Evident from more than 50 years of table-top nonlinear optics, utilizing strong quadratic nonlinearities in integrated photonics can significantly expand the potentials of photonics for applications ranging from sensing to computing. In the past few years, nanophotonic lithium niobate (LN) has emerged as one of the most promising integrated photonic platforms with strong quadratic nonlinearity. In this talk we present some of our recent experimental results on realization and utilizing of dispersion-engineered and quasi-phase-matched devices in nanophotonic LN for intense optical parametric amplification, ultrafast ultra-low-energy all-optical switching, and ultra-low-energy broadband sources in the mid-infrared. We also present some recent experimental and numerical results on how resonators with only strong quadratic nonlinearities exhibit phase transitions in the spectral domain, and pulse compression. We show a path for realization of such nonlinear resonators at the wavelength-scale and discuss how networks of such resonators can lead to topological and non-Hermitian dynamics in the classical and quantum regimes.

A laser adressing unit at 729 nm for up to 10 Ca40+ ions in a 1D-array was developed, focusing down to ca. 1 micron FWHM at a variable pitch of a few microns and allowing for an individual manipulation of each ion arounds its centered position at lowest cross-talk between neighbouring channels. The optical engine comprises of a wave-guide for defining the pitch between the different channels, moveable steering prisms in a telecentric setup and a final common objective to focus onto the ion trap within a vacuum chamber. This unit was now scaled up towards 50 ions, by placing and aligning several of the units precisely with respect to each other, still using the same high performant objective.

Correcting errors in real-time is essential for reliable large-scale quantum computations. Realizing this high-level function requires a system capable of several low-level primitives, including single-qubit and two-qubit operations, mid-circuit measurements of subsets of qubits, real-time processing of measurement outcomes, and the ability to condition subsequent gate operations on those measurements. In this work, we use a ten qubit QCCD (quantum charge-coupled device) trapped-ion quantum computer to encode a single logical qubit using the color code. The logical qubit is initialized into the eigenstates of three mutually unbiased bases using an encoding circuit, and we measure an average logical SPAM error of 1.7(2) 10^{-3}$, compared to the average physical SPAM error 2.4(4) 10^{-3} of our qubits. We then perform multiple syndrome measurements on the encoded qubit, using a real-time decoder to determine any necessary corrections, which are tracked software or applied physically.

Quantum walks are a well-known powerful technique to perform quantum search algorithms, quantum simulations, and universal quantum computation. They have been extensively explored in the optical regime. In our work we have realized an 8x8 two-dimensional square superconducting qubit array with 62 functional qubits. We have used this processor to demonstrate high fidelity multi-particle quantum walks. The programmability of our processor also allows us to implement a Mach-Zehnder interferometer where quantum walkers can coherently traverse both paths of the interferometer before interfering and exiting it. Our work shows an alternate approach for information processing on these NISQ processors.

Programmable arrays of neutral atoms interacting through Rydberg states have arisen over the past few years as a powerful platform to investigate quantum simulation and computation with wide-ranging applications. Their high degree of quantum coherence coupled with the large system sizes achieved, enables the exploration of phenomena beyond the realm of classical simulatability. In this talk, I will describe some of the recent breakthroughs, including their use towards tackling optimization problems and the creation of exotic quantum states. Finally, I will talk about future prospects for the development of this platform and its applications.

We report on the first experimental observation of a non-equilibrium phase of matter, the discrete time crystal (DTC). A DTC breaks time-translational symmetry and displays spatio-temporal quantum order in all of its eigenstates, a feature dubbed “eigenstate order”. We implement Floquet dynamics on a 1D chain of 20 superconducting qubits [2]. Engineered disorders in the two-qubit couplings allow many-body localization (MBL) to occur despite strong external drive, thereby stabilizing the non-equilibrium phase [3]. We carefully validate the phase structure of the DTC by probing the average response of all eigenstates belonging to the Floquet unitary. Using a suitable choice of order parameter, we further identify the location of the MBL-ergodicity crossover via experimentally observed finite-size effects. These results open a direct path to studying quantum phase transitions and critical phenomena on NISQ quantum processors.

Quantum phases with topological order, such as quantum spin liquids, have been the focus of explorations for several decades. Such phases feature a number of remarkable properties including long-range quantum entanglement. Moreover, they can be potentially exploited for the realization of robust quantum computation, as exemplified by the paradigmatic toric code model. While some indications that such phases may be present in frustrated condensed matter systems have been previously reported, so far quantum spin liquids have eluded direct experimental detection. In this talk, I will show how a programmable quantum simulator based on Rydberg atom arrays can be used to realize and probe quantum spin liquid states. In our approach, atoms are placed on the links of a kagome lattice and coherent evolution under Rydberg blockade enables the transition into frustrated quantum states with no local order. We detect the onset of a quantum spin liquid phase of the toric code type by measuring topological string operators in two complementary bases. The properties of this state are further revealed using a lattice with non-trivial topology, representing a step towards the realization of a topological qubit. Our observations open the door to the controlled experimental exploration of topological quantum matter, and could enable the investigation of new methods for topologically protected quantum information processing.

In this research, a new approach of utilizing a 3D Fourier-Transform-assisted Trotter-Suzuki method of time propagation combined with vast cloud computing resources was proposed and used to develop novel theoretical and computational tools capable of simulating ionization induced by nearly relativistic laser fields. Various corrections were included such as the relativistic mass correction and the Nordesick correction, the latter of which accounts for the electron recoil during the absorption of laser photons. The features of the photoelectron distributions change dramatically when the effects of radiation pressure on ionization are considered, which means that they must be included whenever the intensities of the laser fields become relativistically intensive.

Quantized sound waves---phonons---govern the elastic response of crystalline materials, and also play an integral part in determining their thermodynamic properties and electrical response (e.g., by binding electrons into superconducting Cooper pairs). The physics of lattice phonons and elasticity is absent in simulators of quantum solids constructed of neutral atoms in periodic light potentials: unlike real solids, traditional optical lattices are silent because they are infinitely stiff. Optical-lattice realizations of crystals therefore lack some of the central dynamical degrees of freedom that determine the low-temperature properties of real materials. We will discuss our creation of an optical lattice with phonon modes using a Bose-Einstein condensate (BEC) coupled to a confocal optical resonator.

Ultracold neutral atoms in optical lattices have become a powerful platform for the simulation of complex quantum systems. With our quantum gas microscope, we measure snapshots of the quantum many-body wavefunction by fully resolving the position and spin of each atom. I will present the application of this method to two quantum phases: The Haldane spin-1 phase is the prototype of a symmetry-protected topological phase, whose spin order is hidden in conventional two-point correlation functions. On our images, we can evaluate the characteristic string-correlator of this state and directly see the associated edge states of the system. In an antiferromagnetic background, holes get dressed by a surrounding cloud of ferromagnetic correlations. We study these magnetic polarons, which are a building block of the intricate physics of cuprates, by mapping out the spatial structure of the spin environment of individual holes. Upon increased doping, we observe the transition from this unusual polaronic metal to a Fermi Liquid and provide quantum-simulated results of the Fermi-Hubbard model as a benchmark for different theoretical approaches.

In our theoretical research, we investigated how the interaction of graphene with a bi-circular laser field modifies the electronic band structure near the Dirac points. The Dirac-Weyl-Majorana equation, solved using the Floquet theory and the Fourier decomposition, was used to determine the currents induced in graphene. The results show the presence of characteristic current structures with non-zero topological charges, which in turn lead to the generation of high-order harmonics with specific polarization and topological properties.

I will describe our experiments driving spin and orbital resonance of diamond nitrogen-vacancy (NV) centers using the gigahertz-frequency strain oscillations produced within a diamond bulk acoustic resonator. Strain-based coupling between a resonator and a defect center takes advantage of intrinsic coupling mechanisms while maintaining compatibility with conventional magnetic and optical techniques. We demonstrate coherent spin control over both double quantum (Δm=±2) and single quantum (Δm=±1) transitions, providing opportunities for quantum sensing and protection of spin coherence. At cryogenic temperatures, we use orbital-strain interactions driven by a diamond acoustic resonator to study multi-phonon orbital resonance of a single NV center.

Quantum networks rely on the efficient coupling of coherent quantum memories to photonic links. Tin-vacancy centers (SnV) have emerged as promising candidates for the implementation of an optically interfaced quantum memory based on their long spin coherence times at temperatures above 1K. To enhance their interaction with photons SnV centers need to be integrated with high-quality photonic structures. In this work, we report the enhancement of coherent emission of SnV centers by coupling them to a nanophotonic waveguide resonator. We observe strong intensity enhancement of the photon emission when the cavity is resonant with the color center. We demonstrate strong enhancement of the radiative recombination rate of SnV centers resulting in their predominant emission via the coherent zero-phonon line and into the cavity mode. These results are a significant step toward color-center-based quantum information processing applications without the need for dilution refrigerators.

We experimentally demonstrate efficient reduction of heralded telecom single-photon spectral bandwidth by a factor exceeding 220, from 130 GHz to below 550 MHz. The transformation is achieved in an all-fiber setup using dispersive stretching and complex electro-optic phase modulation. The approach is deterministic and results in a 27-fold increase in photon detection probability at the target wavelength. Combined with system transmission of 40% the interface enables increasing the single-photon flux into a spectrally narrowband absorber by a factor of 11. Our approach may enable efficient interfacing matter-based quantum systems with high-rate quantum communication channels and pulsed entangled photon pair sources.

This video was recorded for SPIE Photonics West 2022

Neutral silicon vacancy (SiV0) centers in diamond are promising candidates for quantum networks because of their excellent optical properties and long spin coherence times. In this work, we present the observation of previously unreported optical transitions in SiV0 that are capable of efficiently polarizing the ground state spin. We assign groups of transitions from 825 to 890 nm to higher-lying excited states of SiV0 through a combination of optical and spin measurements. We interpret these spectroscopic lines as transitions to bound exciton states of the defect. Optical spin polarization via these bound exciton states enables the observation of optically detected magnetic resonance of SiV0.

The electromagnetic field from an ordered array of emitters can interfere giving rise to effective emitter-emitter interactions. We study the interaction of light with an array of thulium ions in microring lithium niobate resonators. We show that rare-earth ions arranged into an ordered array can collectively emit light into the micro-resonator. We achieve this via deterministic implantation of ions into the nanofabricated lithium niobate on insulator micro-ring resonators. We show that at cryogenic temperatures, both Purcell enhancement and cooperative enhancement of light emission can be observed in these structures.

This video was recorded for Photonics West 2022

At the core of most quantum technologies is the development of homogeneous, long lived qubits with excellent optical interfaces, and the development of high efficiency and robust optical interconnects for such qubits. To overcome inhomogeneities in semiconductor spin qubits and in their connections, we have been relying on fast photonics inverse design and on optimization of the qubits themselves. We illustrate this approach to scalable semiconductor quantum systems with our results on quantum photonics based on diamond and silicon carbide.

We highlight our work [1] on nanophotonic integration of color centers in semiconductor silicon carbide (SiC). We show ion-assisted implantation and waveguide-integration of silicon vacancy centers (VSi) in 4H-SiC with nearly lifetime limited optical lines and record spin coherence times. We further show controlled coupling to nearby nuclear spin qubits. Our experiments can be performed at high temperatures (T=20 K), thus enabling fast and direct nuclear spin control. Our work shows that VSi centers in SiC are attractive for developing next-generation quantum (computational) networks based on optically interconnected spin-based qubit clusters. [1] Babin et al, Nature Materials, to appear (2021)

The ability of individual photonic qubits to interact via interference lies at the heart of many quantum networking applications. For their development, sources of highly coherent, indistinguishable photons are therefore crucial. Here, we investigate the noise sources that affect InAs/InP quantum dots and limit their coherence. We show that the droplet epitaxy growth mode leads to a quiet environment with 96% of exciton transitions having a coherence time above 100 ps, even under non-resonant excitation. Further, Hong–Ou–Mandel interference reveal a corrected two-photon interference visibility of 98.6±1.6% for these quantum dots, showing their potential for quantum networking applications.

This video was recorded for Photonics West 2022

Twin-field (TF) quantum key distribution (QKD) fundamentally alters the rate-distance relationship of QKD, offering the scaling of a single-node quantum repeater. Although recent experiments have demonstrated the new opportunities for secure long-distance communications allowed by TF-QKD, formidable challenges remain to unlock its true potential. Here, we introduce a novel wavelength-multiplexed stabilisation scheme that overcomes past limitations and can be adapted to other phase-sensitive single-photon applications. In our work, we develop a setup that provides key rates over a record fibre distance of 605 km and increases the secure key rate at long distances by two orders of magnitude to values of practical significance.

Quantum entanglement sources and memories are critical for quantum networking architecture. For high-rate networking, it is important that both technologies are compatible with each other and existing fiber infrastructure. We present our work on the development of a bichromatic photon source (one in the telecom band and one at near-IR) based on warm atomic vapors. We characterize the source, and show our progress towards interfacing it with atomic memories. This paves the way towards building a quantum repeater node based on room-temperature technologies. These narrow linewidth photons are natively well-suited for interfacing with many quantum communication, computation and sensing technologies.

This video was recorded for Photonics West 2022

Quantum networks play a crucial role for distributed quantum information processing, enabling the establishment of entanglement and quantum communication among distant nodes. Firstly, we use a coherently driven quantum dot to experimentally demonstrate a modified Ekert quantum key distribution protocol with two quantum channel approaches: both a 250-m-long single-mode fiber and in free-space, connecting two buildings within the campus of Sapienza University in Rome. Second, we included an independent SPDC source to construct a hybrid network (quantum dot and SPDC) to violating a suitable non-linear Bell inequality, thus demonstrating the nonlocal behavior of the correlations among the nodes of the network.

Quantum optimal control has been shown to improve the performance of quantum technology devices up to their limits in terms e.g. of system size and speed of operation. This talk will review our recent results with a variety of quantum technology platforms, and introduce our newly developed software for automatic calibration of quantum operations - the fundamental building block of next-generation quantum firmware.

The recently demonstrated concept of a quantum pulse gate enables coherent filtering of an individual well-defined optical mode from a noisy signal. This contrasts with the standard technique of spectral filtering followed by temporal gating, which exhibits an inherent trade-off between the efficiency and the noise rejection level. Here we compare the performance of the two techniques in application to quantum key distribution using entangled photon pairs. It shown that coherent filtering may lead to a nearly 10 dB enhancement of the key rate.

Integrated photonics presents an opportunity for low-cost, lightweight and highly-reproducible quantum cryptographic systems. We show that incorporating integrated photonics within pluggable modules a chip-based QKD system operating in real time and with highly competitive secure key rates can be realised with room temperature single photon detectors. The pluggable modules also benefit from their ability to be easily upgraded and replaced so that as the technology matures the system performance can be further enhanced. We also show that our system can be used with standard classical cryptography systems enabling secure data transfer at 100G.

We investigate how accuracy information available in quantum measurement can be used to enhance error correction codes using experimentally obtained state identification data. We test conventional and novel error correction protocols and find a significant reduction of errors in user data transmission. The error reduction and the energy use of error correcting codes compare favorably to that of quantum receivers without error correction. Ordinarily, quantum receivers identify the maximally likely input state out of M possible states. We take advantage of the full vector of Bayesian probabilities available for each act of measurement that supplements the conventional state discrimination.

The process of single-photon subtraction --essentially an implementation of the annihilation operator-- is known to be a useful tool for manipulating quantum optical states. Somewhat surprisingly, subtracting zero photons can also alter the properties of certain quantum states, and forms the bases of a noiseless attenuation process that is useful for quantum communication. Here we review our recent experimental work in this area, which uses parametric down-conversion sources and heralding signals based on the active detection of zero photons.

Future quantum networks will provide multi-node entanglement enabling secure quantum communication on a global scale. Traditional two-party quantum key distribution (2QKD) consumes pairwise entanglement which is costly in constrained networks. Quantum conference key agreement (QCKA) leverages multipartite entanglement within networks to directly produce identical keys among N users, providing up to N-1 rate advantage over 2QKD. In this contribution I will present work on the implementation of QCKA using photonic GHZ states distributed over telecom fibre of up to 50 km combined length. Furthermore, we implemented QCKA on a constrained network consisting of a 6-qubit photonic graph state on which we apply network coding routines to demonstrate the multi-partite advantage over the two-party paradigm.

In this talk, we report the experimental demonstration of direct and efficient verification of entanglement between two multimode-multiphoton systems (one photon in three modes and two photons in three modes) using just two sets of classical correlation tables with and without a discrete Fourier transformation of the optical modes, clearly demonstrating a dramatic reduction in the resources required for entanglement verification. We will also report a novel method to produce a multi-photon parallel state efficiently with reducing excess photon components using multiple heralding single photon sources and N x N active optical switch.

One key challenge in transferring single-photon based quantum technologies from a laboratory environment ‘into the field’ are the limited count rates achievable with today's hardware based on individual detection units. To overcome this limitation we have developed key components pushing beyond the bandwidth-limit of single devices with a massively parallelized (x64) single-photon detection system. Here, detector elements based on superconducting nanowires are optimized for lowest reset times and highest temporal resolution. On-chip (FPGA) data processing over all detector channels provides a viable solution to pre-process the potentially massive amount of initial data which is demonstarted in a QKD experiment.

Quantum Key Distribution (QKD) technology has been considered as the ultimate physical layer security due to its dependencies on the physical laws of physics to generate quantum keys. However, for QKD to become functional for practical scenarios, it must be integrated with the classical optical networking infrastructure. Coping with optical nonlinearity from the classical represents a major challenge for QKD systems. In this paper, we take the advantage of the ultra-low nonlinearity of Hollow Core Nested Antiresonant Nodeless Fibre (HC-NANF) to demonstrate the coexistence of discrete-variable quantum key distribution channel with carrier-grade classical optical channels over a 2 km HC-NANF.

The speed of modern Time-Correlated Single Photon Counting (TCSPC) requires very fast host interfaces and/or real-time data processing. We present a new instrument design with scalability for many channels, an extremely short dead-time, 5 ps resolution and a high speed interface to one or more external FPGAs. Beyond design features and benchmark data we present results from fluorescence lifetime imaging and show a development snapshot of other high throughput applications.