This plenary presentation is about scalable photonic integrated circuits for quantum information applications and was presented at SPIE Photonics West 2023.

This plenary presentation is titled Will quantum technology save the world?: an Australian perspective. It was presented at SPIE Photonics West 2023.

It is now well appreciated that quantum physics can be used to build better sensors. Such sensors can be based on unitary systems like various types of interferometers or open systems based on scattering and lossy transmission channels. I would highlight not only the importance of the quantum states used as probes, but also the importance of the quantum-ness of the measurement schemes. I would bring out the especial importance of the squeezed states of matter and light for sensing studies. I would illustrate the results with experiments.

In questioning the completeness of quantum mechanics, Einstein-Podolsky-Rosen (EPR) claimed that from the outcomes of local experiments performed on an entangled system, it was possible to ascribe simultaneous reality to the values of certain incompatible observables. As EPR acknowledged, the inevitable disturbance of quantum measurements prevents the precise verification of these assertions on a single system. However, the EPR elements of reality can still be tested at the ensemble level through weak measurements by interpreting the EPR assertions as assertions about weak values that follow from the outcomes of projective measurements. Here, we report an implementation of such a test through joint weak measurements followed by postselection on polarization-entangled photon pairs. Our results show that there is a faithful correspondence between the obtained joint weak values and the inferred elements of reality in the polarization version of the EPR assertions.

We report on a robust method for reconstructing multi-mode optical fields. In particular, our method correctly identifies the number and types of modes as well as each mode’s energy, which we demonstrated experimentally. Our method uses multidetector trees and both high-order autocorrelation functions of detected photons (the Glauber function) and high-order autocorrelation functions of no-photon detection. Here we show successful reconstruction for classical and nonclassical multimode fields that contain up to 4 modes whose types are not disclosed to the reconstruction algorithm. This method is significantly more successful than the previously reported one, based on high-order Glauber functions only.

In recent years, photonics has become one of the key contenders in the race to build large-scale quantum computers. The prominence of photonics as a quantum information technology is underscored by the fact that it is one of only a handful of technology platforms which has achieved a quantum advantage, i.e., a large-scale quantum system which outperforms a classical supercomputer at some well-defined computational task [1 2].

In this talk, I will highlight several aspects of the research that my team is doing in this field, focusing on the fundamental science that is enabled by quantum photonic processors, i.e. large-scale, tunable interferometers which are necessary to give these proof-of-principle systems a degree of programmability. We have recently demonstrated [3] the world’s largest quantum photonic processor, i.e. fully tunable multimode optical interferometer, using silicon nitride photonic waveguides, and I will briefly introduce these devices.

Large-scale photonic systems are an interesting novel testbed with which to explore the fundamental concepts and problems of quantum mechanics. Specifically, I will focus on two results obtained in this experimental platform: first, a result in quantum thermodynamics [4], where we experimentally show how to reconcile the apparent contradiction between quantum mechanics, which is time-reversible and information-preserving, and thermodynamics, which is neither, and secondly, a semi-device independent entanglement witness [5] suitable specifically for linear-optical systems.

[1] Zhong et al, Science 370, 6523 (2020).
[2] Zhong et al, arXiv:2106.15534.
[3] Taballione et al, arXiv:2012.05673.
[4] Somhorst et al, arXiv:2201.00049.
[5] v.d Meer et al, arXiv:2112.00067.

Rare-earth ions in solids at cryogenic temperatures are a promising candidate for quantum memory devices due to their long-lived coherence properties. In currently available europium-doped materials, site-to-site variations of the energy levels cause inhomogeneous broadening that prevents resolution of long-lived spin states. These spin states can be resolved with spectral hole burning techniques at the expense of limiting the memory bandwidth and efficiency and by lowering the optical depth. A stoichiometric material has previously demonstrated sufficiently narrow linewidths without sacrificing optical depth. Here we present progress on europium stoichiometric materials as candidates for quantum memory applications.

Nowadays, Superconducting Nanowire Single-Photon Detectors (SNSPDs) play a significant role in quantum technologies and for general faint light detection. Typically, NbTiN SNSPDs are at most 8nm thick to achieve single-photon sensitivity. However, as absorption increases with film thickness, it is desirable to use thicker films of 10 or 12nm considering that the absorption probability places an upper limit for the achievable system detection efficiency. In this contribution, we present for the first time 12nm thick NbTiN SNSPDs that show an increase of system detection efficiency from less than 0.01% to 55% and saturating internal detection efficiency at 4K after irradiation with helium ions. Furthermore, we investigated the influence of helium ion irradiation on the transport properties of the underlying superconducting films.

Collective interactions have been proposed as one avenue to improve the performance of photonic quantum technologies. We present developments in the theory of solid-state systems to model collective emissions in an array of atoms coupled to a 1D ring waveguide, focusing on the impact of array properties on collective emissions. In particular, we examine the role of correlation functions in predicting the onset of collective behaviors. We expand upon prior investigations of correlations in arrays of atoms by examining the influence of a cavity and inhomogeneous broadening on the collective behavior.

Integration of quantum emitters with nanophotonic circuits is crucial for realizing scalable and highly connected multi-qubit photonic chip for quantum information processing. Here, we report our recent experimental progress on hybrid integration of silicon-vacancy centers in nanodiamond with silicon nitride nanophotonics. Enabled by an advanced pick-and-place technique and stepped silicon nitride deposition, we demonstrated deterministic and non-evanescent coupling between silicon-vacancy centers in a preselected nanodiamond and silicon nitride nanophotonic devices. Our method can be applied to virtually any color centers in diamond and pave the way towards high-yield and scalable fabrication of large-scale quantum photonic circuits for the study of many-body quantum physics and photon-mediated entanglement generation.

Current protocols to revert time evolutions of quantum systems suffer either from low success probabilities, or from not being universal - they require knowledge about the evolving state, its evolution, or the interaction through which the evolution is reverted. We overcome these limitations, and demonstrate a novel universal time-reversal protocol by implementing it on a photonic platform using a quantum SWITCH. The schemes universality is verified and a clear quantum advantage with respect to the optimal classical stragegy is shown through running the protocol on a large parameter set and reverting a photons' polarization state with an average state fidelity of over 95%.

We study quantum dynamics of many-qubit systems strongly coupled to a quantized electromagnetic cavity field in the presence of decoherence and dissipation for both fermions and cavity photons, and taking into account the nonuniform cavity field and the spread of qubit transition frequencies. Analytic solutions for quantum state amplitudes and observables are derived for a broad class of open quantum systems. We show that depending on the initial state preparation, the dissipation in a cavity may drive the system into many-qubit entangled dark states that live much longer than the decay time of the cavity field.

Photonics is a powerful platform for quantum computing that offers high clock rates and room-temperature guided-wave interconnects and circuits, without the need for electromagnetic shielding, high vacuum systems or atom trapping and cooling. But generating the entangled states needed for quantum computing with linear optics requires active multplexing, with detection followed by fast feedforward. I will describe ORCA’s approach to addressing this challenge, along with our roadmap to move from NISQ processors to fault-tolerant quantum computers.

With the emergence of different quantum computers approaches, ion-trapped based quantum processors offer a promising concept with coherence times of over 2 s and gate times of 10 to 80 µs with fidelities of over 99.9%. In such systems, ions are being held inside a linear quadrupole trap in defined position zones. The laser induced qubit rotation, initialization and readout operations require an ion individual addressing unit. Our hardware is able to create individual laser foci for 10 captured ions with beam waists of 1.1 µm. Qubit manipulations are realized with a laser wavelength of 396 nm employing 40Ca+ ions.

Photonics integration is a key technology for realizing large-scale photonic quantum information processing. Here, we demonstrate the largest reconfigurable photonic processor based on low-loss silicon nitride waveguide networks for InGaAs quantum dots to date. The 20-mode processor is used for quantum information processing with demultiplexed single photons from a quantum dots source which are detected at the output using single photon detectors. In this talk, we will present the newest results of the system using our programmable quantum photonic processors. Furthermore, we show the challenges of scaling up quantum photonic computers and the range of potential applications and use cases.

Trapped atomic ions are a leading physical platform for quantum computers, featuring qubits with essentially infinite idle coherence times and the highest purity quantum gate operations. Such atomic clock qubits are controlled with laser beams, allowing densely-connected and reconfigurable universal gate sets. The path to scale involves concrete architectural paths based on well-established protocols, from shuttling ions between QPU cores to modular photonic interconnects between multiple QPUs. Full-stack ion trap quantum computers have thus moved away from the physics of qubits and gates and toward the engineering of optical control signals, quantum gate compilation for algorithms, and software-defined error mitigation and correction. I will summarize the state-of-the-art in these quantum computers in both academic and industrial settings, and summarize how they are being used for both scientific and commercial applications.

Semiconductor platforms based on color centers are suitable for implementing scalable quantum systems, based on excellent spin quantum memories with direct photonic interfaces, the possibility to perform high speed and high fidelity quantum gates, combined with expertise in scaling semiconductor circuits. However, there are many remaining challenges to address, including color centers integration into optical structures, as well as the inhomogeneity and imperfections of the qubits resulting from the nonuniform solid state environment. We show how a combination of new computational techniques, nanofabrication approaches, and fast optical and electrical control offers a unique way to address these challenges and to implement scalable quantum systems in diamond and silicon carbide.

Iterative schemes have a great potential for efficient production of complex quantum states of light. This concept refers to generation protocols working on a set of quantum states, starting from basic resources as single-photon states, and building the target in several steps through the implementation of simple quantum operations on the intermediate states. This constitutes a quantum algorithm and requires a basic quantum processor. In this talk we will discuss all-optical architectures for that purpose, based on optical cavities and fast optical switches coupled to a fast-rate single photon source.

Calculating observable properties of quantum chemical systems is a promising application of quantum computers. While most quantum algorithms and experimental demonstrations to date have focused on calculations of electronic structure in molecules, we have recently developed a protocol to study nuclear dynamics processes as well. In this talk, I will describe experiments which use the QSCOUT and IonQ trapped-ion quantum computers to emulate the quantum dynamics and vibrational properties of hydrogen-bonded systems. In our approach, we first treat the proton dynamics as a reduced-dimensional problem on a discretized lattice, then map its Hamiltonian to a sequence of quantum gate operations. Next, we implement these quantum gates on an ion-trap quantum computer to simulate how the proton wavepacket evolves due to the surrounding nuclear framework and electronic potential. Finally, we extract the characteristic vibrational frequencies for the proton motion using the experimentally-simulated dynamics. Our approach offers a new paradigm for simulating quantum dynamics and for computing accurate expectations values, opening the potential to study a range of chemical systems which are otherwise intractable.

Ultracold atomic gases with tunable interactions provide a versatile setting to study quantum systems out of equilibrium. Here, we study Fermi gases following a rapid interaction quench and investigate the subsequent many-body dynamics. For quenches that cross the normal to superfluid phase transition, we observe both pair formation and pair condensation, along with the corresponding timescales to equilibrate. Smaller quenches in the superfluid phase excite the Higgs amplitude mode, which we observe directly using Bragg spectroscopy. The amplitude oscillations provide a measure of the pairing gap through the BCS to BEC crossover and decay according to a power law with a damping exponent that depends upon the interactions.

We generate high-dimensional bi-photon quantum frequency combs with tunable entropies exploiting the second-order nonlinearity of a periodically-poled lithium niobate waveguide through a novel pumping and filtering scheme. Using these quantum states with varying degrees of entanglement, we demonstrate unidirectional bosonic quantum walks, asymmetric energy transfer, and entanglement transport. Our non-maximally entangled quantum states can serve as excellent testbeds for several computational protocols. Moreover, we achieve the steering of the directionality in a scalable format, which enables a new control mechanism for quantum walks as well as novel modification means of joint probability distributions.

This presentation is about applied quantum cryptography: beyond end-to-end encryption and was presented at SPIE Photonics West 2023.

Quantum teleportation allows protected information exchange between distant parties without the need for a physical link, a crucial re- source in future quantum networks. Using high-dimensional quantum states for teleportation offers the promise of higher information capacity channels, protection against optimal cloning machines and improved resilience to noise, but is limited by the commonly used linear optical detection requiring ancillary photons, where the number of entangled photons grows with the dimension to be teleported. Here we overcome this restriction with non-linear optical detection and demonstrate the teleportation of high-dimensional states with just two entangled photons as a quantum resource . Using photonic spatial modes as an example, we demonstrate a 15-dimensional teleportation channel, exceeding the state-of-the-art of three dimensions, offering a scaleable route to high-capacity quantum information transfer.

Quantum technologies have the potential to improve in an unprecedented way the security and efficiency of communications in network infrastructures. We discuss photonic implementations, using encoding in discrete or continuous properties of light, of central quantum network protocols, enabling secret key distribution, flipping coins in the presence of bias, and fingerprinting with real data sets, featuring a provable quantum advantage in security or use of communication resources with respect to their classical counterparts. We also describe our efforts towards the miniaturization of the developed systems and their integration into telecommunication network infrastructures, including with satellite links. These advances enrich the resources and applications of the emerging quantum networks that will play a central role in the context of future global-scale quantum-safe communications.

The Quantum Communications Hub is a major collaboration of academic and industry partners, and national laboratories. As part of the wider UK National Quantum Technologies Programme (UKNQTP), our work focuses on research and development on all aspects of quantum communications. The grand vision of our Hub during the second phase of the UKNQTP (2019-2024) is quantum communications at all distance scales. Starting with a brief summary of our work during the first phase of the UKNQTP (2014-19) for context, I’ll give an overview of our second phase work, including progress to date and objectives for the remainder.

Quantum networks are fundamental for communication protocols and their implementation and validation become hard as the network's complexity increases. In this talk, we show the experimental implementation of a hybrid network exploiting different technologies a quantum dot and a nonlinear crystal. The network is used to share a photonic entangled state among three nodes connected through a 270 m free-space channel and fiber links. We also demonstrate the non-local behavior of the implemented network through the violation of a Bell-like Inequality. Our results pave the way toward the realization of more complex networks and the implementation of quantum communication protocols in urban environments, leveraging the capabilities of hybrid-photonic technologies.

We investigate the coexistence of clock synchronization protocols with quantum signals propagating in a common optical single-mode fiber channel. By measuring optical forward and backward scattering noise in a range from 1510 nm to 1620 nm we demonstrate a potential for up to 100 quantum channels each with 100 GHz bandwidth coexisting with the classical synchronization signals using off-the-shelf optical components. Both “White Rabbit'' and a pulsed laser-based synchronization were characterized and compared.

Quantum enabled optical receivers can achieve a symbol error rate below the shot noise limit and have been extensively studied at visible wavelengths. Yet, the practical use of a quantum receiver requires the implementation at telecom wavelengths. Here we report a successful implementation of a telecom-enabled, versatile time resolving quantum receiver testbed that enables a range of modulation schemes and supports long communication alphabets. We achieved the record sensitivity for a legacy 4-symbol modulation and implemented other modulations and longer alphabets for the first time by combining a highly efficient single photon detector with the time-resolved receiver design.

Practical quantum networks require transform-limited single photon sources that can be synchronized with high timing accuracy over long distances. We build a source of heralded single photons at telecom C-band using a Ti-Sapphire mode locked laser that can be synchronized to an external frequency reference. We target 10 ps single photon pulses so that the independent sources should be carrier frequency matched to within 10 GHz and experience ~3 ps shot-to-shot jitter to independently produce indistinguishable photons. In this progress report, we demonstrate the picosecond single-photon source and discuss high-accuracy precision time protocol (HA-PTP) and pulsed laser-based network synchronization options.

Multi-partite entanglement is a key resource for many applications in quantum information technologies. Based on two material platforms, we consider methods for entanglement generation. In quantum-dot molecules [1], electric-field switching is used, and we characterize the separation between adiabatic and diabatic dynamics in the realization of entangled target states. Our numerical simulations are based on the Bloch-Redfield formalism and are a key step towards the realization of fully quantum-mechanical protocol simulation. As a second platform, we demonstrate the realization of multi-partite entangled states in coupled-cavity arrays and discuss their role in novel quantum machine learning concepts like quantum reservoir computing, to which we provide some insight. [1] Schall et al., Advanced Quantum Technologies 4, 2100002 (2021).

Photonic graph states constitute important resources in quantum information processing. They are used in measurement-based and fusion-based quantum computing and in all-photonic repeaters for quantum networks. However, generating such highly entangled states is challenging due to the fact that photons do not interact with each other. We propose a protocol for generating arbitrary graph states using matter qubits as quantum emitters. Our approach guarantees the minimal number of emitters and provides in polynomial time the quantum circuit for the graph state generation (which includes the non-unitary process of photon emission). We find that the quantum circuit depth for general graphs also scale polynomially with system size.

Manipulation of optically addressable quantum emitters need stable laser sources able to perform complex and high bandwidth waveforms from visible range to infrared. Such waveforms can be generated using an acoustic-optical modulator; however with a limited bandwidth. Here we present a laser source able to deliver high-bandwidth and low noise optical signals at multiple frequencies over a span of a few GHz (scalable to 100 GHz), with a high extinction ratio with respect to all other spurious optical signals. The central wavelengths can be tuned to match the resonance of different optical quantum emitters in the visible or IR.

We present a novel turn-key portable fiber laser for efficient quantum dot excitation to generate single photons. The laser combines a mode-hop-free and alignment-free tunability between 770 nm to 980 nm with a high pulse-to-pulse coherence of 98%. Second order correlation measurements demonstrate a high single photon purity. The laser shows long-term power stability with a standard deviation of less than 0.3% and wavelength stability of better than 5 pm. This development constitutes an essential step for advancing single-photon sources based on quantum dots in terms of integrability and ease of use for research applications and commercial quantum computing.

Semiconductor quantum dots have emerged as excellent artificial atoms to both generate and manipulate quantum light. When embedded in cavities, they can generate single photons and entangled photons with unparalleled efficiency and high quantum purity. In this talk, I will discuss how such devices can be used to generate strings of many entangled photons. The method, leveraging the spin-selective optical transition in a charged quantum dot, leads to the generation of indistinguishable photons in linear cluster states or GHZ states at high rates, realizing an important milestone for scaling-up optical quantum technologies.

I will discuss recent progress of the optical lattice emulators of the Fermi Hubbard model. The new feature of these experiments is availability of snapshots of many-body states with single particle resolution. I will review new insights from these experiments on the properties of doped Mott insulators, including demonstration of magnetically mediated pairing.

Photons play a critical role in future quantum technology, due to its indispensable function as quantum information carrier over long distances. The efficient generation of quantum states is the foundation of quantum photonic technology. In particular, advanced quantum states beyond single-photon pairs are required to meet the rapid development of quantum communication and sensing systems. While integrated photonic circuits provide the unprecedented power to realize complex photon control with minimized structures, most materials used in integrated photonic circuits lack the preferred second-order optical nonlinearity, which limits photon control functionalities and power efficiency. In this talk, I will present our effort in developing advanced photonic states using integrated photonics with second-order nonlinearity. We will start with the first generation of squeezed light on integrated photonic platforms using parametric down-conversion. Then we will focus on the first demonstration of reconfigurable high-dimensional hyper-entanglement. This is achieved through the parallel processing of quantum frequency combs in the path domain. Cavity-enhanced parametric down-conversion with Sagnac configuration is implemented to ensure the spectral indistinguishability. Simultaneous entanglement in path and frequency is realized. On-chip reconfiguration of the entanglement structure in the path domain is also demonstrated. We further present quantum interference in both entanglement degrees of freedom with high visibility.

In this talk, we will present our group’s efforts in developing chip-scale tools for time-frequency domain quantum state engineering. We will present our current work dealing with engineering nanophotonic devices designed for generation and control of quantum states of light in the time and frequency domain on the lithium niobate nanophotonic platform, geared towards applications in simulation and computation.

In this work, we present advanced data preprocessing features of PicoQuant’s latest TCSPC and time-tagging devices and corresponding application examples. One example details a new on-board data reduction for coincidence counting with user-definable balancing between efficient data reduction during the experiment and flexibility in the analysis afterwards. For even more application specific gains the latest generation of our TCSPC-devices can be interfaced to an external FPGA. We demonstrate a QKD-implementation utilizing this feature for real-time analysis of data.

Strong nonlinearities mediated by atomic vapours have the potential to achieve optically controlled switching at room temperature, opening the way to scalable quantum computing. A strong candidate for engineering such interactions is the combination of optical waveguides and cavities with warm alkali vapours. Here we report the implementation of a doubly resonant cavity that translates Kerr nonlinearity induced by an optical control field into a switch for a signal field. We investigate losses and consider optimised designs. We discuss this model in the context of building practical switching devices with applications to logic gate operations at room temperature.

We demonstrate the integration of solid-state single-photon sources in nanophotonic waveguides and show how photons can be collected from several such emitters simultaneously. A lithographic technique here allows us to position solution-based colloidal quantum dots or nanodiamonds hosting defect centers with tens of nanometer accuracy within nanophotonic devices at chip-scale. We further present a novel reinforcement learning-based inverse design approach for realizing photonic integrated circuit components that enable the efficient manipulation of optical modes in devices with minimal footprint. Combining single-photon sources and circuits on a nanophotonic chip will benefit a wide range of applications in quantum technology.

Spectral Hong-Ou-Mandel effect is experimentally revealed between a thermal field and a heralded state. We demonstrate that multi-photon components in heralded state get engaged in quantum interference which is manifested in improved visibilities for certain mean photon numbers of the thermal field. A theoretical relationship is derived for the visibility, showing its compound dependency on the mean photon number per pulse period within the thermal field and the thermal part of the heralded state. Relying on pulsed excitation, we demonstrate an experimental approach for time-efficient acquisition of visibility. The results of this research are of potential application to high-speed remote secret key-sharing and combating the security gaps arising from multi-photon contamination.

We present progress on demonstrating a side-channel-free, decoy-state quantum key distribution (QKD) source based on a polarization-modulator and a wavelength-stable attenuated pulsed laser. By modulating the polarization of the quantum state, a three-state QKD protocol is achieved. The polarization-modulator-based QKD source improves security by removing several sources of side-channel attacks that exist when multiple sources are used to generate different QKD states. Here we present our single-source design, as well as an evaluation of critical subsystems characterized by the quantum bit error rate (QBER), quantum state tomography, and achievable key rates. The QKD source is intended to operate using minimal size, weight, and power (SWaP). In our system, we use a custom KTP waveguide modulator designed and fabricated by AdvR. Our QKD source can be operated using both narrow-band and broadband lasers, and we present characterization data using a He-Ne laser, and a diode laser, respectively. The polarization-modulator-based QKD source has applications in future mobile quantum networks incorporating unmanned aerial vehicles (UAV) and autonomous vehicles, as well as fiber-based quantum networks.

A single photon source (SPS) is an important component for quantum information technology. Among the candidates, S PSs based on epitaxial quantum dot (QD) reported excellent properties of quantum yield, Fourier-transform limited linewidth, multiphoton suppression, and indistinguishability. Despite these good properties, QD itself has poor extraction efficiency of photons to air from high index semiconductor. Micro-pillar and bull’s eye structures greatly improved the brightness. However, due to their high Q-factors, it is challenging to fabricate the SPSs since spectral matching of QD emission and the resonance of the structure is very difficult. In this presentation, we report deterministically fabricated bright and broadband SPS with a double solid immersion lens (SIL) structure on a gold reflector. With finite-difference time-domain simulation, the optimized double SIL structure is expected to provide 88% of maximum brightness in 0.5 NA and more than 80% of brightness in the range of 65 nm. For deterministic fabrication, we find a QD in micro-photoluminescence measurement and cure photoresist at the QD position at low temperature. For two-photon absorption curing, we match the wavelength of the searching and curing laser to reduce the aberration effect. Also, we could perfectly match the beam path by coupling the two lasers into the same single-mode fiber. We fabricated GaAs SIL structure with the cured pattern by an isotropic wet etching process. The second polymer SIL covered the GaAs SIL to complete the double SIL structure. Experimentally, we obtained brightness of 51.6% and g(2)(0)=0.014 in non-resonant excitation with a GaAs SIL structure larger than the optimized one. The presented structure is highly efficient yet easy to fabricate without using e-beam lithography and dry etching equipment, which can pave the way to practical use of SPSs.