This PDF file contains the front matter associated with SPIE Proceedings Volume 12570, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

We discuss the use of time and frequency degrees of freedom of single photons in quantum optics. Such a degree of freedom is generally discretized into modes for experimental reasons, but it is not a physical requirement. The origin of the quantumness of the time and frequency variables can be explained because of the non-commutativity of time and frequency operators - which can be defined properly- when restricted to the one photon per mode subspace. As a consequence, We will show that frequency and time operators can be used to define a universal set of gates in this particular subspace and provide an experimental implementation of such a universal set of gates.

There has been an increased focus on precise time and frequency transmission dissemination at a national and international level recently. We would like to present the situation in the Czech Republic, our strategy, approach, and our experience with a non-commercial, cost-effective solution that utilizes shared optical networks. The presented solution provides accurate time and stable frequency at a lower operational cost, utilizing the shared spectrum of the CESNET3 network infrastructure. We are committed to future developments and upgrades that will include the next wavelength bands and geographic extensions. Additionally, we have implemented bidirectional dark channels on various wavebands, which utilize shared leased fibers and offer bidirectional compensation for fiber losses. However, operating precise time and frequency requires a single path with bidirectional amplification performed by optical amplifiers, which are sensitive to feedback from the fiber line induced by back-scattering, and reflections, and which can cause unwanted oscillations. We have addressed this issue by carefully solving the interference with parallel data transmissions. In summary, we have implemented a cost-effective solution for precise time and frequency dissemination in the Czech Republic, which utilizes shared optical networks. We are committed to future developments, and we are also part of a consortium that plans to realize a Pan-European network to offer time and frequency services to a broad range of users.

We have optimized the performance of the semiconductor single photon detector developed for ESA European Laser Timing (ELT) project. It is used to facilitate transmission of time information from atomic clock onboard the International Space Station (ISS) to ground. These kinds of detectors can achieve single shot time resolution better than 20 ps and propagation delay stability significantly better than 1 ps. Our main focus in improving the detector’s capabilities is the active quenching and gating control circuit (AQGC), which uses a comparator as its main component. The comparator is the most vital component in the circuit and plays a major role in determining the detector’s final characteristics. The previously employed comparator was underperforming, it was especially unsatisfactory when it came to propagation delay temperature stability, so we have chosen to replace it with a more suitable alternative. In our measurement we have achieved linear propagation delay temperature dependence of 150 fs/K without evidently compromising the other characteristics of the detector. Even lower temperature dependence of detection delay is expected to be achievable in the new circuit setup. Our next step is the overall improvement of detector quenching speed, which will be done by removing redundant parts of the AQGC and replacing outdated parts for more modern equivalent versions.

Quantum Key Distribution (QKD) is a promising tool for secure communication in the near future. In combination with one-time-pad technique, it provides an unconditionally secure communication channel (meaning that it is secure against an adversary, even with unlimited computational power), at least in principle. However, a major implementation challenge for some schemes is the reliable creation, transportation, and measurement of entangled photon pairs over long-distance fiber networks. Our project aims to explore the possibilities for distributing quantum information on an existing network infrastructure while measuring the effects of real-world conditions. We characterized a commercial source of entangled photons. We measured its spectrum, brightness (1.6±0.3)×10⁴ pairs/s/μW, and performed quantum state tomography (QST) to reconstruct the density matrix of the quantum state. Our implementation focuses on an all-fiber solution, which would enable a simplified QKD implementation. In laboratory conditions, we achieved the visibility equal to (0.957 ± 0.004) as a mean in both bases with a coincidence rate of (275 ± 4) counts/s and successfully ran QKD protocol with secret key rate of (86 ± 1) bits/s and average quantum bit error rate (QBER) of (4.8 ± 0.7) %.

This work focused on two primary error sources limiting Satellite Laser Ranging accuracy. The detection delay instability spoils ranging data concerning long-term stability. This results in the impossibility of distinguishing if the error has an origin during calibration or ranging; both errors limit the SLR product quality. Another error source arises from threshold detection, where no information about signal amplitude is available for individual laser signal returns. In this contribution, we will introduce a new ranging strategy, where we time-tag photons of interest against so-called clock photons. They are accurately synchronized to a stable atomic clock and generate equidistantly well-defined time intervals as an optical ruler. The clock photons propagate through the same detection chain as ranging photons and effectively remove and calibrate all variable electrical delays during the detection process. In this detection scenario, the biggest drawback may be the increase in noise since the detector noise is added twice. However, the clock photons can be averaged effectively; therefore, the additive clock detection jitter is suppressed. Since the detector response is sampled using 20 GSPS, it allows us to reconstruct a detector response envelope and consequently remove any amplitude to timing modulation from ranging data. Here we will outline the measurement concept and discuss laboratory tests and range measurements obtained from satellite echoes.

The timing jitter of a single-photon avalanche diode (SPAD) can be a limiting factor for the performance of quantum key distribution (QKD). Within QKD receiver modules, single mode fiber coupling has been extensively studied due to optical fiber-based demonstrations. However, multimode fiber, which is utilized for free-space demonstrations because it reduces the coupling losses, has been less extensively studied, but it is understood that multimode fibers will increase the timing jitter and decrease the performance of the detector as the fiber core diameter increases. Here, we present a collection of experimental analysis over the relationships between characteristics of multimode fiber such as length, core diameter and incident spot size and how this impacts the timing jitter of the detector. Additionally, we present analysis on the use of graded-index multimode fibers, mitigating for a portion of the impact on quantum bit error rate (QBER) due to multimode characteristics which can improve implementation of these receiver systems.

Superconducting Microstrips Single Photon Detectors (SMSPDs) are currently of crucial interest in numerous applications thanks to their excellent performance in terms of high detection efficiency, short jitter time, and low dark count rate, meeting the necessities of covering a large active area using low-cost technology. In this scenario, the research of new materials to detect from telecom (1550 nm) to mid-infrared wavelengths plays an important role. In this work we fabricated SMSPDs made of NbRe, an innovative material recently proposed in this field. We realized devices with different microstrip configurations based on single microstrips and pairs of parallel microstrips to investigate the role of the geometry. Single photon detection sensitivity at 1550 nm wavelength at a temperature of 1.79 K was demonstrated. The results obtained are encouraging for the photon detection with NbRe-based devices covering large areas.

We demonstrate proof-of-principle carbon dioxide (CO₂) sensing at 2.05μm wavelength using a commercial infrared superconducting nanowire single photon detector (SNSPD). We verify the absorption peaks within the tuning range of the illumination source and perform a differential measurement within one of these peaks to detect a changing concentration of carbon dioxide within a gas cell. We demonstrate that SNSPDs are a promising enabling technology for short wave and mid wave infrared gas remote sensing.

High count rates (10’s to 100’s to 1000’s of MHz) and very precise timing (10’s of ps FWHM jitter or less) are two of the features which make superconducting nanowire single-photon detectors a revolutionary experimental and engineering tool. Both of these performance metrics also depend on the device bias current level. The maximum count rate is determined by how soon after detection the bias current recharges to the level required for maximum efficiency, while the timing jitter decreases with increased bias current even beyond level which yields maximum efficiency. For a device with a strong detection efficiency plateau, the bias current recharge can push the device into the regime of maximum efficiency at a fractional level of the current required to achieve the desired jitter. Here, we present an experimental analysis of this effect. These results should enable users to consider the trade-off between count rate and timing jitter for various experiments.

We present a single-flux-quantum (SFQ) based digital correlator to trace independent signals from two superconducting single-photon detectors (SSPDs) triggering its inputs. In our design two SSPDs are magnetically coupled to inputs of a readout system where direct current (DC)-to-SFQ converters are used to convert transient SSPD output pulses, triggered by detection of single-photon events, to SFQ pulses. The coincidence verification of SFQ pulses, generated by the two DC-to-SFQ converters, is performed with a modified SFQ coincidence buffer. The coincidence buffer is designed to generate an SFQ output pulse only when its both inputs are triggered simultaneously, or within a preset margin time. The output of the coincidence buffer is connected via, this time, an SFQ-to-DC converter, to a pulse counter operated at room temperature. We performed extensive simulations of both the SSPD equivalent circuit and correlator redout elements for the proposed coincidence scheme, using a WRSpice and PSCAN2 simulation platforms that are specifically designated to model Josephson junctions and widely used to simulate operation of the SFQ circuitry. In particular, we investigated our coincidence correlator scheme for measurements of the second-order correlation function, used to demonstrate the antibunching effect in the single-photon detection of non-classical light.

We present a photodetector capable of detecting both optical and x-ray picosecond pulses, based on our in-house grown cadmium magnesium telluride (Cd,Mg)Te single crystals. We focused on a specific Cd_0.97Mg_0.03Te, In-doped crystal composition, because of its bandgap suitable for 800-nm-wavelength light detection and a single-picosecond optical photoresponse. The detector was fabricated as a planar metal-semiconductor-metal structure with interdigitated electrodes and exhibited a linear, Schottky-free, current-voltage characteristics with <40-pA dark current and up to 20-mA/W responsivity. The detector temporal resolution was measured to be ~200-ps full-width-at-half-maximum transient, in response of ~100-fs-wide pulses consisting of either optical (800-nm wavelength) or x-ray (4.5-keV) photons and was limited by the detector housing and 15-GHz bandwidth of the readout oscilloscope. The latter demonstrates the detector is suitable for coarse timing in x-ray free-electron laser/optical femtosecond pump-probe spectroscopy applications. We also demonstrated that due to its very high stopping power, the Cd_0.97Mg_0.03Te detector responded well to various nuclear gamma sources with energy ranging from 59.6-keV to 660-keV.

In this work, light−matter interaction is explored in a hybrid device, consisting of a microfiber cavity and a plasmonic nanoparticle. Perovskite nanowires are embedded in the microcavity and cylindrical nanoholes are formed on the surface of the structure to facilitate generating a hybrid photonic-plasmonic resonator. A spherical gold nanoparticle with a diameter of 15 nm, coated with a 15 nm polymer layer, is placed inside one of the cylindrical nanoholes on the surface of the microdevice to interact the localized surface plasmons with the cavity’s mode field. The device enables extreme light localization through concurrently coupling the emitter’s light into the optical mode field and strong plasmonic field, causing a significant change in the localized density of the electromagnetic states. Time resolved experiments, based on a single photon counting technique, are performed and single-atom cooperativity parameters procedure is applied to determine the enhancement of the light−matter interaction in the presence of the plasmonic nanoparticle. Consequently, light-matter interaction enhances by a factor of 6.4 upon coupling of the Perovskite nanowire into the hybrid photonic-plasmonic mode.

Quantum key distribution (QKD) allows the sharing of secret cryptographic keys between two distant users, whose intrinsic security is guaranteed by the laws of nature. Nowadays, the most promising technique for the integration of QKD in already deployed long-haul telecommunication fiber networks is the Twin-field QKD (TF-QKD) protocol, but it requires that the communication channel is stable in terms of phase oscillations and it is free from background light, that reduces the transmission key-rate. Recently, we presented a solution to the phase stabilization problem, derived from atomic clocks comparison technology, demonstrating advantages in performances of real word TF-QKD. Here we quantify and characterize the background photons, analyzing in details their effects on the transmission and the practicalities to reduce their contribution to a negligible level.

Considerable advances in the growth process of quantum dots with non-trivial geometries have been obtained. These advances lead to countless applications in quantum optics, quantum information, and biophysics. However, the theoretical investigation of these objects is complex and analytically impossible in most cases. The investigation of pyramidal or conical quantum dots with a comparable height-to-base ratio is one of those problems. That is why the numerical finite element method in the framework of the envelope function approximation has been used to obtain the eigenvalues and eigenfunctions for ZnO pyramidal and conical quantum dots. The mesh domains required for the finite element method calculation were pyramidal domains with bases ranging from an equilateral triangle to an equilateral decagon. Cones were used for the approximation of the mesh objects for pyramidal quantum dots with a larger number of edges. Different head radius to height ratios (½, 1, 2, 3, 4) were considered. The optical transition energies were shown to decrease with the increase in the number of faces. The optical transition strengths were shown to exhibit the opposite behavior. The interband absorption curves generally exhibit a redshift with the increase in the number of edges.

Week radiation and single photon detectors have many applications in different areas of modern science and technology. In this work, we study the multi-layer thermoelectric photodetector’s detection pixel consisting of a heat sink (Bi2223), thermoelectric sensor (CeB₆), absorber (Bi2223), and antireflection layer (SiO₂) for UV single photon detection. We examine heat transfer after 3.1 eV and 7.1 eV energy photon absorption in the SiO₂/Bi2223/CeB₆/Bi2223/Al₂O₃ detection pixel. The operating temperature for this structure is 9 K. Computer simulation was carried out based on the equation of heat distribution from a limited volume using the three-dimensional matrix method for differential equations. We study the detector’s signal, count rate, and noise dependence from detection pixel geometry. Calculations show that after absorption of photons with an energy of 7.1 eV the generated signal maximum will be about 127 nV and achieved up to 10 ps after photon absorption. The full width at half maximum of the obtained signal is from 34 – 228 ps, depending on the detection pixel layer thicknesses. Such a signal can be registered without preliminary amplification. It is shown that this detection pixel provides a gigahertz count rate. Using Bi2223 high-temperature superconductors as an absorber and heat sink allowed less Johnson noise.

Thanks to the significant advances in quantum technologies, the use of single photon detectors (SPDs) is becoming increasingly common. As a result of the excellent photodetection performance of these detectors, they have been utilized in a wide range of fields such as quantum cryptography, astronomy, spectroscopy, and medical applications. There is no doubt that improvements in the performance of these detectors will open new paths to their multidisciplinary applications. Over the years, several different types of SPDs have been developed, such as photomultiplier tubes based on vacuum tubes, avalanche photodiodes (APDs) based on semiconductors, or nanowires based on superconducting technology. Any of these technologies, which are also commercially offered by many companies, has been used according to their advantages and disadvantages for intended applications by making a trade-off. At that point, SPDs based on Silicon APD technology have many advantages including low voltage operation, high reliability, simple electronic requirements, and high detection efficiency. In this study, the TO-8 SAP500 series Silicon APD provided by Laser Component was preferred, and the driving circuit was designed for visible-range sensing applications. The quenching and thermoelectric cooling circuit designs were presented, and the performance of the detector was analyzed according to some important parameters. Our motivation is to investigate the CubeSat compatibility of the detector for space applications.

Theoretical investigation of the intraband transition in a cylindrical quantum dot with the Kratzer confining potential is considered in the presence of external electric and magnetic fields. The intraband linear absorption spectra have been calculated for GaAs cylindrical quantum dots with different sizes. Moreover, the behavior of the linear absorption spectra is observed for various regimes of magnetic quantization and depending on the strength of the electric field. The density probability of the electro has been presented depending on the value of the external electric field. It has been shown that the shift of electron localization area is asymmetric depending on the external electric field direction.

A photon number resolving detector (PNRD) is a device providing a different output depending on the number of incident photons in the single or few photons regime. This tool is crucial in several applications such as quantum communication, boson sampling, photon sources characterization and so on. PNRD are not new players in ultraviolet or visible wavelengths, but superconducting nanostrips provide a performing counterpart also in the near-infrared, where the aforementioned applications would like to operate. In this work we present a comprehensive description of the operation of an eight-pixel PNRD at the telecom wavelength of 1550 nm.

In this study, we focus on the influence of quantum wire material on the geometrical factors and electronic energy band structure The system is defined by parabolic confinement which is exposed in the perpendicular magnetic field and Rashba spin-orbit interaction The dependence of the energy dispersion on the material of quantum wire has been examined Numerical results illustrate that the different materials of quantum wire cause the change in geometrical factors and shifting in the energy dispersion Moreover, the oddity in the physical parameters and energy subbands brings out the shift in the optical properties of quantum wire.