This PDF file contains the front matter associated with SPIE Proceedings Volume 11347, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Structure photons invoke interesting fundamental properties and enable novel applications, e.g. their use as high-dimensional quantum states, which are known to be beneficial in various quantum information tasks. However, to use their full potential the ability to perform any unitary operations is indispensable. Here, we present a scheme to perform arbitrary unitary operation for all transverse spatial modes, i.e. a so-called multiport for high-dimensional quantum states. We realize it by using multiple consecutive phase modulations, which are designed by wave front matching techniques. At first, we perform near-perfect measurements of full-field modes, i.e. modes with an azimuthal and radial structure [1]. Our method only requires three consecutive phase modulations followed by a single mode fiber and is, in principle, error-free and lossless. We achieve an average error of 4.2% for a set of 9 different full-field Laguerre-Gauss and Hermite-Gauss modes with an efficiency of up to 70%. We further demon-strate its potential in a quantum cryptography protocol and in high-dimensional quantum state tomography. We then use the multiport to implement high-dimensional quantum gates [2], such as cyclic and quantum Fourier transformations, known as Pauli X-gates and Hadamard H-gates, respectively. We achieve an aver-age visibility of more than 90% for dimensions up to 5 and obtain a process purity of 99% by means of a full quantum process tomography. We also demonstrate the benefit of the two independent spatial degrees of freedom, i.e. azimuthal and radial, and implement a two-qubit controlled-NOT quantum operation on a single photon. Using more efficient phase modulations will pave the way to novel multi-particle high-dimensional quantum information experiments. [1] M. Hiekkamäki, S. Prabhakar, R. Fickler, Near-perfect measuring of full-field transverse-spatial modes of light, arXiv:1909.01685 [2] F. Brandt, M. Hiekkamäki, F. Bouchard, M. Huber, R. Fickler, High-dimensional quantum gates using full-field spatial modes of photons, arXiv:1907.13002

For future quantum technologies and in particular for quantum architecture systems, one needs to consider scalability and reproducibility in order to be able to handle and add easily as many qubits of information as possible on a given platform. Quantum coherence and quantum information are fragile and thus requires the need for many qubits to be created, to interact and quantum information to be preserved. Currently there are either very good qubits in terms of fidelity but poor scalability or very good scalability but poor fidelity thus requiring more qubits to compensate. Most of the current platforms are in solid-state physics such as superconducting, dopants in silicon, quantum dots in III-V semiconductors or defects in diamond. The main interest for condensed matter systems is the fact that it is potentially scalable as integration is possible in the future, already demonstrated in the ‘classical’ semiconductor industry. Nevertheless, there are some true challenges in controlling and understanding the mechanisms of decoherence, losses and unwanted effects in these ‘dirty’ systems. As quantum technologies require ultimate control of quantum effects such as maintaining quantum coherence, quantum superposition and entanglement, it pushes towards deeper knowledge of underlying material and condensed matter physics. Photons are of particular interest as they are good carriers of quantum information and one aim is to explore a fully integrated photonic quantum circuit which would be a hybrid system made of stationary solid-state qubits (we will call them quantum emitters) coupled together via single photons travelling within a common optical bus that is photonics-ready for quantum communications within a network of quantum nodes. In this presentation, we will present our latest developments and results towards the integration of quantum emitters with photonic structures with nanosize features. In the first part, our platform is described which consists of glass and thus directly compatible with optical fibres. It is based on the technique of exchanged ions within the glass in order to create locally a particular confinement of the light. The next part will concern our latest results on two different quantum emitters. The first one is based on perovskite nanocristals that can be synthetised chemically and giving rise to quantum optics-type of emitters. The second one is based on the so-called silicon-vacancy defect centre in nanodiamonds. These emitters are produced by the high-pressure/high-temperature method and lead to promising results for quantum technologies. Finally, the last part will treat the nanophotonics approaches in order to couple efficiently these emitters with our platform.

We propose a novel platform for the realisation of quantum simulations of spin arrays, providing unprecedented flexibility and allowing one to explore regimes beyond the reach of other platforms. It is based on laser-trapped circular Rydberg levels. The strong van der Waals interaction between the atoms emulates a spin-1/2 XXZ Hamiltonian. All its parameters are experimentally and dynamically tunable over a wide range. A spontaneous-emission inhibiting structure extends the lifetime of individual laser-trapped circular Rydberg atoms to the minute range. Quantum simulations over more than $10^4$ interaction cycles are thus within reach. This enables the observation of adiabatic evolutions through quantum phase transitions, of sudden quenches, and fast modulations of the interaction parameters. After I present the key features of this simulator, I will focus on our latest experimental results regarding the preparation and manipulation of laser-cooled circular Rydberg atoms in the vicinity of an atomchip in an optical-access 4K-cryostat. Lifetime measurements reveal a below-10K microwave blackbody temperature, while Ramsey interferometry shows coherence times solely limited by magnetic field noise. I will finally present our latest results on laser-trapping of circular Rydberg atoms, a decisive step towards the realisation of the proposed quantum simulator.

Ultracold atoms confined in microtrap array is a highly versatile platform for quantum many-body physics, quantum simulation and quantum computation because of the easy scalability, long coherence time, single site addressability and controllable interactions, e.g. using Rydberg states. Here, we demonstrate an novel method to prepare versatile arrays of atomic ensembles by transferring them from a pancake shaped optical reservoir to an array of optical tweezers produced via direct projection of light patterns produced via a digital micromirror device. The size of each ensemble is smaller than the Rydberg blockade radius, such that each one can carry either 0 or 1 (collective) excitations which can then strongly interact with the neighbouring ensembles. Finally I will discuss a recent proposal to use such Rydberg arrays to realise programmable quantum systems in the form of quantum cellular automata (QCA). This opens a path to study many-body quantum dynamics in quantum and classical regimes, to engineer highly entangled quantum states and toward an inherently scalable approach to quantum information processing and computing.

Semi-device dependent characterization of quantum devices

Complete characterization of states and processes that occur within quantum devices is crucial for understanding and testing their potential to outperform classical technologies for communications and computing. However, solving this task with current state-of-the-art techniques becomes unwieldy for large and complex quantum systems. Here we realize and experimentally demonstrate a method for complete characterization of a quantum harmonic oscillator based on an artificial neural network known as the restricted Boltzmann machine. We apply the method to optical homodyne tomography and show it to allow full estimation of quantum states based on a smaller amount of experimental data compared to state-of-the-art methods. We link this advantage to reduced overfitting. Although our experiment is in the optical domain, our method provides a way of exploring quantum resources in a broad class of large-scale physical systems, such as superconducting circuits, atomic and molecular ensembles, and optomechanical systems.

As the reach of quantum technologies extends ever further in communication and information science, a reliable way of transferring quantum information between distant locations becomes ever more crucial. While photons are widely accepted as excellent carriers due to their speed and low decoherence, losses of transmission (in free space or fibre) and the impossibility of cloning quantum information still pose a great challenge. The quantum repeater architecture was suggested as a solution to both problems [1]. In a quantum repeater the information encoded in an input state is transferred to a new one through entanglement swapping, that is then sent on along the channel. In this work we present our advances towards the realisation of a quantum repeater. Our system of choice combines a solid-state quantum memory with a source of photon pairs. The memory is based on a Rare-Earth Doped crystal, where quantum information can be stored in Pr3+ ions as a collective excitation using the Atomic Frequency Comb technique. On demand retrieval of the information is realised by transferring the excitation to a long-lived spin state. Record values of storage times and retrieval efficiencies have been demonstrated in this system [2]. Entangled pairs of single photons are generated by parametric down conversion in a periodically poled crystal placed inside a bow-tie cavity. This allows us to generate narrow band photons pairs, where the signal is spectrally matched to the memory (606nm), while the idler is in the telecom band [3]. Such a configuration allows us to benefit from the high performance of the memory, that also allows for temporal [2] and frequency [4] multimodality, while at the same time overcoming the high optical losses of 606nm photons by pair generation of a telecom photon. The first stepping stone, progress towards which is presented in this work, is the successful demonstration of energy-time entanglement between the telecom idler photon and the signal photon, stored as spin-wave excitation. The entanglement of the original pair is maintained by the memory temporal multimodality. The entanglement analysis will be made through time-bin qubit analysers made of a fibre-based Mach-Zehnder interferometer, for the former, and a solid-state equivalent based on two AFC with different storage times, for the latter [5]. In this direction we have already doubled the efficiency of the AFC storage protocols, that will be beneficial to count rates and signal-to-noise ratio. With respect to [2], we also increased the spectral-matching between the source and the memory [4]. Our experiment will provide an increase in storage time of 3 orders of magnitude with respect to previous demonstrations, as well as introducing for the first time on-demand read-out in a highly multi-mode memory. Demonstration of the successful transfer of quantum information between the signal photon and the long-lived solid-state excitation will open the way to the demonstration of long-distance entanglement between individual nodes in a quantum network.

Single quantum emitters coupled to optical cavities in the Purcell regime can be used as high-efficiency spin-photon interfaces that are essential for building a quantum network. Furthermore, the dynamical control of the spontaneous emission rate of quantum emitters can have important implications in quantum technologies, e.g. for shaping the emitted photons waveform, or for driving coherently the optical transition while preventing photon emission. Here we demonstrate the dynamical Purcell-enhanced emission of a mesoscopic ensemble of erbium ions doped into nanocrystals coupled to a fully-tunable high-finesse fiber-based optical microcavity. Erbium has excellent optical and spin coherence properties at cryogenic temperatures and, in addition, has a transition in the telecom band that can facilitate integration into existing commercial telecom fibers. We show that we can tune the cavity on and out of resonance at a rate of above 8 KHz, which is two orders of magnitude faster that the natural lifetime of the erbium ions (55 Hz), and a factor of five faster than the Purcell enhanced emission (1.8 KHz). This allows us to shape in real time the Purcell enhanced emission of the ions and to achieve full control over the emitted photon’s waveforms. With moderate improvements in our detection efficiency and cavity finesse, this capability will allow for the generation of single telecom photons with controllable wave-shape from single erbium ions and for the realization of quantum processing between rare-earth ion qubits using dipolar interactions.

Future quantum repeater architectures, capable of efficiently distributing information encoded in quantum states of light over large distances, rely on quantum memories for light [1]. Quantum repeaters can benefit from a modal multiplexing implementation of the memory, essentially scaling up the repeater's throughput [2]. In this work we demonstrate a temporally multiplexed quantum repeater node in a laser-cooled cloud of 87-Rb atoms (as proposed in [3]). We employ the DLCZ protocol where pairs of photons and single collective spin excitations (so called spin-waves) are created [4]. The latter can then be efficiently transferred into a second single photon. For selective readout, we need to control the dephasing and rephasing of the spin-waves created in different temporal modes. We achieve this by a magnetic field gradient, which induces an inhomogeneous broadening of the involved atomic hyperfine levels [5]. By employing this steering technique, combined with cavity-enhanced noise suppression and feed forward readout, we demonstrate distinguishable retrieval of up to 10 temporal modes. For each mode, we prove non-classical correlations between the first and second photon. Furthermore, an enhancement in rates of correlated photon-photon pairs is observed as we increase the number of temporal modes stored in the memory. The reported device is a crucial key element of a quantum repeater architecture implementing multiplexed quantum memories. [1] H.-J. Briegel, W. Dür, J. Cirac and P. Zoller; Phys. Rev. Lett. 81 5932 (1998) [2] C. Simon, H. de Riedmatten, M. Afzelius,N. Sangouard, H. Zbinden and N. Gisin; Phys. Rev. Lett. 98 190503 (2007) [3] C. Simon, H. de Riedmatten and M. Afzelius; Phys. Rev. A 82 010304(R) (2010) [4] L. Duan, M. Lukin, J. Cirac and P. Zoller, P; Nature 414 413 (2001) [5] B.Albrecht, P. Farrera, G. Heinze, M. Cristiani and H. de Riedmatten; Phys. Rev. Lett. 115 160501 (2015)

Entangled photon pairs play a crucial role in emerging quantum technologies, acting as tamper-proof padlocks in secure quantum communication, ultra-precise probes in quantum metrology and high-fidelity information carriers in photonic quantum computing. Amongst the many technological possibilities for generating photon pairs, sources based on spontaneous parametric down-conversion (SPDC) in second-order nonlinear crystals are still the workhorse tool in quantum optics laboratories, and more recently even in long-distance quantum communication with satellites. SPDC is a widely used method to create entangled photon pairs. The number of photon pairs that can be detected in a particular choice of collection modes, typically being optical single-mode fibers, depends critically on the spatial characteristics of the multi-mode SPDC emission as well as the particular beam waist of the collection modes. While several studies have already addressed the issue of optimal fiber coupling of SPDC photons in theory and experiment, the results of these studies arrive at different conclusions. Here, we present the results of a comprehensive experimental study on the optimal collection of photon pairs into single-mode optical fibers. Our approach is based on quasi-phase-matched type II SPDC from a periodically poled KTiOPO4 (ppKTP) crystal. We discuss the influence of pump and collection focal parameters on the spectral brightness and heralding efficiency, as well as practical issues of alignment tolerances into an optical single mode fiber. Further, Using two-photon interference (Hong-Ou-Mandel interference), we assess the spectral bandwidth of the photon pairs for variable crystal lengths. The results are in good agreement with our theoretical model, thus providing the recipe of building ultra bright and stable entangled photon sources.

We show that by using the non-classical two-mode squeezed vacuum (TMSV) to illuminate an object, quantum correlations contribute to a detectable enhancement even under regimes of high signal loss and background thermal noise. We also consider a realistic measurement scenario with click detectors, along with sequential Bayesian inference; a single click on one mode of the TMSV produces a vacuum removed thermal state which enhances the probability of subsequent click detection.

This contribution evaluates the potential for SI-traceable measurements of electromagnetic fields from precision measurements of two-photon rovibrational transitions of cold trapped HD⁺ ions interpreted with accurate theoretical models. Zeeman spectroscopy of a hyperfine component of the (v,L)=(0,0)→(2,2) transition is exploited for the measurement of a static magnetic field. The absolute sensitivity and accuracy are estimated at the 10^-10 T level in the case of frequency measurements at the quantum projection noise limit. Measurements of the AC-Stark shifts of different Zeeman components of the (v,L)=(0,0)→(2,0) transition at different orientations of the magnetic field are exploited to measure the polarisation and the intensity of a THz-wave off-resonantly coupled to HD⁺ rotational levels. The sensitivity is estimated at the 10^-7 W/m₂ level. A reference THz-wave with an intensity of 1 W/m² can be calibrated in intensity with a fractional accuracy limited at the 10^-2 level by the accuracy of the theoretical calculations and at the 10^-4 level by the experimental errors. In addition, an approach for retrieval of the full polarisation ellipse is demonstrated with a selected THz-wave. The fractional accuracy estimated for frequency measurements limited by the quantum projection noise is better than 5% for the amplitudes and better than 10% for the phases of the electric field components of the THz-wave.

This presentation will review some of the latest state-of-the-art developments in cold-atom inertial sensors developped at the SYRTE laboratory in Paris.

Microwave Quantum Network Linking Cryogenic Systems for Superconducting-Circuit-Based Quantum Computing

High-dimensional entangled states of light provide novel possibilities for quantum information, from fundamental tests of quantum mechanics to enhanced computation and communication protocols. In this context, the frequency degree of freedom is particular attractive thanks to its robustness to propagation in optical fibers and its capability to convey large scale of quantum information into a single spatial mode. This provides a strong incentive for the development of efficient and scalable methods for the generation and the manipulation of frequency-encoded quantum states. Nonlinear parametric processes are powerful tools to generate such states, but up to now the manipulation of the generated frequency states has been carried out mostly by post-manipulation, which demands complex and bulk-like experimental setups. Direct production of on-demand frequency-states at the generation stage, and if possible using a chip-based source, is crucial in view of practical and scalable applications for quantum information technologies. Here we use an integrated semiconductor chip to engineer the wavefunction and exchange statistics of frequency-entangled photon pairs directly at the generation stage, without post-manipulation. Tuning the pump spatial intensity allows to produce frequency-anticorrelated, correlated and separable states, while tuning the spatial phase enables to switch between symmetric and antisymmetric spectral wavefunctions, leading respectively to bosonic and fermionic behaviors of the photons. We also demonstrate the generation of non-Gaussian entanglement in the continuous variables formed by the frequency and time degrees of freedom of the photon pairs. These results, obtained at room temperature and telecom wavelength, and with a chip-based source, open promising perspectives for the quantum simulation of fermionic problems with photons on an integrated platform, as well as for communication and computation protocols exploiting antisymmetric high-dimensional quantum states.

A simple, room-temperature, cavity- and vacuum-free interface for an efficient photon-matter interaction is implemented. In the experiment a heralded single photon generated by the process of spontaneous parametric down-conversion is absorbed by a single atom-like system, specifically a nitrogen-vacancy color center in diamond. Here phonon-assisted absorption solves the mismatch problem of a narrow absorption bandwidth in a typical atomic medium and broadband spectrum of quantum light. The source is tunable in the spectral range $452-575$ nm, which overlaps well with the absorption spectrum of nitrogen-vacancy centers. This can also be considered as a useful technique paving the way for development of novel quantum information processing and sensing applications.

Integrated quantum photonics: advanced architectures using 3D laser writing

Nanoscale systems possessing long-lived spins and the ability to coherently couple to light are highly demanded for quantum devices implementations. Several approaches, like NV centers in diamond, semiconductor quantum dots are intensively investigated in the field, where an outstanding challenge is to preserve properties, and especially optical and spin coherence lifetimes, at the nanoscale. In this context, chemically synthesized Eu3+ doped Y2O3 nanoparticles have demonstrated great potential for quantum technologies based on their narrow optical homogeneous linewidth, down to the 10 kHz level, and millisecond-long spin coherence time. Here, we investigate an alternative nanoscale material: Pr3+: Y2O3. We first determine the Pr3+ hyperfine structure in Y2O3 by spectral hole burning and then measure photon and spin echoes from nanoparticles down to 150 nm. Spin T2 up to 880 μs was obtained for the ±3/2↔±5/2 hyperfine transition at 10.42 MHz, a value which exceeds that of bulk Pr3+doped crystals so far reported. These promising results confirm nanoscale Pr3+:Y2O3 as a very appealing candidate to integrate quantum devices.

Single molecules for quantum technologies

We review the signal-to-noise properties of two setups for Correlation Plenoptic Imaging (CPI), a novel technique that exploits the correlations of light intensity to perform the typical tasks of plenoptic imaging: refocusing out-of- focus parts of the scene, extending the depth of field, reconstruct 3D objects, As opposed to first-order plenoptic imaging, based on direct intensity measurement, CPI does not entail a loss of spatial resolution. Both setups are based on the properties of chaotic light and employ the concept of ghost imaging in different ways: the first one to image the object, the second one to image the focusing element. We show that the SNR can be easier to control in the second CPI scheme, in which the object is focused by a lens.

Quantum technologies containing key GaN laser components will enable a new generation of high precision quantum sensors, optical atomic clocks and secure communication systems for many applications such as next generation navigation, gravity mapping and timing since the AlGaInN material system allows for laser diodes to be fabricated over a wide range of wavelengths from the u.v. to the visible. We report our latest results on a range of AlGaInN diode-lasers targeted to meet the linewidth, wavelength and power requirements suitable for optical clocks and cold-atom interferometry systems. This includes the [5s²S_1/2-5p²P_1/2] cooling transition in strontium+ ion optical clocks at 422 nm, the [5s₂ ¹S₀-5p¹P₁] cooling transition in neutral strontium clocks at 461 nm and the [5s²s_1/2 – 6p²P_3/2] transition in rubidium at 420 nm. Several approaches are taken to achieve the required linewidth, wavelength and power, including an extended cavity laser diode (ECLD) system and an on-chip grating, distributed feedback (DFB) GaN laser diode.

The ability to generate and manipulate light in pure quantum states is central to the development of quantum enhanced technologies, be it for optical quantum computing or for quantum secure communications. Recently, artificial atoms in the form of semiconductor quantum dots have emerged as an excellent platform to develop efficient components for optical quantum technologies. Using the tools of cavity quantum electrodynamics to harness the quantum light emission and to reduce the decoherence phenomena, single photon wavepackets with very high quantum purity can now be generated. This new generation of devices show unprecedented efficiency and allows scaling-up linear quantum optical technologies [3,4]. The high quality of the artificial atoms also allows to observe new phenomena, such as the generation of light pulses in a pure quantum superposition in the photon number basis [5], or to explore optical non-linearities at the single photon level [6] [1] N. Somaschi, et al. Nature Photonics, 10, 340 (2016) [2] P Senellart, G Solomon, A White, Nature Nanotechnology 12 (11), (2017) [3] C Antón, et al., Optica 6 (12), 1471-1477 (2019) [4] D. Istrati, arXiv 1912.04375 [5] J. C. Loredo, C. Antón, et., Nature Photonics 13 (11), 803 (2019) [6] L. De Santis, et al, Nature Nanotechnology 12, 663 (2017)

This paper describes the proposal of a fully electronic quantum noise source for random number generators for utilize in mathematics and quantum cryptography. It uses the phenomenon of shot noise generated by current carriers in reversely polarized semiconductor junctions due to inner photoelectric effect. Described device is built to present that the shot noise in simple semiconductor devices can be amplified and sampled, to produce a random bits data stream whose quality is sufficient for all above mentioned applications.

Time-to-digital converters are a key component in many photonics systems, ranging from LiDAR, quantum key distribution, quantum optics experiments and time correlated single photon counting applications. A novel efficient timeto- digital converter non-linearity calibration technique has been developed and demonstrated on a Spartan 6 LX150 field programmable gate array (FPGA). Most FPGA based time-to-digital converters either use post processing or have calibration techniques which do not focus on minimizing resource utilization. With the move towards imaging with arrays of single photon detectors, scalable timing instrumentation is required. The calibration system demonstrated minimizes block memory utilization, using the same memory for probability density function measurement and cumulative distribution function generation, creating a look up table which can be used to calibrate the sub-clock timing module of the time-to-digital converter. The system developed contains 16 time-to-digital converters and demonstrates an average accuracy of 21ps RMS (14.85ps single channel) with a resolution of 1.86ps.

We present an experimental demonstration of the 4-state differential-phase-shift quantum key distribution (4S-DPS-QKD) over a 30 km quantum channel with two different approaches, namely path superposition and time-bin superposition. 4S-DPS-QKD offers enhanced security compared to the pulse-train DPS-QKD against individual attacks. We show that the key generation efficiency, and security, improve with an increase in the number of path delays or time bin superpositions. Our implementation establishes an ease in implementation of the time-bin superposition approach, over the path superposition approach. The sender (Alice) uses a photon in a superposition state, corresponding to either 3 spatial paths or temporal bins. In the temporal case, Alice uses a weak coherent source (WCS) with a pulse width of 3 ns, that we interpret as a single wave packet comprising of three time-bins of 1 ns each.. Alice encodes her random key bit [0,1] as a random phase [0,pi] between |a> and |b>, and |b> and |c>, i.e. successive paths or time bins of the WCS, with mean photon number < n>=0.1, and pulse widths of 0.5 ns. The phase encoded bits are then transmitted over a single mode optical fibre. Both path and timb-bin implementations follow similar setups beyond the transmitter. At the receiver, a delay line interferometer (DLI) introduces a delay of 1 ns, measures < a|b> and< b|c>, and recovers the phase introduced by Alice as a detection at one of the 2 output ports of the DLI with a time resolution of 50 ps using a time-to-digital convertor (TDC). Our experiments have yielded a QBER of 21% for path superposition and 17% for time-bin superposition. We thus establish the equivalence of the two approaches, and note that the time-bin approach is easily extended to more than 3 time-bins, and an increased secure key rate.