Electrically tunable bandpass filters based on liquid crystals are designed, built, and characterized using a vector network analyzer. The filters are composed of half-wavelength open stubs and quarter-wavelength connecting lines in an inverted microstrip geometry. The filters are modeled using computational electromagnetics software utilizing the finite integration technique. Photolithography and thin-film deposition processes are employed, and standard liquid-crystal cell-assembly techniques are used to make the final filter structures. The three-stub filters with passband central frequencies of 30, 50, and 85 GHz are filled with the nematic liquid crystal, LC1917, and tested. 10% tuning of the central frequency is achieved with a 14-volt peak-to-peak ac bias across the $38\text{−}\mu \mathrm{m}$ liquid-crystal layer (electric field of $0.19\mathrm{V}/\mu \mathrm{m}$). At 50 GHz, the insertion loss is $-3.76\mathrm{dB}$, while the return loss ranges from $-9$ to $-25\mathrm{dB}$, indicating a good impedance match for a proof-of-concept device. The passband widths of the 30-, 50-, and 85-GHz filters are 5, 9, and 14 GHz, respectively, resulting in a $Q$ factor of 6. The filter devices presented in this study, although intended for microwave signal-processing applications, furnish an effective methodology for characterizing the dielectric properties of liquid-crystal materials (and fluids or solids in general) up to the terahertz frequency range.

Statistics of photons emitted by mobile excitons in individual carbon nanotubes are investigated. Photoluminescence spectroscopy is used to identify the chiralities and suspended lengths of air-suspended nanotubes, and photon-correlation measurements are performed at room temperature on telecommunication-wavelength nanotube emission with a Hanbury-Brown-Twiss setup. We obtain zero-delay second-order correlation $g^{(2)}(0)$ less than 0.5, indicating single-photon generation. Excitation power dependence of the photon antibunching characteristics is examined for nanotubes with various chiralities and suspended lengths, where we find that the minimum value of $g^{(2)}(0)$ is obtained at the lowest power. The influence of exciton diffusion and end quenching is studied by Monte Carlo simulations, and we derive an analytical expression for the minimum value of $g^{(2)}(0)$. Our results indicate that mobile excitons in micrometer-long nanotubes can in principle produce high-purity single photons, leading to new design strategies for quantum photon sources.

Light scattering by nanostructures has facilitated research on various optical phenomena and applications by interfacing the near fields and free-propagating radiation. However, direct quantitative measurement of far-field scattering by a single nanostructure on the wavelength scale or less is highly challenging. Conventional back-focal-plane imaging covers only a limited solid angle determined by the numerical aperture of the objectives and suffers from optical aberration and distortion. Here, we present a quantitative measurement of the differential far-field scattering cross section of a single nanostructure over the full hemisphere. In goniometer-based far-field scanning with a high signal-to-noise ratio of approximately 27.4 dB, weak scattering signals are efficiently isolated and detected under total-internal-reflection illumination. Systematic measurements reveal that the total and differential scattering cross sections of a Au nanorod are determined by the plasmonic Fabry-Perot resonances and the phase-matching conditions to the free-propagating radiation, respectively. We believe that our angle-resolved far-field measurement scheme provides a way to investigate and evaluate the physical properties and performance of nano-optical materials and phenomena.

Semiconductor nanowires offer the possibility to grow high-quality quantum-dot heterostructures, and, in particular, CdSe quantum dots inserted in ZnSe nanowires have demonstrated the ability to emit single photons up to room temperature. In this paper, we demonstrate a bottom-up approach to fabricate a photonic fiberlike structure around such nanowire quantum dots by depositing an oxide shell using atomic-layer deposition. Simulations suggest that the intensity collected in our $\mathrm{NA}=0.6$ microscope objective can be increased by a factor 7 with respect to the bare nanowire case. Combining microphotoluminescence, decay time measurements, and numerical simulations, we obtain a fourfold increase in the collected photoluminescence from the quantum dot. We show that this improvement is due to an increase of the quantum-dot emission rate and a redirection of the emitted light. Our ex situ fabrication technique allows a precise and reproducible fabrication on a large scale. Its improved extraction efficiency is compared to state-of-the-art top-down devices.

We demonstrate the full functionality of a circuit that generates single microwave photons on demand, with a wave packet that can be modulated with a near-arbitrary shape. We achieve such a high tunability by coupling a superconducting qubit near the end of a semi-infinite transmission line. A dc superconducting quantum interference device shunts the line to ground and is employed to modify the spatial dependence of the electromagnetic mode structure in the transmission line. This control allows us to couple and decouple the qubit from the line, shaping its emission rate on fast time scales. Our decoupling scheme is applicable to all types of superconducting qubits and other solid-state systems and can be generalized to multiple qubits as well as to resonators.

We study the generation of Greenberger-Horne-Zeilinger (GHZ) states of three path-encoded photons. Inspired by the seminal work of Bouwmeester et al. [Phys. Rev. Lett. 82, 1345 (1999)] on polarization-entangled GHZ states, we find a corresponding path representation for the photon states of an optical circuit, identify the elements required for the state generation, and propose a possible implementation of our strategy. Besides the practical advantage of employing an integrated system that can be fabricated with proven lithographic techniques, our example suggests that it is possible to enhance the generation efficiency by using microring resonators.

Harnessing multimode waves allows high information capacity through modal expansions. Although passive multimode devices for broadband responses have been demonstrated in momentum or frequency domains, the difficulty in achieving collective manipulation of all eigenmodes has hindered the implementation of digital multimode devices such as switching. Here we propose building blocks for digital switching of spatially random waves based on parity-converted supersymmetric pairs of multimode potentials. We reveal that unbroken supersymmetric transformations of any parity-symmetric potential derive the parity reversal of all eigenmodes, which allows the complete isolation of random waves in the “off” state. With two representative solvable potentials, building blocks for binary and many-valued logics are then demonstrated for random waves: a harmonic pair for binary switching of arbitrary wave fronts and a Pöschl-Teller pair for multilevel switching which implements fuzzy membership functions. Our results realizing the transfer of arbitrary wave fronts between wave elements will lay the foundation of high-bandwidth data processing.

It is known that the eye’s scotopic photodetectors, rhodopsin molecules, and their associated phototransduction mechanism leading to light perception, are efficient single-photon counters. We here use the photon-counting principles of human rod vision to propose a secure quantum biometric identification based on the quantum-statistical properties of retinal photon detection. The photon path along the human eye until its detection by rod cells is modeled as a filter having a specific transmission coefficient. Precisely determining its value from the photodetection statistics registered by the conscious observer is a quantum parameter estimation problem that leads to a quantum secure identification method. The probabilities for false-positive and false-negative identification of this biometric technique can readily approach ${10}^{-10}$ and ${10}^{-4}$, respectively. The security of the biometric method can be further quantified by the physics of quantum measurements. An impostor must be able to perform quantum thermometry and quantum magnetometry with energy resolution better than ${10}^{-9}\hslash$, in order to foil the device by noninvasively monitoring the biometric activity of a user.

We study the optical transmission of a waveguide that is side coupled to a high-$Q$ circular microresonator. The coupling is critical if the intrinsic resonator losses equal the coupling losses to the waveguide. When this happens, the transmittance of the waveguide displays resonance dips with maximal depth as the frequency is swept through the resonators’ resonances. We show that multiple configurations, parametrized by the minimal distance between the resonator and the waveguide, can lead to critical coupling. Indeed, for a sufficiently large resonator radius, the flow of power between the waveguide and the resonator can change sign several times within a single pass. This leads to an oscillatory coupling parameter as a function of the separation distance. As a result, multiple geometrical configurations can lead to critical coupling, even if the waveguide lies in the equatorial plane of the resonator. These results are explained using coupled-mode theory and full-wave numerical simulations. In the vicinity of secondary or higher-order critical coupling, the depth of the transmittance dip is very sensitive to the environment. We discuss how this effect can be exploited for sensing purpose. Alternatively, by actively controlling the environment in the secondary critical configuration, the waveguide-resonator system can be driven as an optical switch.

${\mathrm{Eu}}^{2+}$-doped alkali or alkali earth iodide scintillators with energy resolutions $\le 3\%$ at 662 keV promise the excellent discrimination ability for radioactive isotopes required for homeland-security and nuclear-nonproliferation applications. To extend their applications to x-ray imaging, such as computed tomography scans, the intense afterglow which delays the response time of such materials is an obstacle that needs to be overcome. However, a clear understanding of the origin of the afterglow and feasible solutions is still lacking. In this work, we present a combined experimental and theoretical investigation of the physical insights of codoping-based defect engineering which can reduce the afterglow effectively in ${\mathrm{KCaI}}_3:{\mathrm{Eu}}^{2+}$ single-crystal scintillators. We illustrate that ${\mathrm{Sc}}^{3+}$ codoping greatly suppresses the afterglow, whereas ${\mathrm{Y}}^{3+}$, ${\mathrm{Gd}}^{3+}$, or ${\mathrm{La}}^{3+}$ codoping enhances the afterglow. Meanwhile, a light yield of $57000\mathrm{photons}/\mathrm{MeV}$ and an energy resolution of 3.4% at 662 keV can be maintained with the appropriate concentration of ${\mathrm{Sc}}^{3+}$ codoping, which makes the material promising for medical-imaging applications. Through our thermoluminescence techniques and density-functional-theory calculations, we are able to identify the defect structures and understand the mechanism by which codoping affects the scintillation performance of ${\mathrm{KCaI}}_3:{\mathrm{Eu}}^{2+}$ crystals. The proposed defect-engineering strategy is further validated by achieving afterglow suppression in ${\mathrm{Mg}}^{2+}$ codoped ${\mathrm{KCaI}}_3:{\mathrm{Eu}}^{2+}$ single crystals.

Nonreciprocal microwave devices, such as circulators, are useful in routing quantum signals in quantum networks and protecting quantum systems against noise coming from the detection chain. However, commercial, cryogenic circulators, now in use, are unsuitable for scalable superconducting quantum architectures due to their appreciable size, loss, and inherent magnetic field. We report on the measurement of a key nonreciprocal element, i.e., the gyrator, which can be used to realize a circulator. Unlike state-of-the-art gyrators, which use a magneto-optic effect to induce a phase shift of $\pi$ between transmitted signals in opposite directions, our device uses the phase nonreciprocity of a Josephson-based three-wave-mixing device. By coupling two of these mixers and operating them in noiseless frequency-conversion mode, we show that the device acts as a nonreciprocal phase shifter whose phase shift is controlled by the phase difference of the microwave tones driving the mixers. Such a device could be used to realize a lossless, on-chip, superconducting circulator suitable for quantum-information-processing applications.

We optimize the design of NbN nanowire superconducting single-photon detectors using the recently discovered position-dependent detection efficiency in these devices. This optimized design of meandering wire NbN detectors maximizes absorption at positions where photon detection is most efficient by altering the field distribution across the wire. In order to calculate the response of the detectors with different geometries, we use a monotonic local detection efficiency from a nanowire and optical absorption distribution via finite-difference-time-domain simulations. The calculations predict a trade-off between average absorption and absorption at the edge, leading to a predicted optimal wire width close to 100 nm for a 1550-nm wavelength, which drops to a 50-nm wire width for a 600-nm wavelength. The absorption at the edges can be enhanced by depositing a silicon nanowire on top of the superconducting nanowire, which improves both the total absorption efficiency and the internal detection efficiency of meandering wire structures. The proposed structure can be integrated in a relatively simple cavity structure to reach absorption efficiencies of 97% for perpendicular and 85% for parallel polarization.

Low-power, high-speed, and bright electrically driven true single-photon sources, which are able to operate at room temperature, are vital for the practical realization of quantum-communication networks and optical quantum computations. Color centers in semiconductors are currently the best candidates; however, in spite of their intensive study in the past decade, the behavior of color centers in electrically controlled systems is poorly understood. Here we present a physical model and establish a theoretical approach to address single-photon emission dynamics of electrically pumped color centers, which interprets experimental results. We support our analysis with self-consistent numerical simulations of a single-photon emitting diode based on a single nitrogen-vacancy center in diamond and predict the second-order autocorrelation function and other emission characteristics. Our theoretical findings demonstrate remarkable agreement with the experimental results and pave the way to the understanding of single-electron and single-photon processes in semiconductors.

Color centers in diamond provide a promising platform for quantum optics in the solid state, with coherent optical transitions and long-lived electron and nuclear spins. Building upon recent demonstrations of nanophotonic waveguides and optical cavities in single-crystal diamond, we now demonstrate on-chip diamond nanophotonics with a high-efficiency fiber-optical interface achieving $>90\%$ power coupling at visible wavelengths. We use this approach to demonstrate a bright source of narrow-band single photons based on a silicon-vacancy color center embedded within a waveguide-coupled diamond photonic crystal cavity. Our fiber-coupled diamond quantum nanophotonic interface results in a high flux (approximately 38 kHz) of coherent single photons (near Fourier limited at $<1\text{−}\mathrm{GHz}$ bandwidth) into a single-mode fiber, enabling possibilities for realizing quantum networks that interface multiple emitters, both on chip and separated by long distances.

We develop a modified two-temperature (2T) model of laser-matter interaction in dielectrics based on experimental insight from picosecond-pulsed high-frequency temperature-controlled second-harmonic (515 nm) generation in periodically poled stoichiometric ${\mathrm{LiTaO}}_3$ crystal and required for computational treatment of short-pulsed nonlinear optics and materials processing applications. We show that the incorporation of an extended set of recombination-kinetics-related energy-release and heat-exchange processes following short-pulsed photoionization by two-photon absorption of the second harmonic allows accurate simulation of the electron-lattice relaxation dynamics and electron-lattice temperature evolution in ${\mathrm{LiTaO}}_3$ crystal in nonlinear laser-frequency conversion. Our experimentally confirmed model and detailed simulation study show that two-photon ionization with the recombination mechanism via ion-electron-lattice interaction followed by a direct transfer of the recombination energy to the lattice is the main laser-matter energy-transfer pathway responsible for the majority of the crystal lattice heating (approximately 90%) continuing for approximately 50 ps after laser-pulse termination and competing with effect of electron-phonon energy transfer from the free electrons. This time delay is due to a recombination bottleneck which hinders faster relaxation to thermal equilibrium in photoionized dielectric crystal. Generally, our study suggests that in dielectrics photoionized by short-pulsed radiation with intensity range used in nonlinear laser-frequency conversion, the electron-lattice relaxation period is defined by the recombination-stage bottleneck of a few tens of picoseconds and not by the time of the electron-phonon energy transfer. This modification of the 2T model can be applied to a broad range of processes involving laser-matter interactions in dielectrics and semiconductors for charge density reaching the range of ${10}^{21}–{10}^{22}{\mathrm{cm}}^{-3}$.

We propose to use graphene-based Josephson junctions (GJJs) to detect single photons in a wide electromagnetic spectrum from visible to radio frequencies. Our approach takes advantage of the exceptionally low electronic heat capacity of monolayer graphene and its constricted thermal conductance to its phonon degrees of freedom. Such a system could provide high-sensitivity photon detection required for research areas including quantum information processing and radio astronomy. As an example, we present our device concepts for GJJ single-photon detectors in both the microwave and infrared regimes. The dark count rate and intrinsic quantum efficiency are computed based on parameters from a measured GJJ, demonstrating feasibility within existing technologies.

Reliable generation of single photons is of key importance for fundamental physical experiments and to demonstrate quantum protocols. Waveguide-based photon-pair sources have shown great promise in this regard due to their large spectral tunability, high generation rates, and long temporal coherence of the photon wave packet. However, integrating such sources with fiber-optic networks often results in a strong degradation of performance. We answer this challenge by presenting an alignment-free source of photon pairs in the telecommunications band that maintains heralding efficiency $>50\%$ even after fiber pigtailing, photon separation, and pump suppression. The source combines this outstanding performance in heralding efficiency with a compact, stable, and easy-to-use “plug-and-play” package: one simply connects a laser to the input and detectors to the output, and the source is ready to use. This high performance can be achieved even outside the lab without the need for alignment which makes the source extremely useful for any experiment or demonstration needing heralded single photons.

The development of quantum relays for long-haul and attack-proof quantum communication networks operating with weak coherent laser pulses requires entangled photon sources at telecommunication wavelengths with intrinsic single-photon emission for most practical implementations. Using a semiconductor quantum dot emitting entangled photon pairs in the telecommunication O band, we demonstrate a quantum relay fulfilling both of these conditions. The system achieves a maximum fidelity of 94.5% for implementation of a standard four-state protocol with input states generated by a laser. We further investigate robustness against frequency detuning of the narrow-band input and perform process tomography of the teleporter, revealing operation for arbitrary pure input states, with an average gate fidelity of 83.6%. The results highlight the potential of semiconductor light sources for compact and robust quantum-relay technology that is compatible with existing communication infrastructures.

We explore a photonic-integrated-circuit platform that implements optical-frequency measurements and timekeeping with a perspective towards next-generation portable and spaceborne frequency references and optical-clock networks. The stoichiometric-silicon-nitride waveguides we create provide an efficient and low-noise medium for nonlinear spectral broadening and supercontinuum generation with fiber-based optical-frequency combs. In particular, we demonstrate detailed control over supercontinuum emission to target specific atomic-transition wavelengths and perform an optical-clock comparison using on-chip supercontinuum sources. We report a clock-limited relative frequency instability of $3.8\times {10}^{-15}$ at $\tau =2\mathrm{s}$ between a 1550-nm cavity-stabilized reference laser and NIST’s calcium atomic-clock laser at 657 nm using a two-octave waveguide-supercontinuum frequency comb.