This PDF file contains the front matter associated with SPIE Proceedings Volume 12446, including the Title Page, Copyright information, Table of Contents, and Conference Committee listings.

Quantum Kerr optical frequency combs are generated by pumping a high-Q microresonator with a continuous-wave resonant laser. Below threshold, two pump photons are symmetrically up- and down-converted as twin-photons via spontaneous four-wave mixing, thereby generating up to a hundred paired channels. These chipscale, high-dimensional and room-temperature systems are expected to play a major role in quantum engineering. However, their theoretical description is still unclear. Here, we present an explicit description of this quantum system via a frequency-bin state and its density operator, and explore the properties of the eigenkets.

We show that the strategy of ghost imaging can be successfully adapted to the estimation of parameters of open systems. Two entangled qubits, so that only one of them interacts with a depolarizing sample, may lead to precision in the estimation of the depolarizing parameter better than the one corresponding to sending the pair, entangled or not, through the sample. As opposed to ghost imaging, entanglement plays an essential role in this case, as does the entangling detection procedure, consisting in projecting the final state on a Bell-state basis. For isotropic depolarization, quantum advantage is obtained only when the final state, after interaction with the sample, is not entangled anymore.

Neutral atoms trapped in arrays of optical tweezers are a versatile platform for multiple research efforts in quantum information science. In particular, alkaline-earth atoms like Yb offer many advantages including ultralong coherence nuclear spin states for quantum information storage and high-fidelity qubit operations, optical transitions to Rydberg states for generating strong, long-range interactions between atoms, and narrow optical transitions for efficient cooling and metrology. Additionally, optical transitions from the long-lived ³P₀ nuclear spin state have frequencies in the telecommunications band making integration with fiber networks and silicon nanophotonics an attractive avenue of exploration for hybrid quantum systems and quantum communication. Finally, these optical tweezer platforms are well suited for investigating the cooperative response of an array of atoms, where the proximity of neighboring atoms fundamentally alters the systems optical response which has implications for quantum information storage. Here I will present efforts to investigate these research avenues using alkaline-earth atoms in optical tweezers.

Broadband time-energy entangled photons feature strong temporal correlations with potential for precision delay metrology, but previous work has leveraged only time-of-flight information ultimately limited by the detection jitter and resolution of the time-tagging electronics. Firstly, our work pushes the entanglement-based nonlocal delay metrology from the conventional time-of-flight measurement to a new direction—two-photon interferometry with subpicosecond sensitivity independent of detection resolution. Next, we show the selective sensitivity of frequency-bin encoded Bell states to the sum and difference of biphoton-delays by using a novel reconfigurable setup capable of switching between the Bell states by successively employing single and dual spectral-line pumps.

Photonic quantum memories will play an essential role in several quantum information protocols, including distributed quantum computing, quantum sensing, and the synchronization of repeater nodes. Most photonic memories operate by storing the photon in matter-based systems, but those approaches have limitations. Namely, they are inherently narrow bandwidth, often require costly overhead in the form of cryogenics, and typically have low retrieval efficiency into single-mode fiber. In this work, we develop a photonic quantum memory that operates at room temperature in free space, allowing us to avoid the aforementioned drawbacks.

We consider the Quantum Natural Gradient Descent (QNGD) scheme which was recently proposed to train variational quantum algorithms. QNGD is Steepest Gradient Descent (SGD) operating on the complex projective space equipped with the Fubini-Study metric. Here we present an adaptive implementation of QNGD based on Armijo’s rule, which is an efficient backtracking line search that enjoys a proven convergence. The proposed algorithm is tested using noisy simulators on three different models with various initializations. Our results show that Adaptive QNGD dynamically adapts the step size and consistently outperforms the original QNGD, which requires knowledge of optimal step size to perform competitively. In addition, we show that the additional complexity involved in performing the line search in Adaptive QNGD is minimal, ensuring the gains provided by the proposed adaptive strategy dominates any increase in complexity. Additionally, our benchmarking demonstrates that a simple SGD algorithm (implemented in the Euclidean space) equipped with the adaptive scheme above, can yield performances similar to the QNGD scheme with optimal step size. Our results are yet another confirmation of the importance of differential geometry in variational quantum computations. As a matter of fact, we foresee advanced mathematics to play a prominent role in the NISQ era in guiding the design of faster and more efficient algorithms.

The rapidly growing field of integrated photonic quantum computing has recently seen enormous breakthroughs, with integrated photonic devices capable of generating highly entangled states of photons on-chip. To scale these devices, simulation tools that model realistic sources and manufacturing imperfections are required when designing quantum building blocks that meet fidelity requirements and fault tolerance thresholds. To address this, we introduce a quantum circuit solver capable of calculating the heralded source biphoton wavefunction and corresponding frequency domain response of photonic integrated circuits in the Fock basis, yielding the fidelity of the output state and probability of success for a given measurement outcome.

Recent results on the Quantum Approximate Optimization Algorithm (QAOA) have cast pessimism on its potential to exhibit practical quantum speedups. For instance, QAOA’s locality limits its performance on tasks such as coloring bipartite graphs—which is easy for classical methods. Motivated by these limitations, the Recursive QAOA was introduced to overcome the locality and symmetry of QAOA. Despite being more powerful than QAOA, RQAOA is fully classically simulable at level-1 depth (p = 1). We report results on RQAOA in this classically simulable regime, benchmarked on random Quantum Unconstrainted Binary Optimization (QUBO) problems with up to 100 variables. We find that RQAOA generally matches the performance of classical simulated annealing and significantly outperforms ordinary QAOA.

The quantum annealing devices, which encode the solution to a computational problem in the ground state of a quantum Hamiltonian, are implemented in D-Wave systems with more than 2,000 qubits. However, quantum annealing can solve only a classical combinatorial optimization problem such as an Ising model, or equivalently, a quadratic unconstrained binary optimization (QUBO) problem. In this paper, we formulate the QUBO model to solve elliptic problems with Dirichlet and Neumann boundary conditions using the finite element method. In this formulation, we develop the objective function of quadratic binary variables represented by qubits and the system finds the binary string combination minimizing the objective function globally. Based on the QUBO formulation, we introduce an iterative algorithm to solve the elliptic problems. We discuss the validation of the modeling on the D-Wave quantum annealing system.

The vision of a global network that enables quantum communications between any point on Earth is known as the quantum internet. One crucial element of this network is the use of quantum repeater chains, which have the potential to overcome transmission losses and implement entanglement or quantum key distribution protocols over extended distances. There are various proposals for quantum repeaters, but they can generally be evaluated based on two main figures of merit: the average time for end-to-end entanglement delivery and the associated average fidelity. However, characterizing these quantities can be difficult due to factors such as feedback loops, decoherence, entanglement generation being a probabilistic process, and the potential failure of subprotocols. In this talk, I will discuss algorithmic and analytical methods for computing these quantities for relevant families of protocols.

Modern cryptography is more than sending secret messages, and quantum cryptography is more than quantum key distribution. One example is oblivious transfer, which is interesting partly because it can be used to implement secure multiparty computation. We discuss a protocol for quantum XOR oblivious transfer, and how non-interactive quantum oblivious transfer protocols can be “reversed”, so that oblivious transfer is still implemented from a sender to a receiver, but so that it is the receiver who sends a quantum state to the sender, who measures it, instead of the other way round. This is useful when one party can only prepare and send quantum states, and the other party can only measure them, which is often the case in practical quantum communication systems. Both the “original” XOR oblivious transfer protocol and its reversed version have been implemented optically. We also discuss how quantum random access codes can be connected with quantum oblivious transfer.

Quantum key distribution (QKD) is the first commercial application of the second quantum revolution. Although QKD systems have already been developed and implemented all around the world, some open challenges are limiting the overall deployment of this technology (limited key rate, limited link distance, etc.). By improving the current QKD protocols, it is possible to increase the final secret key generation rate. In this work, we compare 1-decoy with 2-decoy methods in BB84 protocol over an underwater optical fiber link connecting Malta to Italy, showing that 2-decoy can achieve more than twice the key rate of 1-decoy method.

In the absence of quantum repeaters, the only way to increase the quantum communication capacity at a given reach is to employ more modes for encoding and transmitting information. While frequency, temporal, and polarization modes have already been exploited for this purpose, the use of many spatial modes over long distances has only recently become possible owing to the development of low-loss few-mode fibers (FMFs). We will discuss the development of two key enablers of quantum communication over FMF spatial modes: 1) spatiallyentangled photon-pair generator and 2) dynamically-reconfigurable spatial-mode de-multiplexer that can perform projective measurements alternating between two sets of mutually unbiased bases in a given spatial mode space. Both devices are based on the spatial-mode-selective quantum frequency conversion process, implemented in either χ(2) (multimode LiNbO3 waveguide) or χ(3) (custom-made FMF) nonlinear medium.

The coexistence of classical and quantum signals over the same optical fiber is critical for quantum networks operating within the existing communications infrastructure. Here, we characterize the quantum channel that results from distributing approximate single-photon polarization-encoded qubits simultaneously with classical light of varying intensities through a 25 km fiber-optic channel. We use spectrally resolved quantum process tomography with a newly developed Bayesian reconstruction method to estimate the quantum channel from experimental data, both with and without classical noise. Furthermore, we show that the coexistent fiber-based quantum channel has high process fidelity with an ideal depolarizing channel if the noise is dominated by Raman scattering. These results aid future development of quantum repeater designs and quantum error-correcting codes which benefit from realistic channel error models.

Precision synchronization is vital for robust long-distance quantum networking over fiber and free-space channels for which high-fidelity entanglement swapping between separate sources via an optical Bell state measurement requires temporal overlap of photonic qubits arriving from either source. This challenge is particularly distinct in satellite-based entanglement distribution in which relative motion, channel effects, and propagation delay must be addressed. This work presents a precision synchronization method for free space entanglement distribution, and reports on risk reduction testing in a quantum networking testbed at MIT Lincoln Laboratory. Primary consideration is for a dual-uplink architecture in which photons from entanglement sources at two ground locations interact in an optical Bell-state measurement implemented on a satellite in a low-earth orbit. The control approach uses independent entanglement sources at each ground location supplemented with a synchronization signal for feedback control from a timing discriminant measured at the spacecraft. The approach is being implemented in a laboratory testbed using 1-GHz repetition rate 1550-nm band entanglement sources generating ~10-MHz source entanglement rates with few-ps photon pulse lengths. The paper describes both fundamental architectural considerations and practical implementation details.

Quantum networks between mobile platforms enable secure communication, distributed quantum sensors, and distributed quantum computing. As progress towards a future quantum internet continues, connecting mobile platforms (e.g., unmanned drones, smart vehicles, ships, and planes) to quantum networks remains a challenge. For instance, engineering constraints for real-world mobile platforms require low size, weight, and power (SWaP) for quantum systems. Additionally, single photons must be routed to platforms that are in motion and experience vibrations. In this effort, we discuss progress toward developing and demonstrating quantum communication links, including decoy-state quantum key distribution (QKD), between mobile drone and vehicle platforms in several configurations (drone-to-drone, drone-to-moving vehicle, and vehicle-to-vehicle). We will discuss and analyze critical subsystems including our decoy-state QKD source based on resonant cavity light emitting diodes (LED), compact optical system design, pointing, acquisition, and tracking (PAT) subsystem, single-photon detectors, field-programmable gate array-based time-tagger, and a novel time-synchronization algorithm. In addition, we present system performance including tracking performance under multiple conditions and mobile platform configurations.

Frequency-encoded quantum information offers intriguing opportunities for quantum communications networks, with the quantum frequency processor (QFP) paradigm promising scalable construction of quantum gates. Yet all experimental demonstrations to date have relied on discrete fiber-optic components that occupy significant physical space and impart appreciable loss. We introduce a model for designing QFPs comprising microring resonator-based pulse shapers and integrated phase modulators. We estimate the performance of frequency-bin Hadamard gates, finding high fidelity values sustained for relatively wide-bandwidth frequency bins. Our simple model and can be extended to other material platforms, providing a design tool for future frequency processors in integrated photonics.

One of the most advanced technologies within the field of quantum mechanics is quantum key distribution (QKD), which allows the secure generation of secret keys among remote users. In order for QKD to be more widely adopted, it must be integrated into existing classical communication systems. However, this can be difficult due to the use of various technologies and channels in deployed networks. Recently, we developed a QKD network in the metropolitan area of Padova, which connects various nodes across the city through a combination of fiber and free-space links. By utilizing a modular design based on the iPOGNAC encoder and the Qubit4Sync method, we have realized portable and adaptable systems that operate in the C and O bands. This allowed us to deploy and test the compatibility of both research and commercial QKD systems by ThinkQuantum with classical communication over a variety of links, as well as their ability to switch between free-space and fiber connections. Finally, we developed and experimentally implemented complex network configurations such as star networks, where a fiber-based transmitter and free-space transmitter could operate with a single receiver.

We report the development of an optical assembly and driving electronics for a low-SWaP polarization encoder design for use in satellite-to-ground quantum communication. The optical design multiplexes multiple lasers, which are selectively excited to produce a polarization encoded output. This implementation is intrinsically stable due to the use of only polarization maintaining fiber in the combining optics. The transmitter, provides a low-cost, low-power and high-speed platform to produce polarization encoded pulses. We use the transmitter to generate 4 polarization states with 2 intensity levels via multiplexing of 8 pulsed light sources. The module can generate the polarization states H, V, D and A, which correspond to polarization angles of 0, 90, 45 and -45 degrees respectively, forming two mutually unbiased bases. The transmitter is characterized via a polarization decoder over a free-space link within a laboratory setting. We characterize the source for varying optical channel loss which is introduced between the transmitter and receiver. The transmitter employs the T12 decoy-state BB84 protocol. We explore the performance of the system with commercially available single photon detectors for two clock rates of 500 MHz and 1 GHz. We find a similar secure key rate for both repetition rates, despite the expected 3 dB gain at 1 GHz. This is a result of detector jitter hindering the performance of the QKD system, resulting in a larger QBER when detection events leak into the adjacent time bins and ultimately reduces the secure key rate.

Efficient transcontinental entanglement distribution is necessary to build a global quantum network. Without quantum repeaters, distribution through optical fibers is assailed by loss and scattering, limiting the network’s reach to around 100 kilometers. This distance can be greatly extended however, by transmitting photons through free space, where the transmission falls only as the reciprocal square of the propagation distance. Our experiment aims to prove the viability of one proposed satellite intermediary scheme: a down-link architecture using entanglement swapping. In this scheme, a satellite generates a pair of entangled photons that are spectrally unentangled – and therefore able to interfere with other photons. The satellite transmits the telecom pump and the entangled telecom photons down to a station on Earth’s surface; by doing so, both channels experience the same temporal drift due to Doppler shift and dispersion. The transmitted pump can then be collected, reamplified, and used to pump a second terrestrial entanglement source. Synchronizing and interfering the satellite and terrestrial entangled, telecom photons will then swap entanglement to the unused photons. We are implementing this by pumping non-degenerate entanglement sources, which produce daughter photons at 773 nm and 1588 nm, with a 520 nm pump, generated from third harmonic generation.

Quantum Key Distribution (QKD) is a potential mitigation against quantum computation advances that threaten currently deployed public key cryptosystems. QKD is also a precursor to large-scale quantum communications and the quantum internet. Terrestrial direct transmission of quantum photonic signals is restricted by optical fibre exponential absorption, hence space-based systems have been proposed for intercontinental distribution of quantum keys with lower-loss free-space transmission through vacuum. However, challenges remain for the practical deployment of satellite QKD (SatQKD) such as weather, background light, and terminal deployment. In particular, the short transmission time inherent in low Earth orbit satellite QKD and finite statistical uncertainty can limit secure key generation. Here we illustrate the analysis of these effects and some practical engineering design and operational constraints on key length using the Satellite Quantum Modelling and Analysis (SatQuMA) software package being developed for satellite quantum communications research.

Wavelength tunability in quantum key distribution (QKD) is desirable as noise can be introduced to the quantum channel if there is significant traffic near the same wavelength as the quantum data. Today’s QKD systems are typically of fixed wavelength due to the single-mode lasers used to encode qubits. We present a new optical transmitter design consisting of a multi-modal Fabry Perot laser optically injection locked by a tunable laser, capable of encoding qubits at GHz speeds with a tunable wavelength of < 65 nm and perform proof-of-principle BB84 QKD over a range of wavelengths with secure bit rates of order Mb/s.

Packet-switching has been widely used in classical fiber networks for better network scalability and efficiency. Recently, we introduced packet-switching as a new paradigm in quantum networking, where quantum payloads are packetized with classical headers to construct hybrid classical-quantum data frames, and the routing decisions are made in a decentralized fashion at individual routers dynamically. While there are tremendous challenges in building a generic packet-switched quantum network, our study shows that it is possible to develop metropolitan size packet-switched quantum key distribution (QKD) networks with today’s technology. This may greatly facilitate the integration of QKD with classical communication networks and accelerate its adoption.

Trapped ions are one of the leading platforms for quantum computation and networking, with long coherence times and high fidelity qubit operation. We exploit these attributes to build a quantum network with the use of three trapped ion modules connected via photonic links. In each of these systems, we have ions of different species for use as memory or communication qubits. Our newest module contains two high NA=0.8 objectives, each covering 20% of the solid angle of photons emitted from the ion, making it suitable for use as the middle node of the network. Photons are collected from each chamber, and we generate remote entanglement between each pair of nodes by interfering the photons on a beamsplitter. This heralds entanglement of the communication ions in the pair of network nodes, which we then swap onto the memory qubits. I will discuss our progress towards creating a Greenberger–Horne–Zeilinger (GHZ) state between the memory ions in the three nodes.

Intermodal quantum key distribution (IM-QKD) enables the integration of fiber networks and free-space connections, which can be ground-to-ground links or involve satellite nodes in orbit. IM-QKD permits to extend the reach of free-space links without trusting any additional node, but this requires to efficiently couple the free-space signal into a single-mode fiber (SMF). We present the implementation of different IM-QKD networks realized in Padova and Vienna, exploiting km-long deployed fibers and free-space channels up to 620 meters. We show that such an intermodal scheme is compatible with both in-house QKD systems and commercially available solutions exploiting polarization encoding at 1550 nm. Remarkably, we realized different QKD tests in daylight and also in rainy conditions.

Advancing quantum information and communication requires the control of quantum correlations in complementary degrees of freedom. In this work, we generate electron-photon pair states via inelastic scattering of free electrons at a high-Q photonic-chip-based microresonator. In analogy to spontaneous parametric down-conversion, time- and energy-resolved detection of both particles enables various heralding schemes. We experimentally characterize this new heralded source of single photons and free electrons. Ultimately, these results underpin the recent progress in free-electron quantum optics, promising electron-photon entanglement, tailored photon Fock states, and quantum-enhanced electron imaging.

A single-photon source has been developed using a single quantum dot to mimic a two-level atom. Low noise is achieved by operation at low temperature, the use of very high quality material, and by embedding the quantum dots in a diode structure. A single quantum dot is tuned into resonance with an open microcavity, a highly miniaturised Fabry-Perot cavity. The Purcell factor is approximately ten resulting in a radiative lifetime of just 50 ps. An end-to-end efficiency of above 50% is achieved and the photons exhibit two-photon interference with a visibility of 98%.

The performance of modular, networked quantum technologies will be contingent upon the quality of their light-matter interconnects. Solid-state colour centres, and in particular T centres in silicon, offer competitive technological and commercial advantages as the basis for quantum networking and distributed quantum computing. These newly rediscovered silicon defects offer direct telecommunications-band photonic emission, long-lived electron and nuclear spins, and proven integration into industry-standard, CMOS-compatible, silicon-on-insulator (SOI) photonic chips at scale. Here we present recent advances with T centre devices towards high-performance, large-scale distributed quantum technologies based upon T centres in silicon.

We report our recent progress in the generation and verification of a transverse-mode Bell state using spontaneous four-wave mixing in a few-mode polarization-maintaining fiber. Utilizing spatial light modulators, we show control over individual four-wave mixing processes through precise beam shaping of the pump transverse mode and verify entanglement via quantum state tomography. We discuss challenges in temporal and frequency distinguishability and illustrate how they can be resolved. This work represents a first step towards creating a versatile fiber-based transverse mode-entangled photon-pair source matched to fiber infrastructure.

Near-field interactions among quantum emitters can be leveraged to realize useful novel light sources. One example is superradiance, the cooperative enhancement of spontaneous emission, which enables lasing with high spectral purity. Here, we numerically demonstrate suppression of superradiance in atomic arrays resonant with the epsilon-near-zero region of a nearby indium tin oxide (ITO) thin film. By exploiting the field-effect tunability of ITO, we show how cooperative emission can be switched on and off using a simple gold-oxide-ITO thin-film structure.

High degrees of security have been achieved in quantum communication over fiber using entangled photons. For communications applications, it is crucial to produce high-yield entangled photon pairs (EPPs) with the capacity to transmit them over telecommunication-length fiber distances and the ability to integrate and transmit them over existing classical communication systems. Since spatial division multiplexing (SDM) is currently being demonstrated to replace single-mode communication systems, it only makes sense to demonstrate the generation and transferring EPPs in various fiber modes suitable for generation and transmission in SDM fibers. We experimentally demonstrate EPP formation in a 25-km-long graded-index few-mode fiber (three modes) via the intermodal spontaneous four-wave mixing (SFWM) effect. Mode profile and phase matching condition have been confirmed through experiments.

NASA is developing quantum metrology capabilities for potential space-based quantum components in future navigation and communications systems. Innate knowledge of component operation is key for the space qualification of these components. This paper focuses on the measurement and analysis of an important characteristic of an entanglement source, the joint spectrum. We describe a spectrometer based on dispersive optical fibers and present experimental measurements of the joint spectrum of a highly non-degenerate SPDC-based entanglement source that emits entangled photons in the near-infrared and telecommunications bands. How the analysis of such a joint spectrum could be applied to the modeling and simulation of entanglement swapping operations as possible extensions of quantum networks is examined. Lastly, we discuss how the separability of the two-photon state is quantified via Schmidt decomposition and how the degree of separability impacts the spectral purity of heralded single-photon emissions.

Single-frequency laser sources serve as a backbone for quantum computing and metrology technologies and applications. Availability of integrated laser systems delivering tailored multiwavelength emission across optical spectrum is a defining factor for further quantum technologies development and commercialization. Modulight demonstrates highly integrated ML6600 laser system for cooling and repumping of 87Rb atoms at D2 line. Such laser system incorporates driving electronics, internal spectroscopy-locked reference laser source, heterodyne detection coupled with frequency stabilization and tuning functionalities of outputs. In-house semiconductor engineering and fabrication of DBR, DFB and external cavity lasers allows ML6600 system to cover wavelength ranges from UV to IR.

Laser diode solutions for quantum systems have highly variable requirements, depending on the technology and purpose of the laser used in the application – for instance, quantum control of particles or molecules and excitation of the quantum systems. Requirements of the laser systems used in the mentioned applications are highly demanding, such as single-mode operation, frequency stability over operation lifetime and narrow spectral linewidth. Narrow spectral linewidth can be achieved with distributed Bragg reflector (DBR), distributed feedback (DFB) lasers and external cavities. Wavelength of laser diode and light output power can vary depending on the intended application. For further efficiency, such emitters can be fabricated in arrays, eliminating the need for multiple single-emitter chips. Individually addressability of the emitters further enhances the efficiency. Examples of different applications using laser diodes described earlier are frequency comb generation, single-photon emitters, and timing sources for picosecond pulses. In this work, we present our results in 780 nm region DFB laser diodes. Gratings were implemented within the structure, including overgrowth step. Multiple variants in grating pitch were introduced for structure and design optimization purposes. Future improvements in the device processing, design, and reliability are discussed. 780 nm region laser diodes are used in multiple quantum systems, such as atomic clocks and manipulation of Rb atoms. In addition, it can be used for terahertz wave generation.

We report two Quantum Random Number Generator (QRNG) schemes based on Amplified Spontaneous Emission (ASE) to address the problems of previous ASE based QRNGs – (1) costlier and commercially difficult-to-get components, (2) higher electrical noise in the faster detectors. Our ASE-ASE-beating based QRNG, built using commercially available components, clocks in 7.437Gbps random number generation rate. Our second QRNG scheme based on the beating of LASER and ASE of similar power generates random bits at a speed of 7.80Gbps. The minimum entropy values for these schemes are 5.21 and 5.4990 respectively for an 8-bit ADC. The second scheme reports higher entropy because its random numbers has a higher variance than first one. We use the Toeplitz extractor on the raw bits to generate unbiased random numbers and verify them with the help of industry-standard statistical tests.

A major challenge in photonic quantum technologies is two-photon interference from distinct quantum emitters on the same chip. Here, we show and discuss recent results on Hong–Ou–Mandel interference experiments using couples of single organic molecules within few tens of microns, yielding post-selected visibilities of up to 97%. In particular, we discuss the potential interest for future realizations of measurement-device independent quantum key distribution protocols for information-theoretic secure communication.

Photonic integrated circuits (PICs) are a technology with a growing interest in a wide range of applications in quantum information, from computation to communications and sensing. Amongst the various types of PICs, universal quantum photonic processors (UQPPs) are programmable photonic integrated circuits on which any arbitrary unitary transformation can be implemented on a given input quantum photonic state, sometimes also referred to as quantum photonic FPGAs. Various kinds of UQPPs have been reported, mostly with a reconfigurable Mach-Zehnder interferometer (MZI) as building block, in photonic platforms ranging from silicon nitride to glass-based direct laser writing. Among them, femtosecond laser writing (FLW) is a versatile fabrication technique that allows for cost-effective fabrication of waveguides in glass substrates with low insertion losses (down to 0.1 dB cm⁻¹ for the propagation and 0.2 dB per facet for the coupling) over a wide wavelength range, a key requirement for quantum applications. Moreover, FLW allows for the fabrication of microstructures in the substrate such as trenches, which can act as thermal isolation structures that significantly reduce the power dissipation of thermal phase shifters and their thermal crosstalk. In this work, firstly we present the fabrication of a 6-mode FLW-UQPP in a rectangular MZI mesh layout operating at 785 nm with average amplitude fidelity as high as 0.9963 when implementing switching unitaries and 0.9979 when implementing Haar random unitaries. Secondly, we demonstrate the versatility of the FLW platform by fabricating another 6-mode UQPP with waveguides optimized for operation at 1550 nm wavelength, with similar performance, on which we repeated the implementation of Haar random unitaries with 0.9969 average amplitude fidelity.

Two main challenges for quantum networks are state preservation and scaling current infrastructure. Photonic polarization qubits are susceptible to effective decoherence via polarization mode dispersion in optical fibers. This can be circumvented by encoding qubits in the photon’s arrival time, i.e., time-bin encoding. Here, we present measurements on a thin-film lithium niobate integrated-optic device, designed to analyze telecom-wavelength photonic time-bin qubits. By thermo-optically tuning the phase and amplitudes of interfering processes traversing the photonic circuit on the device, we are able to obtain ∼ 83% interference visibility, marking significant progress towards efficient time-bin encoding and analysis with integrated photonics.

Machine learning has paved the way for many breakthroughs in recent years, from the development of powerful computers to advances in image and voice recognition. The technique is applied to a wide range of problems, yet it is particularly useful in solving biomedical questions such as how cells function, get infected by microorganisms, and respond to drugs. Separately, biophotonics is the science involving the mixing of life and light sciences, where numerous health challenges have been studied. However, regardless of outstanding achievements and contributions to life sciences and medicine, there are significant challenges and exciting opportunities in both biophotonics and machine learning. In this article, we provide an overview of machine learning in biophotonics and its applications in cell imaging, diagnosis, and treatment. In biophotonics, machine learning is used to classify, detect, and segment cells or pathogens. Our group aims to use machine learning to detect, classify, and differentiate normal cells from diseased cells for diagnostics at point-of-care settings. It is noteworthy that we focus on the challenges of working with high-dimensional data such as continuous spectra from optical imaging and spectral features from cell imaging. The resulting increase in technological advances has led to an ever-increasing need for easy access, faster, and more efficient methods of healthcare. Applications of machine learning in biophotonics are already providing the answer in many ways, and through it, there is the potential to fully transform healthcare in the future.

The Josephson junction is a device and constructed by separating two superconductors with an insulating barrier, which is thin enough so that super-electrons or Cooper pairs, composed of two electrons with opposite momenta and spins, formed by the electron-phonon interaction, and described as probability waves, can quantummechanically tunnel through the barrier and cause superposition and interferences. Due to the quantum e↵ect, a Josephson junction can generate and sustain an indefinitely long direct electric current or potential di↵erence, which becomes alternating one when an external voltage is applied. This paper investigates the Josephson pulse including its generation, property, and control. The results obtained from this study indicate that perturbations of the superconductors can generate strong superconducting pulses in the Josephson junction. The electric potential and field in the barrier oscillate when the two superconductors have di↵erent physical properties including the number densities of super-electrons, the phases of superconductors, and their rates of variations. The oscillations can be harmonic, combined, and purely pulse-like. The strength of the pulse increases with the di↵erences in the rates of densities and phases between the two superconductors. The pulse can be stronger in strength and narrower in width when one superconductor has lower density but higher rates of density and phase than another superconductor. Light or electromagnetic waves can perturb the superconductivities of superconductors and hence can be applied to control the pulses of Josephson junctions.

Classification of MRI images of brain cancer using deep learning methods such as CNN’s is an increasingly popular method to detect cancer and its spread. In the present work, we perform a competitive analysis of hybrid quantum CNN based methods to classify the MRI images of brain cancer into three different classes using quantum simulators. The quantum image processing is done via three encoding schemes, viz. QCNN, FRQI and NEQR. We see that QCNN has higher accuracy of 87% with a precision of 81%. The NEQR and FRQI encoding schemes have an accuracy of 79% and 75%, respectively. The computational time for QCNN, FRQI and NEQR are considerably less than the conventional CNN method, which was tested by running the same dataset through DenseNet121 Keras architecture.