Small-scale quantum networks currently operating in a number of countries are able to distributed entanglement between the various modestly spaced nodes of those networks. In the longer term we know that a future quantum internet should give us the required infrastructure to distribute and process quantum information on the planetary scale. However, in today environment quantum networking resources are far more limited (and scarce) that the NISQ processors and quantum sensors/clocks that may want to operate on them. In this talk we will introduce the concepts of quantum edge and quantum fog computing as key steps in the development of tomorrows quantum internet while noting that they are stand-alone approaches in their own right. We will discuss how these architectures will enable various forms of distributed quantum information processing and what technologies will be required to realize them.

Quantum states of light have been shown to be able to provide an advantage over classical ones in a variety of tasks. Our work shows how practical enhancement can be obtained, using quantum correlations, in the readout of classical information from a digital memory. The quantum advantadge is preserved also in a more complex tasks, namely pattern recognition, highlighting the potential for applications of the techniques proposed.

A quantum internet holds promise for accomplishing distributed quantum sensing and large-scale quantum computer networks, as well as quantum communication among arbitrary clients all over the globe. The main building block is efficient distribution of entanglement over a quantum network. This could be achieved by aggregating quantum repeater protocols. However, the existing protocol requires point-to-point entanglement generation not only to suppress the error, depending on the size of the whole network, but also to be run more than necessary. Here we present an aggregated quantum repeater protocol which works with minimum cost. We also introduce the concept of concatenation of it to achieve arbitrary long-distance communication with fixed error over the network, independently of its size.

Nonlinear quantum interferometers enable us to extract the optical properties of a sample with a detection frequency different from the probe frequency using quantum correlation and quantum interference. In this work, we investigate both theoretically and experimentally the nonlinear quantum interferometer with pulsed laser excitation where a dispersive spectrometer is used for identifying the detection frequency. We show that the visibility of interferometric fringes in the spectral domain is reduced when the interferometric path difference is increased. This result will be useful for the design of pulsed laser-excited nonlinear interferometer for quantum sensing applications.

Frequency-entangled photon pairs are attracting lots of attention due to their potential for novel quantum applications, namely quantum sensing. In the first part, we report the efficient spatial separation of collinearly emitted broadband frequency-correlated photon pairs. We find that broadband photon pairs with a bandwidth of about 90 nm can be efficiently separated using time-reversed Hong-Ou-Mandel (HOM) interference. In the second part, we report the application of such broadband frequency entangled photons for quantum sensing applications.

Diamond color centers like the nitrogen-vacancy (NV) have shown much promise for nanoscale sensing of magnetic and electric fields and temperature. So far however the quantum properties of the NV have not been used to full advantage, for example quantum entanglement of NV qubits has rarely been used for sensing. In this talk I will review recent advances in the fabrication of NVs, and other magnetic color centers in diamond, and in the growth of high quality nanodiamonds. Combining these advances, I will discuss the future prospects of engineering quantum-enhanced sensors in nanodiamonds.

We propose a metasurface-based shadow tomography protocol to measure the properties of quantum states efficiently. We design Si nano-disks based metagratings that act as efficient polarizing beam splitters and can distinguish orthogonal polarizations. This allows for the measurement of all necessary quantum state observables in parallel, reducing the time needed to characterize quantum states of photonic qubits and minimizes the impact of decoherence. We validate our protocol by performing numerical simulations of a two-qubit system with metasurface as quantum detectors. We show that the protocol can accurately estimate properties of quantum states with few measurements and without reconfiguring optical setups.

Infrared (IR) imaging is important in many disciplines but is limited by inefficient, noisy and expensive cameras. Nonlinear interferometers (NLI) enable imaging with undetected photons, where correlated visible-IR photon pairs convey information about an object illuminated in the IR but detected by a visible camera. We introduce compact PPLN based Michelson-style NLI sand discuss their operation in the context of a comprehensive model, exploring the influence of internal losses, IR seeding, and parametric gain on interferometer contrast and visibility. We show that NLI performance can be enhanced for samples with low transmission even in the presence of significant experimental losses.

Magneto-optical spectroscopies are increasingly powerful probes of spin excitations in quantum materials, but at cryogenic temperatures, the laser excitation can be highly non-perturbative. While balanced photodetection can be used to suppress classical noise sources, the photon shot noise limit fundamentally constrains the measurement sensitivity for a given laser power. Here, we have used a two-mode squeezed light source to suppress noise below the shot noise level for magnetic circular dichroism measurements, thus enabling lower power measurements with reduced photothermal effects. We also describe the fundamental sensitivity limits for quantum enhanced interferometric and intensity-difference magneto-optical Kerr effect and circular dichroism spectroscopies.

The distribution of entanglement between the nodes of a quantum network will allow new advances e.g. in long-distance quantum communication, distributed quantum computing and quantum sensing. But the realisation of large-scale quantum networks is faced with the same problem of its classical counterpart: the attenuation in optical fibres. The quantum repeater has been introduced in quantum communication to solve this same problem, since the amplification of quantum states is not possible without a critical decrease in the qubit fidelity due to the no-cloning theorem. The nodes of such a quantum repeater are matter systems that should efficiently interact with quantum light, allow entanglement with photons (ideally at telecommunication wavelengths) and serve as a quantum memory allowing long-lived, faithful and multiplexed storage of (entangled) quantum bits. In this talk, after introducing the current efforts tand architectures for a quantum internet, I will describe our recent progress towards the realisation of quantum repeater nodes with multiplexed ensemble-based quantum memories, using cryogenically-cooled rare-earth ion doped solids. They can be considered a solid-state version of an atomic ensemble, with billions of ions trapped inside a crystalline matrix. They have long been used as a powerful platform for light-matter interaction due to their long coherence times at cryogenic temperatures and great potential for massive multiplexing, which can be harnessed for quantum communication. I will describe how we have employed this system to demonstrate the basic requirements for a quantum repeater node, namely the generation of light-matter entanglement, the demonstration of remote matter-matter entanglement between two memories and the implementation of quantum teleportation with active feedforward. Finally, I will explain our current work towards deploying our system outside of the laboratory environment, and how we plan to build quantum processing nodes using single rare-earth ions in nanoparticles. • N. Maring et al., Photonic quantum state transfer between a cold atomic gas and a crystal. Nature 551, 485 (2017). • D. Lago-Rivera et al., Telecom-heralded entanglement between multimode solid-state quantum memories, Nature 594, 37 (2021). • J. V. Rakonjac et al., Entanglement between a Telecom Photon and an On-Demand Multimode Solid-State Quantum Memory. Phys. Rev. Lett. 127, 210502 (2021). • J. V. Rakonjac et al., Storage and Analysis of Light-matter Entanglement in a Fiber-integrated System. Science Advances 8, eabn3919 (2022). • A. Ortu, et al. Multimode capacity of atomic-frequency comb quantum memories. Quantum Sci. Technol. 7, 035024 (2022). • D. Lago-Rivera et al., Long-distance multiplexed quantum teleportation from a telecom photon to a solid-state qubit. Nature Communications 14, 1889 (2023). • J. V. Rakonjac et al., Transmission of light-matter entanglement over a metropolitan network. arXiv 2304.05416 (2023).

We demonstrate optical-to-microwave conversion using a yttrium-iron-garnet (YIG) sphere. The spin mode in the ferrimagnetic material can interact with both microwave and optical light and mediate conversions between the two. In the conversion system, the generated microwave has clear dependence on the polarization angle of the infrared laser input. We also demonstrate conversion in a cryogenic environment, where microwave measurement precision is enhanced due to reduced noise.

The quantum frequency conversion plays important roles in quantum technologies. Various systems can be implemented to transfer frequency of the photon into another frequencies. We investigate the microwave bandwidth frequency conversion by utilizing mechanical mode. The system of the device to be investigated is cavity electromechanical system. We theoretically analyze the system to obtain the optimal parameters. To realize this system, we fabricate the superconducting cavity electromechanical device. Also, microwave photon to be converted is squeezed vacuum state generated by Josephson parametric amplifier.

We propose a coincidence measurement scheme of combining on-off and photon number resolving (PNR) detectors to discriminate the presence or absence of a low-reflectivity target using Gaussian entangled states in a strong thermal-noise environment. We show that, for coincidence counting, it is better to take the PNR detector on a signal mode and the on-off detector on an idler mode than to take the other way, which can exhibit a similar performance of the coincidence counting using PNR detectors.