography protocols for partially distinguishable bosons and measures of bosonic indistinguishability. 

Expected results include (i) the development of more efficient protocols for tomography of photonic states prepared by linear optical interferometers and imperfect photon sources, (ii) the application to the characterization of unitary errors and particle distinguishability in photonic experiments, (iii) the connection of measures of photon indistinguishability to the complexity of simulation of linear optical experiments and (iv) the analysis of complexity of classical simulations of boson sampling with partial distinguishability and limited adaptivity.

The objective is to implement distributed quantum computing based on SPSs and integrated photonics. The DC will first be trained on the design of quantum photonic architecture, schemes for distributed quantum computing with a specific focus on the photonics platform and further developing advanced photonic platforms (comprising high efficiency photon sources, integrated circuits interfaced with single modes fibers with path-polarization converters and single-photon detectors). The DC will then work on the implementation of distributed quantum computing with up to three nodes.

Expected results include (i) assembly of an advanced photonic quantum computing platform distributed over 3 nodes (in cooperation with DC13), (ii) design of a blind quantum computing architectures with 3 nodes and (iii) implementation of the developed protocols.

The objective is to perform experimental demonstration of VQAs in an advanced photonic platform. The DC will be trained in the relevant theoretical (VQAs) and experimental skills (advanced photonic comprising high efficiency SPSs, integrated circuits and single-photon detectors) via secondments at three partners. The DC will then demonstrate the application of a variational approach to optimize the implementation of quantum computing primitive within the assembled system. 

Expected results include (i) assembly of an advanced photonic quantum computing platform (in cooperation with DC12), (ii) identification and optimization of variational quantum approaches to obtain a specifically tailored toolbox for photonic quantum computing apparata including noise models and (iii) implementation of the identified toolbox to control and program the assembled photonic platform for quantum computing applications.

The objective is to experimentally demonstrate quantum computing protocols with linear cluster states generated with InGaAs QDs operating at 925 nm. After working on maximizing the cluster state length and dimension as well as the photon indistinguishability, we will implement small scale quantum information processing protocols using the generated states. We will target both quantum communication protocols and quantum computing protocols. 

Expected results include (i) enhanced rate, fidelity and length photonic cluster states based on spin-photon or linear gates, (ii) experimental demonstration of a quantum communication protocol based photonic cluster states, (iii) demonstration of delegated and/or secure quantum computing protocol with photonic cluster states at 925 nm and (iv) exploration of quantum frequency conversion for long distance demonstrations.

The objective is to demonstrate on-demand storage and retrieval of photons from a QD SPS in an atomic quantum memory for applications in quantum networks. Based on prior works of the host with 90% storage-and-retrieval efficiency in a Rb large atomic ensemble, the quantum memory will be optimized to efficiently store the short temporal wavepacket of the single photons via control pulse shaping. 

Expected results include (i) efficient storage and on-demand read out of photons from a QD source in atomic quantum memory, (ii) demonstration that the single-photon purity of the photons retrieved from the memory is unchanged from the input and (iii) demonstration that the indistinguishability of the photons retrieved from the memory is higher than the input due to mode filtering.