Heterodyne Research Articles

The finite-size effect on continuous-variable quantum key distribution in the Terahertz band

We conducted a study to evaluate the performance of Terahertz Continuous Variable Quantum Key Distribution (THz CVQKD) systems in atmospheric conditions with the finite-size effect. Our research delved into the secure key rates and transmission distances of THz CVQKD for four protocols, all based on Gaussian coherent states, under the threat of collective Gaussian attacks. These protocols include reverse reconciliation with homodyne detection, direct reconciliation with homodyne detection, reverse reconciliation with heterodyne detection, and direct reconciliation with heterodyne detection. Our findings revealed distinctive key rates across various THz frequencies and protocols. However, the finite-size effect had a diminishing impact on the key rates of CVQKD in all scenarios. Fortunately, by harnessing a substantial number of keys exceeding 1012 and optimizing modulation variance, both the key rates and transmission distances of THz CVQKD can be significantly extended. This research contributes to advancing the practicality of THz CVQKD technology.

Photon Frequency Conversion in High-Q Superconducting Resonators: Axion Electrodynamics, QED, and Nonlinear Meissner Radiation

Abstract High-Q superconducting resonators have been proposed and developed as detectors of light-by-light scattering mediated by the hypothesized axion or virtual electron–positron pairs in quantum electrodynamics: the Euler–Heisenberg (EH) interaction. Photon frequency and mode conversion is central to the scheme for detecting such rare events. Superconducting resonators are nonlinear devices. The Meissner screening currents that confine the electromagnetic fields to the vacuum region of a superconducting RF cavity are nonlinear functions of the EM field at the vacuum–superconducting interface, and as a result can generate source currents and frequency conversion of microwave photons in the cavity. In this report we consider photon frequency and mode conversion in superconducting resonators with high quality factors from Meissner currents in single- and dual-cavity setups proposed for axion and QED searches based on light-by-light scattering. In a single cavity with two pump modes, photon frequency conversion by the Meissner screening current dominates photon generation by the EH interaction for cavities with $Q \lesssim 10^{12}$. The Meissner currents also generate background photons that limit the operation of the resonator for axion detection in three-mode, single-cavity setups. We also consider the leakage of photons from pump modes into the signal mode for both axion- and EH-mediated light-by-light scattering. Photon frequency conversion by the EH interaction can compete with Meissner and leakage radiation in ultra-high-Q cavities that are beyond the current state of the art. Meissner radiation and leakage backgrounds can be suppressed in dual-cavity setups with appropriate choices for pump and spectator modes, as well as the single-cavity setup proposed for heterodyne detection of galactic axion dark matter.

Broad Instantaneous Bandwidth Microwave Spectrum Analyzer with a Microfabricated Atomic Vapor Cell

We report on broad instantaneous bandwidth microwave spectrum analysis with hot Rb87 atoms in a microfabricated vapor cell in a large magnetic field gradient. The sensor is a MEMS atomic vapor cell filled with isotopically pure Rb87 and N2 buffer gas to localize the motion of the atoms. The microwave signals of interest are coupled through a coplanar waveguide to the cell, inducing spin-flip transitions between optically pumped ground states of the atoms. A static magnetic field with large gradient maps the frequency spectrum of the input microwave signals to a position-dependent spin-flip pattern on absorption images of the cell recorded with a laser beam onto a camera. In our proof-of-principle experiment, we demonstrate a microwave spectrum analyzer that has ≈1 GHz instantaneous bandwidth centered around 13 GHz, 3 MHz frequency resolution, 2 kHz refresh rate, and a −23 dBm single-tone microwave power detection limit in 1 s measurement time. A theoretical model is constructed to simulate the image signals by considering the processes of optical pumping, microwave interaction, diffusion of Rb87 atoms, and laser absorption. We expect to reach more than 25 GHz instantaneous bandwidth in an optimized setup, limited by the applied magnetic field gradient. Our demonstration offers a practical alternative to conventional microwave spectrum analyzers based on electronic heterodyne detection. Published by the American Physical Society 2024

Active near‐infrared laser heterodyne system based on supercontinuum laser source for gas measurement

AbstractThe performance of an active near‐infrared laser heterodyne system using a supercontinuum light source as the light signal for trace species detection is experimentally demonstrated. The characteristics of the supercontinuum light source and the data processing methods used in heterodyne detection are described. The measurement of methane (CH₄) and carbon dioxide (CO₂) absorption spectra was carried out in the laboratory to evaluate the active laser heterodyne system, and high‐resolution absorption spectra of CO₂ and CH₄ were recorded simultaneously. The stability of the active laser heterodyne system is analyzed using Allan variance analysis of long‐term observation data. The active near‐infrared laser heterodyne detection system demonstrated in this paper has great potential for the development of industrial parks' gas monitoring.

Visible-wavelength and monostatic coherent transceiver with independent second harmonic generation on transmitted and local lights using commercially available lithium niobate phase modulator for fundamental wavelength

We propose what we believe to be a new monostatic coherent transceiver at visible wavelength using a commercially available lithium niobate (LiNbO3) phase modulator (LNPM), which is capable of high-speed, wide-bandwidth, and multi-functional operation. The proposed coherent transceiver needs successful heterodyne detection with independent second harmonic generation (SHG) on transmitted and local lights. Experimental proof of the above-mentioned heterodyne detection is shown. The proof of concept for the application to continuous-waves coherent Doppler lidar (CW-CDL) is shown in both free-space and underwater sensing. Preliminary experiment regarding high-speed and wide-bandwidth modulation is additionally shown.

Implementation of Optical Coherent Communication System between Two Computers

This work represents implementation and investigation of optical coherent communication system between two computers. A single mode optical fiber is selected as transmission medium. The data are sent via the RS-232 standard interface with a bit rate of 9.6 kbps from personal computer (PC1) by line receive to convert the data from electrical levels (-12/+12 V) into TTL level (0/5 V). The modulation of this data was accomplished by internal modulation using laser diode type (HFCT-5208M) 1310 nm wavelength. The optical D-coupler was used to combine the optical signal that come from laser source with optical signal of laser local oscillator (OTS-304XI) at 1310/1550 nm wavelength to obtain coherent (homodyne and heterodyne) detection respectively. A PIN photodetector (HFCT-5208M) is used. Calculations of Signal to Noise Ratio (SNR) and Bit Error Rate (BER) for coherent detection were measured at different length of the optical fiber. Result show that high SNR and low BER for heterodyne detection than for homodyne detection.

Design and inversion study of a 1.6 µm high resolution non-modulated laser heterodyne radiometer (NM-LHR).

Laser heterodyne detection boasts exceptional advantages such as high spectral resolution and high signal-to-noise ratio (SNR). It excels at capturing spectral line broadening information of upper atmospheric molecules, which presents substantial research value in the realms of greenhouse gas profile measurement and the assessment of laser propagation effects in the atmosphere. This paper delves into the investigation of the processing method for heterodyne signals, adopting a non-modulated signal processing method to construct a near-infrared non-modulated laser heterodyne radiometer. This innovative design significantly enhanced the response speed and SNR. The radiometer achieved a spectral resolution of 0.006 cm-1 and an SNR of 300. This facilitated the acquisition of vertical profile distribution and column concentration of CH4 by measuring the absorption spectrum. Comparative tests revealed compelling advantages of the non-modulated device. The modulated device collected data 6 times in 6 minutes, yielding an SNR of 58. In contrast, the non-modulated device demonstrated superior efficiency by collecting data 6000 times in 2 minutes, resulting in a remarkable SNR of 103. In the process of inversion, the influence of the solar spectrum was coupled to improve the accuracy of inversion results. The inversion results of the CH4 column concentration from the laser heterodyne radiometer were compared with those from the Fourier transform spectrometer (EM27/SUN), with average concentrations of 1.946 ppmv and 1.930 ppmv, and exhibited an overall deviation of approximately 0.8%. The non-modulated laser heterodyne radiometer provides a new reference for the rapid, accurate and high spectral resolution measurements of greenhouse gas concentration.

Design an RF Up-Down Convertor using Software Defined Radio and GNU Radio

There is a need to design of RF Up/Down- converter for (from C/Ku to/from L band) signals required for several applications. The proposed work presents a design of an RF Up/Down Converter that makes use of GNU Radio and Software-Defined Radios (SDRs). The converter helps in frequency translation between the C/Ku and L bands, leading to a cost-effective and versatile solution for RF signal processing. By using open-source GNU Radio software, the proposed system enhances accessibility, enabling its deployment in diverse applications, from satellite communication to radar systems. The converter’s unique features include real-time processing capabilities, customization through an intuitive graphical interface, and Python scripting. This idea presents the design considerations, signal processing techniques, and performance evaluation of the RF Up/Down Converter. The advantages of an open-source solution over other available alternatives are in terms of cost, flexibility, and rapid prototyping. Simulation and hardware results demonstrate the efficacy of the proposed work.

RIS-aided free-space optics communications in A2G networks over inverted Gamma–Gamma turbulent channels

Reconfigurable intelligent surfaces (RISs) have revolutionized free-space optics (FSO) communication by dynamically optimizing the propagation environment. This study proposes a framework to analyze RIS-assisted FSO communication over inverted Gamma–Gamma (IGGG) distributions. We use the IGGG distribution for air-to-ground networks, accurately modeling atmospheric turbulence. Consequently, we derived performance metrics in terms of Meijer’s G by employing an asymptotic analysis to provide deeper insights. Our results demonstrate incorporating RISs into our proposed network enhances outage performance by 52.08% at a SNR of 30 dB. Furthermore, the results highlight the importance of heterodyne detection in mitigating the adverse effects of pointing errors.

Quantum enhancement detection techniques for FMCW LiDAR.

Interferometric LiDAR is a device that is used to achieve distance, velocity and phase estimation with high precision and resolution through the use of frequency-modulated continuous wave (FMCW). In this instance, we study quantum enhancement detection techniques for a Mach-Zender interferometer with a FMCW coherent state input. Various quantum detection methods-including NOON state detection, coincidence detection, and sum of parity detection-are applied to the FMCW coherent state and compared against the classical heterodyne detection technique. The findings reveal the potential to trade maximum detectable distance for resolution enhancement. Furthermore, classical Fisher information is utilized to validate and quantify the precision limits of each detection technique. In scenarios characterized by high losses, it is observed that the precision limits of coincidence detection, sum of parity detection, and classical detection techniques are comparable. Therefore, this study offers practical guidance for designing quantum-enhanced receivers for FMCW LiDAR systems.

Advance directives in the intensive care unit: An eight-year vanguard cohort study

PurposeTo investigate the frequency, content, and clinical translation of advance directives in intensive care units (ICUs). Material and methodsRetrospective cohort study in a Swiss tertiary ICU, including patients with advance directives treated in ICUs ≥48 h. The primary endpoint was the violation of directives. Key secondary endpoints were the directives' prevalence and their translation into clinical practice. ResultsOf 5′851 patients treated ≥48 h in ICUs, 2.7 % had documented directives. Despite 92 % using templates, subjective or contradictory wording was found in 19 % and 12 %. Nine percent of directives were violated. Patients with directive violations had worse in-hospital outcomes (p = 0.012). At admission, 64 % of patients experiencing violations could not communicate, and directives were missing/unrecognized in 30 %. Mostly, directives were not followed regarding life-prolonging measures (6 %), ICU admission (5 %), and mechanical ventilation (3 %). Kaplan Meier statistics revealed a lower survival rate with directives recognized at admission (p = 0.04) and when treatment was withheld (p < 0.001). ConclusionsAdvance directives are available in a minority of ICU patients and often contain subjective/contradictory wording. Physicians respected directives in 90 % of patients, with treatment adapted following their wishes. However, violation of directives may have serious consequences with unfavorable in-hospital outcomes and decreased long-term survival with treatment adaption following directives.

Digital pre-equalization for performance improvement in coherent PON

As high-speed optical access networks continue to evolve, reducing the computational complexity at the optical network unit (ONU) side remains a significant challenge in coherent passive optical network (PON). This paper presents a pre-equalization scheme that integrates digital pre-distortion (DPD) and pre-emphasis techniques in 200 Gb/s coherent PON. The focus is on three modulation formats: 50 Gbaud polarization multiplexing-quadrature phase shift keying (PM-QPSK), 50 Gbaud Alamouti-coded polarization multiplexing-16 quadrature amplitude modulation (PM-16QAM), and 25 Gbaud PM-16QAM, using intradyne or heterodyne detection schemes. Through comprehensive numerical simulations and experimental evaluations, we demonstrate significant improvements in receiver sensitivity and power budget in 200 Gb/s coherent PON. When the number of digital filtering taps is small, the proposed scheme achieves power budget enhancements of 3 dB for 50 Gbaud PM-QPSK, 7 dB for 50 Gbaud Alamouti-coded PM-16QAM, and 2.1 dB for 25 Gbaud PM-16QAM in the downlink. These findings highlight the potential of pre-equalization techniques in enhancing the performance of next-generation coherent PON with low computational complexity at the ONU side.

End-to-end performance of mixed RF/UWOC system with AF protocol based on Mellin transform

This paper proposes an approach to analyze the performance of radio frequency/underwater wireless optical communication (RF/UWOC) systems using Mellin inverse transform. This analysis combines heterodyne detection (HD) and direct detection /intensity modulation (IM/DD). The RF link from land ground stations to relay stations follows the Nakagami-m distributed channel. The UWOC link from the relay station to the underwater vehicle is modeled using the double generalized gamma (GG) distribution with pointing errors. By employing the Fixed Gain Amplification and Forwarding (AF) protocol, the probability density function (PDF) of the RF/UWOC link is derived using the Mellin inverse transform, expressed in the form of the Meijer-G function. In addition, expressions for the outage probability and bit error rate of the mixed system were derived. Monte Carlo simulations validated the analysis results, which demonstrated the superior performance of HD technology.

Bidirectional TFDM 200 G coherent PON with a simplified receiver at the ONU side.

Time and frequency division multiplexing (TFDM) coherent passive optical networks (PONs) are considered as a promising candidate for future optical access networks due to the advantage of high sensitivity, high spectral efficiency, and flexibility. We propose a novel, to our knowledge, bidirectional TFDM 200-Gb/s coherent PON architecture based on the digital subcarrier multiplexing (DSCM) technology. A polarization-insensitive simplified coherent receiver is achieved at the ONU side by Alamouti coding and heterodyne detection. We experimentally demonstrate this bidirectional coherent PON with a 50-Gbaud Alamouti 16 quadrature amplitude modulation (QAM) signal transmitted downstream and a 25-Gbaud dual polarization (DP)-16QAM signal transmitted upstream. By using a single sideband modulated transmitter, only one laser source is required at the optical network unit (ONU) side. The power budgets can reach 30 and 33 dB for the downstream and upstream, respectively, after the 20-km transmission over a standard single-mode fiber (SSMF).

Flexible bandwidth-efficient simplified coherent IFoF using twin-OSSB with a dually modulated EML.

A bandwidth-efficient coherent analog IFoF approach is proposed, employing a twin optical sideband (twin-OSSB) modulation of intermediate frequency-over-fiber (IFoF) signals in a dually modulated electroabsorption modulated laser (EML), demonstrating stable modulations without complex bias drifts and polarization control. The receiver (Rx) utilizes heterodyne detection with phase noise canceling (PNC) techniques for channel recovery. Successful demonstrations include an 8 Gbps aggregated twin-OSSB of two QPSK channels, with measurements covering optical back-to-back (btb) and 25 km of a standard single-mode fiber (SSMF), revealing sensitivities of -30 dBm and -25 dBm for QPSK-twin-OSSB and 16-QAM single user, respectively.

Composite Raman-Nath heterodyne interferometry with relevance for precise spectroscopy

Atomic state manipulation by laser radiation requires a stringent control of the light phase noise for precise spectroscopy and quantum computation. In this work we describe a novel interferometer recently employed to estimate the phase noise induced by an optical frequency shifter based on serrodyne modulation for optical frequency standards (Barbiero et al., 2023). The interferometer is generated by an acousto-optic actuator working in the Raman-Nath regime. Using the heterodyne detection between the unshifted optical mode with the first higher order optical components, we are able to detect the differential phase between the two optical path arms with a reduction of the phase noise induced by the actuator. We provide a theoretical treatment for the actuator phase suppression and we estimate the detectable phase noise limit of this interferometer. Finally, we discuss the possible applications in the field of precision measurements.

Heterodyne coherent detection of the electric field temporal trace emitted by frequency-modulated comb lasers

Frequency-modulated (FM) combs are produced by mode-locked lasers in which the electric field has a linearly chirped frequency and nearly constant amplitude. This regime of operation occurs naturally in certain laser systems and constitutes a valuable alternative to generate spectra with equidistant modes. Here, we use a low-noise fs-pulse comb as the local oscillator and combine dual comb heterodyne detection with time domain analysis of the multi-heterodyne signal to reveal the temporal trace of both amplitude and phase quadratures of FM comb lasers’ electric field. This technique is applied to both a dense and a harmonic mid-infrared free-running quantum cascade laser frequency comb and shows direct evidence of the FM behavior together with the high degree of coherence of these sources. Our results furnish a deeper insight on the origin of the FM combs and pave the way to further improvement and optimization of these devices.

Temporal Refraction in an Acoustic Phononic Lattice.

In this Letter, we present the first experimental demonstration of the temporal refraction of acoustic waves in a phononic lattice. A step change in grounding stiffness results in a discontinuous change in group velocity across a so-called temporal boundary. This leads to frequency translation of incident signals, which maintain constant wavelength. We use the system to construct phononic analogs of the classical Snell and Fresnel relationships for temporal boundaries, providing evidence of temporal refraction. Last, we propose the ability to design systems to achieve tunable slow sound.

Conditional Dynamics in Heterodyne Detection of Superradiant Lasing with Incoherently Pumped Atoms.

In this Letter, we use quantum trajectory theory to simulate heterodyne detection of narrow bandwidth superradiant lasing from an incoherently excited atomic ensemble. To this end, we describe the system dynamics and account for stochastic measurement backaction by second-order mean-field theory. Our simulations show how heterodyne measurements break the phase symmetry, and initiate the atomic coherence with a random phase and a long temporal phase coherence. More importantly, our theory allows direct simulation of experimental procedures for extraction of spectral information which do not lend themselves to evaluation with the quantum regression theorem.

Digital Homodyne and Heterodyne Detection for Stationary Bosonic Modes.

Homo- and heterodyne detection are fundamental techniques for measuring propagating electromagnetic fields. However, applying these techniques to stationary fields confined in cavities poses a challenge. As a way to overcome this challenge, we propose to use repeated indirect measurements of a two-level system interacting with the cavity. We demonstrate numerically that the proposed measurement scheme faithfully reproduces measurement statistics of homo- or heterodyne detection. The scheme can be implemented in various physical architectures, including circuit quantum electrodynamics. Our results pave the way for implementation of quantum algorithms requiring linear detection of stationary modes, including quantum verification protocols.