Simple Pulse Shape Research Articles

Programmable Shaping of Femtosecond Pulses Using a One-Dimensional Photonic Crystal with Continuous Spatial Modulation of the Refractive Index

An algorithm for constructing the structure of a one-dimensional photonic crystal, which forms a femtosecond pulse of an arbitrary given shape when reflecting a Gaussian pulse, is developed theoretically. The method is illustrated by the example of a rectangular pulse. Examples for simple pulse shapes are given, the influence of optical absorption is evaluated, and a method for its compensation is proposed.

Simple framework for systematic high-fidelity gate operations

Semiconductor spin qubits demonstrated single-qubit gates with fidelities up to benchmarked in the single-qubit subspace. However, tomographic characterizations reveal non-negligible crosstalk errors in a larger space. Additionally, it was long thought that the two-qubit gate performance is limited by charge noise, which couples to the qubits via the exchange interaction. Here, we show that coherent error sources such as a limited bandwidth of the control signals, diabaticity errors, microwave crosstalk, and non-linear transfer functions can equally limit the fidelity. We report a simple theoretical framework for pulse optimization that relates erroneous dynamics to spectral concentration problems and allows for the reuse of existing signal shaping methods on a larger set of gate operations. We apply this framework to common gate operations for spin qubits and show that simple pulse shaping techniques can significantly improve the performance of these gate operations in the presence of such coherent error sources. The methods presented in the paper were used to demonstrate two-qubit gate fidelities with in Xue et al (2022 Nature 601 343). We also find that single and two-qubit gates can be optimized using the same pulse shape. We use analytic derivations and numerical simulations to arrive at predicted gate fidelities greater than 99.9% with duration less than, where is the difference in qubit frequencies.

Programmable Shaping of Femtosecond Pulses Using a One-Dimensional Photonic Crystal with Continuous Spatial Modulation of the Refractive Index

An algorithm for constructing the structure of a one-dimensional photonic crystal, which forms a femtosecond pulse of an arbitrary given shape when reflecting a Gaussian pulse, is developed theoretically. The method is illustrated by the example of a rectangular pulse. Examples for simple pulse shapes are given, the influence of optical absorption is evaluated, and a method for its compensation is proposed.

Saturable Purcell filter for circuit quantum electrodynamics

We consider a typical circuit QED setup where an artificial atom encodes a qubit and is dispersively coupled to a measurement resonator that in turn is coupled to a transmission line. We show theoretically that by placing another artificial atom in this transmission line to act as a filter, the Purcell decay of the qubit into the transmission line is suppressed. When strong control fields are applied in the transmission line, the filter is saturated and effectively switched off. Such a Purcell filtering capability permits both the control and measurement of the qubit using the single transmission line, while maintaining the long coherence time of the qubit in the absence of the control pulses. We show that high fidelity Pauli ${\ensuremath{\sigma}}_{x}$ gates on the qubit can be realized using simple pulse shapes. For devices that already use one transmission line both for control and measurement of the qubit, our paper provides a way to completely filter out the qubit frequency without removing the possibility of controlling the system. Further, combining the proposed filter with frequency multiplexing potentially enables both control and measurement of several qubits using a single Purcell-filtered transmission line. This will enhance the scalability of superconducting quantum processors by decreasing the number of the required transmission lines.

Sub-voxel identification of gamma-ray interaction positions within a pixelated CZT detector through signal analysis

Cadmium zinc telluride (CZT) detectors are well suited for use in gamma-imaging applications thanks to their room-temperature operation, excellent energy resolution, and three-dimensional sensitivity to positions of gamma-ray interactions. In many of these applications, the position resolution with which gamma-interactions can be determined is crucial for high quality imaging. Finer pixelisation may provide improved position resolution but at the cost of increased fabrication cost and degradation of spectral performance. This work sets out to instead investigate and quantify the three-dimensional position resolution achievable using pulse shape analysis applied to digital signals, arising from interactions in a CZT detector pixelated into 2 × 2 × 5 mm3 voxels. The signal response of a 3 × 3 pixel cluster has been characterised as a function of gamma-ray interaction position using collimated 57Co and 137Cs sources. Simple pulse shape analysis algorithms using the transient image and charge collection signals have been developed and applied to achieve a sub-voxel position resolution of better than 0.65 × 0.69 × 1.26 mm3 at 122 keV and of 1.06 mm through 5 mm depth at 662 keV, not accounting for divergence of the collimated beam used to evaluate performance. Assessment of the contribution of the diverging gamma-beam placed a lower bound on the achievable positions resolution in 5 mm thickness at 122 keV of 290μm.

Ultrashort laser-driven dynamics of massless Dirac electrons generating valley polarization in graphene

We identify and describe how intense short light pulses couple to massless Dirac fermions in two-dimensional systems. The ensuing excitation dynamics exhibits unusual scaling with the wavelength of the light due the linear dispersion of the band structure and the fact that light coupling is efficient only close to the Dirac points. We exploit these features to achieve valley polarization of more than 70 % with simple pulse shapes. Quantitative results are given for pristine graphene.

A simple pulse shape for an EM pulse propagating through dispersive strong turbulence

The shape of an EM pulse wave propagating through dispersive-strong turbulence shows asymmetry, with an exponentially decaying tail due to different pulse components traversing different paths. An accurate expression for the pulse shape is identified in literature, but is intricate and seldom used. Instead, the model of a Gaussian convolved with an exponential is assumed more often. Here, a relatively simple expression is derived for the average intensity of a narrowband Gaussian pulse propagating through dispersive-strong turbulence, which reduces to familiar results in the absence of dispersion. Comparisons to well known pulse models is carried out. Furthermore, the validity of the proposed model is tested for the application of radiations from pulsars.

The Thousand-Pulsar-Array programme on MeerKAT – II. Observing strategy for pulsar monitoring with subarrays

ABSTRACT The Thousand-Pulsar-Array (TPA) programme currently monitors about 500 pulsars with the sensitive MeerKAT radio telescope by using subarrays to observe multiple sources simultaneously. Here we define the adopted observing strategy, which guarantees that each target is observed long enough to obtain a high-fidelity pulse profile, thereby reaching a sufficient precision of a simple pulse shape parameter. This precision is estimated from the contribution of the system noise of the telescope, and the pulse-to-pulse variability of each pulsar, which we quantify under some simplifying assumptions. We test the assumptions and choice of model parameters using data from the MeerKAT 64-dish array and the Lovell and Parkes telescopes. We demonstrate that the observing times derived from our method produce high-fidelity pulse profiles that meet the needs of the TPA in studying pulse shape variability and pulsar timing. Our method can also be used to compare strategies for observing large numbers of pulsars with telescopes capable of forming multiple subarray configurations. We find that using two 32-dish MeerKAT subarrays is the most efficient strategy for the TPA project. We also find that the ability to observe in different array configurations will become increasingly important for large observing programmes using the Square Kilometre Array telescope.

Synaptic Plasticity in Semiconducting Single‐Walled Carbon Nanotubes Transistors

Recently, a significant amount of attention went toward the development of artificial neural networks, as these promise efficient computation for specific tasks that are expensive or even impossible for von Neumann architecture‐based computers. A considerable amount of research is conducted to develop materials in which layered neural networks are easily implemented. Herein, the use of high‐quality semiconducting single‐walled carbon nanotube (s‐SWCNT) inks to fabricate artificial synapses using simple bottom‐gate field‐effect transistors (FETs) is reported. The synapse utilizes the otherwise often dreaded hysteresis, the characteristic of the device structure, and the materials used therein. This hysteresis spans several orders of magnitude. The SWCNT synaptic transistor exhibits a clear spike‐time‐dependent plasticity (STDP), indicating the ability to achieve learning, following a mechanism similar to biological systems: asymmetric anti‐Hebbian learning. The very well‐defined STDP shape, together with the use of a simple pulse shape to obtain it, has not been reported previously for synaptic transistors. This represents a crucial step for further development of actual hardware implementation of artificial synapses in a more extensive, layered network. Interestingly, the time response of the device is very similar to the one of biological synapse, opening opportunities unavailable to CMOS emulations.

Emittance Reduction of RF Photoinjector Generated Electron Beams by Transverse Laser Beam Shaping

Laser pulse shaping is one of the key elements to generate low emittance electron beams with RF photoinjectors. Ultimately high performance can be achieved with ellipsoidal laser pulses, but 3-dimensional shaping is challenging. High beam quality can also be reached by simple transverse pulse shaping, which has demonstrated improved beam emittance compared to a transversely uniform laser in the ‘pancake’ photoemission regime. In this contribution we present the truncation of a Gaussian laser at a radius of approximately one sigma in the intermediate (electron bunch length directly after emission about the same as radius) photoemission regime with high acceleration gradients (up to 60 MV/m). This type of electron bunch is used e.g. at the European XFEL and FLASH free electron lasers at DESY, Hamburg site and is being investigated in detail at the Photoinjector Test facility at DESY in Zeuthen (PITZ). Here we present ray-tracing simulations and experimental data of a laser beamline upgrade enabling variable transverse truncation. Initial projected emittance measurements taken with help of this setup are shown, as well as supporting beam dynamics simulations. Additional simulations show the potential for substantial reduction of slice emittance at PITZ.

Operation and intrinsic error budget of a two-qubit cross-resonance gate

We analyze analytically, semi-analytically, and numerically the operation of Cross-Resonance (CR) gate for superconducting qubits (transmons). We find that a relatively simple semi-analytical method gives accurate results for the CNOT-equivalent gate duration and compensating single-qubit rotations. It also allows us to minimize the CNOT gate duration over the amplitude of the applied microwave drive and find dependence on the detuning between the qubits. However, full numerical simulations are needed to calculate intrinsic fidelity of the CR gate. We decompose numerical infidelity into contributions from various physical mechanisms, thus finding the intrinsic error budget. In particular, at small drive amplitudes the CR gate fidelity is limited by imperfections of the target-qubit rotations, while at large amplitudes it is limited by leakage. The gate duration and fidelity are analyzed numerically as functions of the detuning between qubits, their coupling, drive frequency, relative duration of pulse ramps, and microwave crosstalk. The effect of the echo sequence is also analyzed numerically. Our results show that the CR gate can provide intrinsic infidelity of less than $10^{-3}$ when a simple pulse shape is used.

Acoustic analysis methods for particle identification with superheated droplet detectors

A superheated droplet detector (SDD) consists of a uniform dispersion of over-expanded, micrometric-sized halocarbon droplets suspended in a hydrogenated gel, each droplet of which functions as a mini-bubble chamber. Energy deposition by irradiation nucleates the phase transition of the superheated droplets, generating millimetric-sized bubbles that are recorded acoustically. A simple pulse shape validation routine was developed in which each pulse is first amplitude demodulated and the decay constant then determined through an exponential fit. Despite this, low amplitude (< 3 mV) events embedded at naked eye in the noise level are not counted for calibration purposes with neutron and alpha sources. The solution found was to filter the data with a low band-pass filter in the region that the bubbles nucleate (typically from 450 to 750 Hz). After this, a peak finding algorithm to count all the events was implemented. The performance demonstrates better than a factor 40 reduction in noise and an extra factor 10 reduction with the filtering application. The lowering of noise and discovery of low signal amplitudes by the acoustic instrumentation and acoustic analysis permits a capability of discriminating nucleation events from acoustic backgrounds and radiation sources and, having a 95% confidence level on identifying and counting events in substantial data sets like in calibrations.

Use of neural networks to analyze pulse shape data in low-background detectors

Pacific Northwest National Laboratory has accumulated years of data with ultra-low-background proportional counters collected in an on-site shallow underground laboratory. This large dataset of events is exploited to study the impact of using neural networks for data analysis compared to simple pulse shape discrimination (PSD). The PSD method can introduce false positives for overlapping event distributions; however, a neural network can separate and correctly classify these events. This paper describes the training, testing, and validation of a neural network, analysis of challenge datasets, and a comparison between the standard PSD approach and a dense, fully-connected neural network.

Fast microwave-driven three-qubit gates for cavity-coupled superconducting qubits

Although single- and two-qubit gates are sufficient for universal quantum computation, single-shot three-qubit gates greatly simplify quantum error correction schemes and algorithms. We design fast, high-fidelity three-qubit entangling gates based on microwave pulses for transmon qubits coupled through a superconducting resonator. We show that when interqubit frequency differences are comparable to single-qubit anharmonicities, errors occur primarily through a single unwanted transition. This feature enables the design of fast three-qubit gates based on simple analytical pulse shapes that are engineered to minimize such errors. We show that a three-qubit ccz gate can be performed in 260 ns with fidelities exceeding $99.38%$, or $99.99%$ with numerical optimization.

Scattering analysis of LOFAR pulsar observations

We measure the effects of interstellar scattering on average pulse profiles from 13 radio pulsars with simple pulse shapes. We use data from the LOFAR High Band Antennas, at frequencies between 110 and 190~MHz. We apply a forward fitting technique, and simultaneously determine the intrinsic pulse shape, assuming single Gaussian component profiles. We find that the constant $\tau$, associated with scattering by a single thin screen, has a power-law dependence on frequency $\tau \propto \nu^{-\alpha}$, with indices ranging from $\alpha = 1.50$ to $4.0$, despite simplest theoretical models predicting $\alpha = 4.0$ or $4.4$. Modelling the screen as an isotropic or extremely anisotropic scatterer, we find anisotropic scattering fits lead to larger power-law indices, often in better agreement with theoretically expected values. We compare the scattering models based on the inferred, frequency dependent parameters of the intrinsic pulse, and the resulting correction to the dispersion measure (DM). We highlight the cases in which fits of extreme anisotropic scattering are appealing, while stressing that the data do not strictly favour either model for any of the 13 pulsars. The pulsars show anomalous scattering properties that are consistent with finite scattering screens and/or anisotropy, but these data alone do not provide the means for an unambiguous characterization of the screens. We revisit the empirical $\tau$ versus DM relation and consider how our results support a frequency dependence of $\alpha$. Very long baseline interferometry, and observations of the scattering and scintillation properties of these sources at higher frequencies, will provide further evidence.

Time-over-threshold for pulse shape discrimination in a time-of-flight phoswich PET detector

It is well known that a PET detector capable of measuring both photon time-of-flight (TOF) and depth-of-interaction (DOI) improves the image quality and accuracy. Phoswich designs have been realized in PET detectors to measure DOI for more than a decade. However, PET detectors based on phoswich designs put great demand on the readout circuits, which have to differentiate the pulse shape produced by different crystal layers. A simple pulse shape discrimination approach is required to realize the phoswich designs in a clinical PET scanner, which consists of thousands of scintillation crystal elements. In this work, we studied time-over-threshold (ToT) as a pulse shape parameter for DOI. The energy, timing and DOI performance were evaluated for a phoswich detector design comprising mm LYSO:Ce crystal optically coupled to mm calcium co-doped LSO:Ce,Ca(0.4%) crystal read out by a silicon photomultiplier (SiPM). A DOI accuracy of 97.2% has been achieved for photopeak events using the proposed time-over-threshold (ToT) processing. The energy resolution without correction for SiPM non-linearity was % and % FWHM at 511 keV for LYSO and LSO crystal layers, respectively. The coincidence time resolution for photopeak events ranges from 164.6 ps to 183.1 ps FWHM, depending on the layer combinations. The coincidence time resolution for inter-crystal scatter events ranges from 214.6 ps to 418.3 ps FWHM, depending on the energy windows applied. These results show great promises of using ToT for pulse shape discrimination in a TOF phoswich detector since a ToT measurement can be easily implemented in readout electronics.

Improving the power efficiency of SOA-based UWB over fiber systems via pulse shape randomization

A simple pulse shape randomization scheme is considered in this paper for improving the performance of Ultra Wide band (UWB) communication systems using On Off Keying (OOK) or Pulse Position Modulation (PPM) formats. The advantage of the proposed scheme, which can be either employed for Impulse Radio (IR) or for carrier-based systems, is first theoretically studied based on closed-form derivations of power spectral densities. Then, we investigate an application to an IR-UWB over optical fiber system, by utilizing the 4-th and 5-th orders of Gaussian derivatives. Our approach proves to be effective for 1 Gbps-PPM and 2 Gbps-OOK transmissions, with an advantage in terms of power efficiency for short distances. We also examine the performance for a system employing an in-line Semiconductor Optical Amplifier (SOA) with the view to achieve a reach extension, while limiting the cost and system complexity.

Entanglement generation through local field and quantum dissipation

Entanglement in a Gaussian two-mode system can be generated by local driving if additional non-local features are introduced to the dynamics. We demonstrate that weak to moderate ohmic friction arising from a dissipative environment can enable entanglement generation in a driven system. This synergy of driving and dissipation is highly sensitive to the pulse shape; several simple pulse shapes fail to produce this effect at all or deposit large amounts of energy in the system as a side effect. Complex pulse shapes, determined by optimal control techniques, however, are effective without detrimental side effects.

Compact high power mid-infrared optical parametric oscillator pumped by a gain-switched fiber laser with "figure-of-h" pulse shape.

We demonstrate a compact high power mid-infrared (MIR) optical parametric oscillator (OPO) pumped by a gain-switched linearly polarized, pulsed fiber laser. The gain-switched fiber laser was constructed with a piece of Yb doped polarization maintaining (PM) fiber, a pair of fiber Bragg gratings written into the matched passive PM fiber and 6 pigtailed pump laser diodes working at 915 nm with 30 W output peak power each. By modulating the pulse width of the pump laser diode, simple pedestal-free pulse shape or pedestal-free trailing pulse shape ("figure-of-h" as we call it) could be achieved from the gain-switched fiber laser. The laser was employed as the pump of a two-channel, periodically poled magnesium oxide lithium niobate-based OPO system. High power MIR emission was generated with average output power of 5.15 W at 3.8 μm channel and 8.54 W at 3.3 μm channel under the highest pump power of 45 W. The corresponding pump-to-idler conversion efficiency was computed to be 11.7% and 19.1%, respectively. Experimental results verify a significant improvement to signal-to-idler conversion efficiency by using "figure-of-h" pulses over simple pedestal-free pulses. Compared to the master oscillator power amplifier (MOPA) fiber laser counterpart, the presented gain switched fiber laser is more attractive in OPO pumping due to its compactness and simplicity which are beneficial to construction of OPO systems for practical MIR applications.

Comments on the stochastic characteristics of fission chamber signals

This paper reports on theoretical investigations of the stochastic properties of the signal series of ionisation chambers, in particular fission chambers. First, a simple and transparent derivation is given of the higher order moments of the random detector signal for incoming pulses with a non-homogeneous Poisson distribution and random pulse heights and arbitrary shape. Exact relationships are derived for the higher order moments of the detector signal, which constitute a generalisation of the so-called higher order Campbelling techniques. The probability distribution of the number of time points when the signal exceeds a certain level is also derived. Then, a few simple pulse shapes and amplitude distributions are selected as idealised models of the detector signals. Assuming that the incoming particles form a homogeneous Poisson process, explicit expressions are given for the higher order moments of the signal and the number of level crossings in a given time interval for the selected pulse shapes.