The allure of quantum computing stems from the extraordinary computational power attainable for solving certain problems that are intractable with classical computers. Given the relative ease in which coherence, interference, and entanglement can be generated, photonics has served as an important platform for testing the principles of quantum computing that are now being exploited in a variety of physical systems. The last 20 years gave several important schemes towards universal photonic quantum computing and, since then, we have gained a better understanding of their strengths and weaknesses. Practical universal quantum computers require fault-tolerance, or the ability to correct errors, and remain a long-term goal. In the interim, much attention has been placed on non-universal models of photonic quantum computing which can be used to perform dedicated tasks. In the same vein, Gaussian Boson Sampling is a strong candidate for demonstrating a quantum computational advantage over classical computers, and finds utility in solving classes of graph problems and simulating molecular vibronic spectra, among other applications. This review will provide an outline of these schemes for photonic quantum computing, including some key theoretical and experimental progress to-date, as well as an outlook towards future directions and challenges.

Quantum computation with trapped ions in a QCCD (quantum charge coupled device) architecture is a leading candidate for early-to-market universal quantum computers. In this architecture, ion species are held in a vacuum with a combination of radio frequency and direct current electric fields in Quantinuum’s H-series ion traps. Multiple lasers and advanced optical and optomechanical systems are needed for delivering light to ions for photoionization, laser cooling, repumping, and qubit-ion addressing and manipulation resulting in fast, high fidelity quantum operations.

Two-dimensional arrays of optically trapped neutral atoms are a promising candidate for scalable quantum computation. In particular, nuclear spin qubits for which each qubit state is encoded in a magnetic sublevel of the ground manifold realize exceptionally long qubit coherence times as a result of the naturally occuring long-lived states of the atom. In this talk, I will discuss techniques and tools we are using to assemble an array of up to 100 strontium-87 atoms in our first system: Phoenix. I will also discuss our implementation of single-site addressable, high-fidelity, single and multi-qubit gates.

High-performance quantum memories are an essential enabling component for long-distance quantum networking with key applications including quantum key distribution, network-enhanced quantum sensing, distributed quantum computing, etc. Here, we present such a quantum device engineered to meet real-world challenges, while relying solely on room temperature (vacuum-free, cryogen-free) technologies. Based on a warm rubidium vapor, our device offers high-fidelity (94%), high efficiency (5%), long coherence time (up to 1ms), and low operation error (< 10−3), while only takes a standard 2U rackmount space. This result marks an important step transitioning from academic research into integrated and scalable devices working in the fields.

Alter UK offers a range of highly integrated, miniaturized and robust narrow-linewidth lasers for quantum applications. One such example is the compact Flame laser module. The Flame presents an internal saturation absorption spectroscopy (SAS) arm consisting of a rubidium vapor cell which the laser can subsequently be locked to, reducing the need for external optical components. The following work focuses on how we characterize the Flame in terms of frequency stability, by use of a three-cornered hat experimental setup. Additionally, the laser locked to its internal SAS is compared to an external setup.

We introduce a quantum token scheme that relies neither on quantum memories nor on space-time constraints. While this is achieved by limiting the flexibility, our protocol still exhibits interesting advantages. Among them is the capability of enhancing the protection against screen monitoring while retaining user privacy. To show that our protocol is secure even in realistic scenarios, we implemented it utilising an asymmetric SPDC source.

Patent filing is in progress.

The goal of quantum communication is to transmit quantum states between distant sites. The key aspect to achieve this goal is the generation of entangled states over long distances. Such states can then be used to faithfully transfer classical and quantum states via quantum teleportation. This is an exciting new direction which establishes the fundamentals of a new quantum internet. The big challenge, however, is that the entanglement rates generated between two distant sites decreases exponentially with the length of the connecting channel. To overcome this difficulty, the new concepts of entanglement swapping, and quantum repeater operation are needed. In this talk we will show our progress towards building a quantum network of many quantum devices capable of distributing entanglement over long distances connecting Stony Brook University and the Brookhaven National Laboratory on Long Island, New York. We will show how to produce photonic quantum entanglement in the laboratory and how to store it and distribute it by optically manipulating the properties of atomic clouds. Finally, we will discuss our recent experiments in which several quantum devices are already interconnected forming elementary quantum cryptographic and quantum repeater networks.

Integrated photonics technology has achieved the degree of scalability and complexity needed for building up photonic quantum computers based on optically-addressable spin qubits such as color centers. However, at present none of the industry-standard photonics materials host high quality color centers. Silicon Carbide has the potential to become a technologically-mature platform that can close this longstanding gap between classical and quantum photonics devices. I will discuss the recent progress of Silicon Carbide integrated photonics with color centers and the prospect for large-scale color center quantum networks on-chip.

Quantum states of many indistinguishable photons are key resources in optical quantum technologies. However, experimentally quantifying the multi-photon indistinguishability of a state of many photons is not trivial. We introduce a universal method to measure the genuine indistinguishability of n photons using a low-depth, cyclic interferometer with N = 2n modes. We demonstrate this method using an 8-mode integrated interferometer fabricated using femtosecond laser micromachining, and four photons from a quantum dot single-photon source. We measure a four-photon indistinguishability up to 0.81±0.03. This value decreases as we intentionally alter the photon pairwise indistinguishability. The low-depth and low-loss multiport interferometer design provides an efficient and scalable path to evaluate the genuine indistinguishability of resource states of increasing photon number.

Nanoscale diamond optomechanical devices allow light to be coherently coupled to mechanical resonators, providing a rich platform for optical information processing, with applications including memory, switching, sensing, and wavelength conversion. Recently, it’s been shown that they also allow optical control of quantum memories formed by impurities in the diamond crystal lattice. This interface, in which nanoscale vibrations excited by light drive spin transitions of the defects, enables photons at telecommunication wavelength to address quantum memories with no intrinsic optical transitions.

We are building a certified random number generation service based on a photonic loophole-free Bell test. This is a true "Quantum 2.0" application based on a simple quantum network that has a clear quantum advantage over classical systems. However, taking a system out of the lab and turning it into a service is a major challenge. In this talk I will discuss some of the lessons we have learned, what are some of the photonic components holding us back, and why there is still a place for table-top or free-space optical systems.

Quantum Imaging is a discipline that seeks methods to capture images with properties that exceed the standard limit for image acquisition. Goals of this work include forming images with spatial resolution that exceeds the standard Abbe or Rayleigh form, forming high-quality images using a small number of photons, or forming an image at a wavelength different from the wavelength of the light that illuminates the object to be imaged. Another goal is the form high-quality images when a scattering medium is located between the object and the observer. In this talk I will describe some of the results of research performed in this area. I will pay special attention to the process of ghost imaging. Some implementations of ghost imaging make use of the quantum methods, whereas other use entirely classical methods. Both methods make use of two highly correlated light waves. One of these waves interacts with the object to be imaged, and if it is not absorbed by the object it is detected by a detection system with broad angular acceptance, that is, is detected by a bucket detector. The other wave falls onto a camera with good spatial resolution. By correlating the signals from the two detectors, an image of the object is built up. In this talk I will first describe in more detail how the process of ghost imaging operates and will then provide a critique of when ghost imaging is useful and when it is not.

Scientific CMOS (sCMOS) cameras have developed rapidly over the past decade benefiting from the advancement and wide usage of CMOS image sensors. The talk will provide an overview on Hamamatsu’s latest sCMOS camera, ORCA-Quest quantitative CMOS (qCMOS) camera, with photon number resolving capability and discuss some of the camera’s features. We discuss the role of sCMOS cameras in emerging quantum applications ranging from quantum optics to neutral atoms.

Optically levitated nanoobjects have been shown to be excellent force or torque sensors. Due to their excellent isolation from the thermal environment, they promise unique sensing applications for fundamental research, such as searching for modifications to gravity or Coulomb’s law or detection of dark matter. In this talk I will show how to greatly enhance the sensing performance with a trap array of optically levitated nanoparticles. I will present increased sensitivity to external noises by exploiting direct coupling mechanisms between particles. Finally, I will discuss how this platform can be used to investigate decoherence due to weak forces.

When a resonant photon traverses a sample of absorbing atoms, how much time do atoms spend in the excited state? Does the answer depend on whether the photon is ultimately absorbed or transmitted? In particular, if it is not absorbed, does it cause atoms to spend any time in the excited state at all? In a recent experiment probing single-photon–level optical nonlinearities [PRX Quantum 3, 010314 (2022)], we attempted to measure this time, and found a result which at first surprised us. I will describe this measurement, and more recent theoretical work which attempts to explain our observation. The theory makes further counter-intuitive predictions which we are planning to test experimentally. I will discuss the relationship of these times with other related effects and timescales.

Recent work in our group will be presented on compact and chip-scale wavelength references based on optical transitions in alkali and alkaline earth atomic vapors. These vapors are confined in millimeter-scale silicon-glass microfabricated cells that are easily manufactured in large numbers. We will also describe various applications of such devices including precision timekeeping and dimensional metrology.

Next-generation quantum sensors are currently being investigated in laboratories for a variety of applications. One application area that will benefit from increased precision in sensors is positioning, navigation, and timing (PNT). Current laser technologies are not deployable and are generally constrained to the lab due to sensitivities to thermal and acoustic perturbations. In this effort, we focus on an optical clockwork that will aid both civilian and military applications including improved GPS instabilities and navigation in GPS-denied environments.

Quantum sensors that harness atomic defects within diamond as quantum bits have attracted much interest in application areas including photonics, quantum computing and biosensing. Of the hundreds of defects within diamond, the Nitrogen Vacancy (NV) defect is being widely studied due to the ability to optically address and read out its quantum spin state and its ability to sense magnetic fields, electric fields, temperature and chemical oxidation states at room temperature. In this talk, I will present results demonstrating the sensing capabilities of NV defects within diamond in biochemical solutions, mammalian cells, molecular cages and photomagnetic materials. Finally, I will discuss future prospects for sensitivity enhancement and miniaturisation leveraging photonic structures.

Integrated quantum photonics uses classical integrated technologies for quantum applications. As in classical photonics, chip-scale integration has become critical for scaling up and translating laboratory demonstrators to real-life technologies. Efforts are centered around the development of monolithically, hybrid or heterogeneously integrated platforms. We discuss the value that integrated photonics brings to quantum technologies and discuss what may become possible overcoming current roadblocks. Our aim is to stimulate further research by outlining not only the scientific challenges of materials, devices and components associated with integrated photonics for quantum technologies but also those related to the development of the necessary manufacturing infrastructure and supply chains.

Researchers around the world focus on the search for the perfect quantum system suitable for applications of ‘quantum technology 2.0’. Those range from quantum sensing, via communication to computing using qubits. Even though these applications are based on quantum effects, their diversity brings with them a variety of different requirements on the ideal quantum systems. A prominent class of quantum systems are defect centers in solid-state materials. They have been found to represent quantum systems that are precisely measurable and controllable, while being almost unaffected by their environment. Characterization of the different systems at various ambient conditions using photoluminescence excitation (PLE) spectroscopy techniques helps identifying the right quantum system for the respective target application. In this context we discuss the design and unique performance characteristics of widely tunable CW optical parametric oscillators as novel laser light sources for PLE.

Diamond is a highly attractive material as a solid-state host for a class of crystal defect spin-based qubits, which exhibit high stability even under ambient conditions. Carbon-12 isotopically purified diamond is also spin free, allowing for exceptionally long coherence times. We will introduce the challenges and opportunities in this field and present the results of our ongoing efforts to improve the charge state stability of defects through the controlled growth of doped diamond and our efforts to identify new defects in diamond with long coherence times and high-quality optical interfaces.

Entanglement is an essential resource for secure communication, quantum networks, quantum computing and quantum sensing. However, the leading photon technology to generate entangled photon pairs is based on probabilistic approaches such as spontaneous parametric down-conversion (SPDC) in a non-linear crystal. Here, we overcome this limitation and generate entangled photons on-demand with near-unity fidelity using quantum dots in photonic nanowire devices. We show that by minimizing dephasing processes in the quantum dot and using a fast detection system with precise timing resolution (<20 ps), the entanglement fidelity approaches unity. Finally, we will present a viable route to reaching pair-extraction efficiencies that are two orders of magnitude brighter than SPDC sources without degrading the entanglement fidelity. Such high extraction efficiencies will lead to enhanced secure key rates, longer distance quantum communication, and faster quantum computation.

Quantum cryptography can provide security guarantees against adversaries with unlimited computational power, which motivates research towards a quantum internet. However, widely used Poisson-distributed sources limit the maximal security level and communication rate of such quantum networks. This can be overcome by quantum dot single-photon sources. We show that quantum dots provide additional security benefits based on the tunability of number state coherence. We identify the optimal quantum dot setting for the main quantum-cryptographic primitives and benchmark their performance. Our work is extended by results on network-based blind quantum computing with classical clients, which will make secure quantum computing more accessible.

In a future quantum network consisting of memories, processors, and communication channels operating at disparate frequencies, optical frequency conversion will be important. Our group works on tricks of phase matching to change the usual behavior of nonlinear optical frequency conversion. In this talk, I will discuss two concepts that may have importance for quantum frequency conversion: adiabatic frequency conversion — an analog of rapid adiabatic passage in nonlinear optical wave mixing that can be used for efficient, broadband conversion and precise photonic state preparation, and photon homogenization, the use of phase matching to eliminate spectral distinguishability during frequency conversion.

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%.

Photonic systems are among the most promising for robust quantum information processing. Photonic entangled graph states can be used to encode quantum information in a way that is robust against loss and decoherence, and they serve as a fundamental resource for various quantum applications, e.g., quantum computing, communication and sensing. In this talk, we will present our recent progress toward deterministically generating photonic graph states using quantum emitters. We propose several schemes for generating microwave graph states using transmon qubits, and we provide explicit superconducting circuit designs and fidelity estimates. Furthermore, we will also discuss the required resources for generating photonic graph states deterministically from other types of emitters such as quantum dots or color centers. We will present our algorithm for determining the minimum number of quantum emitters and the precise operation sequence needed to generate an arbitrary target photonic graph state. The algorithm and the operation sequence both scale polynomially in the size of the photonic graph state.