I will give my personal perspective on recent progress in quantum networks and quantum communication, including some mentions of efforts towards distributed quantum computing. In this context of moving towards a global 'quantum internet', I will discuss some of our own recent work on quantum repeaters, quantum memories, quantum transducers, and sources of entanglement. I will discuss approaches to the quantum internet involving satellites. Finally I will also briefly talk about our work on the question whether there could be quantum networks in our brains.

Our team has an on-going quantum satellite network project in Canada called Quantum Encryption and Science Satellite (QEYSSat). I would like to introduce our polarization-based entangled photon source suitable for the satellite QKD. The idea of satellite based QKD came up to overcome the technical limitation of using fibers. We are aiming for our quantum source to sufficiently overcome the current distance limits. In order to achieve, my quantum source will have to meet the following three criteria; high pair production rate, narrow bandwidth of wavelengths and high stability. We require the minimum pair production rate for the satellite communication to be above 100 MHz (meaning 100 million photon pairs per second). Meanwhile, most telecommunication through the ground these days uses optical fibers which transports signals with 1550nm or 1310nm wavelengths. We have specifically chosen 791nm signal wavelength which can have 1550nm idler wavelength as its pair. Another focus was bandwidths of the photon wavelengths. A broad photon wavelength will require a large bandwidth filter to be used to detect the signal. This will result in our filter allowing more photon backgrounds which will reduce the accuracy of the measurements. The condition may become even worse in the daylight operations. After having conducted an extensive study in how a crystal changes its shape from thermal expansions caused by the pumped beam, or interactions with its surroundings, we have chosen the periodically poled lithium niobate crystal (PPLN), and experimentally confirmed its performance in the lab. Efficiency tests for the candidate source will soon be run outside the laboratory during the daytime to verify whether it meets the requirement. An entangled photon source with high stability and robustness can be achieved by improving the photon interferometer that is necessary to make the correlated photon pairs. Beam splitters are one of the optical devices commonly used to make such interferometers. The disadvantage of beam splitters that it is difficult to separate the pair with that much of large separation in the wavelengths (which is the wavelength difference between 791nm and 1550nm). Our team has come up with an alternative interferometer design which uses two beam displacers instead of the beam splitters, and its performances have been verified. Ground-to-ground EB QKD has already been well established, and now we are about to make a next huge step by applying it to a satellite networking system.

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The quantum interconnect (QuIC) is critical for scalable quantum ecosystems. An ideal QuIC should contain: a quantum memory, an entanglement source and a quantum transducer. With the development of these functionalities, current leading qubit technologies, such as superconducting and photonic qubits, will become viable and a hybrid approach to quantum computing can be taken, solving the current scalability crisis. We here describe our approach using an optical quantum memory and optically-heralded distribution of microwave entanglement based on a ladder-type two photon optical process and coupling to electron spin sublevels in an ensemble of Er ions in a stoichiometric host crystal.

The ability to distribute entanglement across a quantum network may lead to new capabilities like teleporting information over a difficult-to-access quantum channel or higher resolution quantum sensors. However, there are many outstanding challenges to realizing such a quantum network. One of these challenges is how to interface disparate quantum technologies effectively and efficiently. Ultimately a quantum network will be used to connect different types of devices, much like the current internet does today, and this requires one to seamlessly connect qubit technologies operating in vastly different environments. This talk will present results on interfacing trapped ion qubits to quantum integrated photonic circuits and discuss challenges related to interfacing trapped ion qubits with superconducting qubits.

Quantum networking is upon us, which brings unforeseen rewards and challenges. One such challenge is the coexistence of quantum and classical information in a single standard fiber. Indeed, most quantum networking protocols rely on auxiliary classical communication be it synchronization or a “public channel” traffic – a classical link that operates contemporarily with the quantum link. I will talk about the coexistence limits due to classical-quantum crosstalk and introduce a concept of classical communication via a quantum channel and with the help of quantum measurement. I will show how quantum measurement can improve energy-per-bit requirements and introduce the first quantum-enabled error correction protocol.

Photon-number-revolving detectors are the ultimate measurement tool of light. However, few detectors to date can provide high-fidelity photon number resolution at few-photon levels. Here, we demonstrate an on-chip detector that can resolve up to 100 photons by spatiotemporally multiplexing an array of superconducting nanowires along a single optical waveguide. The unparalleled photon number resolution paired with the high-speed response exclusively allows us to unveil the quantum photon statistics of a true thermal light source in an unprecedented level, which is realized by direct measurement of higher-order correlation function g^(N) with N up to 15 and the observation of photon-subtraction-induced photon number enhancement. Our detector provides a viable route towards various important applications, including photonic quantum computation and quantum metrology.

We perform coherent population trapping (CPT) experiments to measure the hyperfine coupling of a single 73GeV to be A∼37 MHz. Furthermore, we identify a rapid nuclear spin diffusion process that occurs when performing CPT and leverage it to realise all-optical, arbitrary nuclear state preparation at timescales approaching ∼1 μs. This is a fundamental step in gaining access to the otherwise difficult to address nuclear spin qudit and paves the way for future research regarding the 73GeV.

Optically active point defects in the wide-bandgap semiconductors, diamond and SiC, are of interest as solid-state qubits for quantum photonics and metrology. The negatively charged silicon-vacancy (VSi) point defect in the 4H polytype of SiC consisting of a vacancy on a silicon site in SiC is a prominent defect qubit that has attractive features such as single-photon emission and long spin coherence times relevant for magnetic and temperature sensors, and single photon emitters. This work investigates ion irradiation protocols for the generation of defect qubits and their addressability by optical techniques of photoluminescence spectroscopy and optically detected magnetic resonance.

Bell’s nonlocality—correlations between two distant, entangled particles that cannot be explained by physical theories based on local causality—remains one of the most celebrated results of quantum foundations. However, these nonlocal correlations eventually disappear under the presence of noise. In this experimental work, we show that nonlocality, inaccessible for noisy states in the two-party scenario, can be activated in a three-node photonic network structure with a single copy of the state. Our results open up new possibilities for network-based quantum information processing applications, central for the development of a future quantum internet.

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We study the fluctuations of photon currents in a non-linear optical model. These are found to exhibit very exhotic on/off switching behavior as a consequence of the quantum features of the model. We use the tools of full counting statistics to explain why this switching behavior leads to an exponential growth of fluctuations.

The development of on-chip, CMOS-compatible quantum photonics is critical for future scalable quantum communications, quantum computing, and quantum sensing. Integrated photonic waveguides, photonic resonators, and single-photon emitters are essential building blocks for such a purpose. In this talk, I will present how machine learning (ML) can enhance the quantum properties of these building blocks, specifically the indistinguishability (I) of the generated single photons, with a further decrease in quantum decoherence.

A single-photon source is presented using a single quantum dot in an open microcavity. The performance metrics are promising for applications in quantum technology. In particular, the coherence of the photons is high (two-photon interference visbility 98%) and is maintained over long strings of photons. The end-to-end efficiency, the probability of creating a photon at the output of the device's final optical fibre, is above 50%.

In this work we demonstrate a record optical power of single-photon flux measured on a SI-traceable trap detector. This flux was measured using a single-mode fiber, allowing for the absolute calibration of a superconducting nanowire detector. Single-photons from an InGaAs quantum dot in a micropillar cavity were determined to have an optical power of 1.06±0.03pW or 4.95±0.14 ×10^6 photons per second as measured on the calibrated trap detector. The single-photon fluorescence at this power exhibited a g(2)(τ=0) of 0.13.

In this talk, I will discuss how photons generated by a semiconductor quantum dot can be used to implement quantum teleportation, entanglement swapping, and quantum key distribution protocols.

Quantum dots (QDs) embedded inside indium-phosphide (InP) nanowires have the potential to be bright, on-demand sources of perfect polarization-entangled photon pairs fabricated with near-unity yields. However, to date very high degrees of entanglement have not yet been measured from such devices. By performing quantum state tomography with state-of-the-art superconducting nanowire single-photon detectors (SNSPDs) and two-photon resonant excitation of the QD, we show that these sources are indeed capable of producing near-unity entangled photon pairs. We measure a raw peak fidelity of 97.5% +/- 0.8% and a lifetime-weighted fidelity of 0.94% +/- 0.04%. These results conclusively demonstrate that the majority of the degradation from unity-measured entanglement fidelity in earlier studies was not due to spin-spin dephasing from the large 9/2 nuclear spin of indium. These results solidify InP nanowire QDs as a promising platform for future quantum photonics applications.

Entangled photons are an important resource for quantum optics. Quantum dots are a source of on-demand and highly entangled photon pairs at a high repetition rate. However, fine structure splitting (FSS) in the biexciton-exciton cascade causes the photons to be emitted in a time-dependant state instead of an ideal Bell state. Current techniques to remove the FSS include applying a strain, electric, or magnetic field and require post-processing of the quantum dots which reduces device yield. We use a novel all-optical approach implemented by emulating a fast-rotating half-wave plate in a Lithium Niobate waveguide using a electro-optic modulator. This method allows us to frequency shift single photons and produce a time-independant entangled photon.

In this presentation I will describe a new laser driving scheme for triggered high-brightness single photon emitters using a chirped laser pulse possessing a spectral hole resonant with the transition energy of the emitter, referred to as Notch-filtered adiabatic rapid passage (NARP). When combined with optimized photonic structures for enhanced collection efficiency and commercial filters we estimate that NARP would provide less than 10-8 scattered photons per emitted photon with a 4% detection loss together with resonant driving for high photon indistinguishability. We demonstrate inversion using NARP experimentally in a single semiconductor QD.

In this research, we presents a novel design for an all-electrical single photon emitter that utilizes a single electron pump and a lateral p-n junction based on an AlGaAs/GaAs heterostructure. The fundamental promise of single photon emission is achieved by injecting one and only one electron into the p-n junction, where one photon is generated after e-h radiative recombination. This ensures an intrinsically on-demand and deterministic single photon source. Up to GHz repetition rate is expected given the single electron pump has demonstrated quantized generation of electrons in the GHz range. We will present some promising stable EL emission after overcoming the charge accumulation problem in our dopant-free architecture.

We will discuss how to realize QDs based on the well-developed GaAs-platform capable of emitting in the telecom O- and C-bands [1]. Advanced nanofabrication techniques for the realization of optical resonators utilized to greatly enhance the source brightness will be discussed [2,3]. In(Ga)As QDs operating in the telecom C-band and integrated into circular Bragg grating cavities will be shown. The Purcell-enhanced sources reached a fibre-coupled single-photon count rate of 13.9 MHz (excitation rep. rate: 228 MHz, first-lens collection efficiency ~17%), with a multi-photon contribution as low as g(2)(0) = 0.0052. Operation at elevated temperature will be demonstrated [4]. [1] S. L. Portalupi, et al. Sci. Technol. 34, 053001 (2019). [2] M. Sartison, et al., Appl. Phys. Lett. 113, 032103 (2018). [3] S. Kolatschek, et al., Nano Lett. 21, 7740 (2021). [4] C. Nawrath et al., arXiv:2207.12898 (2022).

Reconfigurable photonic integrated circuits (PICs) for quantum optical information processing are now routinely made in foundries with hundreds or thousands of components. Such PICs rely on linear optics, meaning that all photon-photon interactions are probabilistic, and hence that complex protocols that, for example, produce entanglement or graph states do so at low rates. Adding optical nonlinearities to the circuits provides an activation function analogous to that of classical neural networks, enabling deterministic photon-photon protocols in ideal circuits. Here, we discuss the next step: quantum photonic neural networks made from more realistic – i.e. imperfect – components. We show that these are capable of near-deterministic entanglement generation, when imperfections such as losses, imperfect routing and sub-optimal nonlinearities are balanced against network size. These results provide a design for real quantum photonic neural networks and demonstrate that they may play important roles in emerging quantum technologies.

Silicon photonic foundries offer one path for scaling quantum photonic integrated circuits to useful sizes composed of many circuit elements. I will describe our group’s work using the AIM Photonics Foundry to create sources of high-dimensional entanglement across many waveguide paths by exciting arrays of microring-resonator photon pair sources in parallel. Aspects of our work include efficient schemes to certify and quantify entanglement, multi-mode components for manipulating high-dimensional states of light, and advanced packaging techniques.

Any optical quantum computer will comprise systems on a very large scale. With a focus on photon-based and integrated systems, we will talk about the kinds of large-scale hardware that is needed, from photonics to control systems, about their current limitations, and some possible ways forward.

The development of a useful quantum computer requires quantum error correction on a large scale. At PsiQuantum, our goal is to implement a fusion-based fault-tolerant quantum computer using photonics hardware. I will outline the applications we are targeting and the architecture we are pursuing. I will then highlight some progress we have made in creating an integrated photonics platform that meets the requirements of our application.

Quantum emitters enable the deterministic generation of entangled photons, a key resource for photonic quantum computing. We will talk about how these systems can be used to implement fault-tolerant quantum computing, how architectures can be tailored to them, and what are the hardware requirements needed to reach fault-tolerance.