Classical Noise Model Research Articles

Detecting and eliminating quantum noise of quantum measurements

Quantum measurements are crucial for extracting information from quantum systems, but they are error-prone due to hardware imperfections in near-term devices. Measurement errors can be mitigated through classical post-processing, based on the assumption of a classical noise model. However, the coherence of quantum measurements leads to unavoidable quantum noise that defies this assumption. In this work, we introduce a two-stage procedure to systematically tackle such quantum noise in measurements. The idea is intuitive: we first detect and then eliminate quantum noise. In the first stage, inspired by coherence witness in the resource theory of quantum coherence, we design an efficient method to detect quantum noise. It works by fitting the difference between two measurement statistics to the Fourier series, where the statistics are obtained using maximally coherent states with relative phase and maximally mixed states as inputs. The fitting coefficients quantitatively benchmark quantum noise. In the second stage, we design various methods to eliminate quantum noise, inspired by the Pauli twirling technique. They work by executing randomly sampled Pauli gates before the measurement device and conditionally flipping the measurement outcomes in such a way that the effective measurement device contains only classical noise. We numerically demonstrate the two-stage procedure’s feasibility on the Baidu Quantum Platform. Notably, the results reveal significant suppression of quantum noise in measurement devices and substantial enhancement in quantum computation accuracy. We highlight that the two-stage procedure complements existing measurement error mitigation techniques, and they together form a standard toolbox for manipulating measurement errors in near-term quantum devices.

Ancillary Gaussian modes activate the potential to witness non-Markovianity

We study how the number of employed modes impacts the ability to witness non-Markovian evolutions via correlation backflows in continuous-variable quantum dynamics. We first prove the existence of non-Markovian Gaussian evolutions that do not show any revivals in the correlations between the mode evolving through the dynamics and a single ancillary mode. We then demonstrate how this scenario radically changes when two ancillary modes are considered. Indeed, we show that the same evolutions can show correlation backflows along a specific bipartition when three-mode states are employed, and where only one mode is subjected to the evolution. These results can be interpreted as a form of activation phenomenon in non-Markovianity detection and are proven for two types of correlations, entanglement and steering, and two classes of Gaussian evolutions, a classical noise model and the quantum Brownian motion model.

A Hahn-Ramsey scheme for dynamical decoupling of single solid-state qubits

Spin systems in solid state materials are promising qubit candidates for quantum information in particular as quantum memories or for quantum sensing. A major prerequisite here is the coherence of spin phase oscillations. In this work, we show a control sequence which, by applying RF pulses of variable detuning, allows to increase the visibility of spin phase oscillations. We experimentally demonstrate the scheme on single NV centers in diamond and analytically describe how the NV electron spin phase oscillations behave in the presence of classical noise models. We hereby introduce detuning as the enabling factor that modulates the filter function of the sequence, in order to achieve a visibility of the Ramsey fringes comparable to or longer than the Hahn-echo T2 time and an improved sensitivity to DC magnetic fields in various experimental settings.

Enhanced quantumness via non-Markovianity

In this paper we address the issue of the interplay between non-Markovianity and nonclassicality of a system. We consider the action of non-Markovian environments on a qubit in terms of two models, a classical noise model and a quantum microscopic collision model. For both models we use the violation of the Leggett-Garg inequality (LGI) as a criterion of quantumness, and use the conditional past-future (CPF) correlation to measure non-Markovianity. Besides for the collision model, we also use violation of the nondiscord generating and detecting (NDGD) dynamics to indicate the nonclassicality. And LGI, NDGD, and CPF are probed sequentially by three projective measurements of some observable. In this paper we do not use the assumption of the quantum regression theorem and especially for the collision model the measurement backaction on the environment is explicitly considered. We find that compared with the Markovian dynamics the LGI can be violated for a longer time interval, i.e., non-Markovianity can preserve the quantumness of the system. For the collision model we find that for a specific measurement process, although the non-Markovianity cannot be detected, the effect of non-Markovianity of the system dynamics on nonclassicality still exists, and for some specific measurement operator the violation of both LGI and NDGD will never occur for the Markovian dynamics while with the enhancement of non-Markovianity the quantumness can appear. We also find that the NDGD can be violated for a much wider parameter regime than the LGI.

Computation of quantum‐corrected noise in graphene‐SiC‐based impact avalanche transit time diode

AbstractHere, we have performed the noise analysis in graphene‐SiC‐based double drift region impact avalanche transit time diode (IMPATT). For computation, the noise model taken is based on drift diffusion with quantum corrections. Though noise computation in IMPATT with classical drift diffusion model are widely followed in earlier years, noise analysis through classical drift diffusion model is independent of confinement and tunneling due to quantum effects. Noise computed through quantum‐corrected noise model is more accurate to measured noise. Therefore, by considering quantum effects in addition to classical noise model, we have computed the noise in graphene‐SiC IMPATT. The noise performance of graphene‐SiC IMPATT is evaluated at Ka band which ranges from 26.5 to 40 GHz. The obtained noise from graphene‐SiC IMPATT is in good agreement with earlier reported noise behavior in IMPATT. We observed that the graphene‐SiC IMPATT is giving the better noise performance in terms of lower computed noise and noise spectral density as compared to other materials reported so far.

When can quantum decoherence be mimicked by classical noise?

Quantum decoherence arises due to uncontrollable entanglement between a system and its environment. However, the effects of decoherence are often thought of and modeled through a simpler picture in which the role of the environment is to introduce classical noise in the system's degrees of freedom. Here, we establish necessary conditions that the classical noise models need to satisfy to quantitatively model the decoherence. Specifically, for pure-dephasing processes, we identify well-defined statistical properties for the noise that are determined by the quantum many-point time correlation function of the environmental operators that enter into the system-bath interaction. In particular, for the exemplifying spin-boson problem with a Lorentz-Drude spectral density, we show that the high-temperature quantum decoherence is quantitatively mimicked by colored Gaussian noise. In turn, for dissipative environments, we show that classical noise models cannot describe decoherence effects due to relaxation through spontaneous emission of photons/phonons. These developments provide a rigorous platform to assess the validity of classical noise models of decoherence.

Quantum corrected drift diffusion based noise model for impact avalanche and transit time diode

We have proposed a quantum corrected drift diffusion based noise model for the more accurate noise computation of double drift region Impact Avalanche Transit Time diode, operating from 26.5 GHz to 40 GHz. Measuring of noise in IMPATT is very difficult task as the noise arises from the generation of electron and hole pairs, due to major effects like hot carriers transport in semiconductor, phonon scattering, inter band scattering of electrons and holes, quantum confinement, quantum tunneling. Conventional classical drift diffusion noise model is independent of these effects. However, in this paper a quantum corrected noise model has been presented in addition to the classical drift diffusion noise model by considering the quantum effects. For the validation of the proposed noise model, silicon has considered as the base material of IMPATT diode. The results obtained from the proposed model are in good agreement with the experimentally measured noise performance and giving better response as compared to that of classical noise model.

Noise spectroscopy of a quantum-classical environment with a diamond qubit

Knowing a quantum system's environment is critical for its practical use as a quantum device. Qubit sensors can reconstruct the noise spectral density of a classical bath, provided long enough coherence time. Here we present a protocol that can unravel the characteristics of a more complex environment, comprising both unknown coherently coupled quantum systems, and a larger quantum bath that can be modeled as a classical stochastic field. We exploit the rich environment of a Nitrogen-Vacancy center in diamond, tuning the environment behavior with a bias magnetic field, to experimentally demonstrate our method. We show how to reconstruct the noise spectral density even when limited by relatively short coherence times, and identify the local spin environment. Importantly, we demonstrate that the reconstructed model can have predictive power, describing the spin qubit dynamics under control sequences not used for noise spectroscopy, a feature critical for building robust quantum devices. At lower bias fields, where the effects of the quantum nature of the bath are more pronounced, we find that more than a single classical noise model are needed to properly describe the spin coherence under different controls, due to the back action of the qubit onto the bath.

Novel Method for Investigating Broadband Velocity Fluctuations in Axisymmetric Screeching Jets

The noise spectrum of screeching jets consists of broadband and discrete components referred to as turbulent mixing noise and screech tones, respectively. The screech tones make the application of classical noise models to screeching jets difficult for modern problems. These problems involve an additional treatment of the near-field dynamics before the broadband noise prediction schemes can be implemented. This paper proposes an a posteriori approach for studying the velocity fluctuations associated with the broadband turbulent mixing noise in screeching jets. The broadband and screeching fluctuations associated with the broadband and discrete noise components are calculated using proper orthogonal decomposition to decouple the velocity variables. The methodology is demonstrated on velocity fields of screeching jets measured with high-resolution particle image velocimetry. Jets dominated by the precessing flapping (B) and helical (C) instability screech modes are considered, which correspond to ideally expanded Mach numbers of , 1.45, and 1.59, respectively. Axially averaged spatial wavelength spectra of the broadband velocity fluctuations are compared to illustrate the decomposition. The proper-orthogonal-decomposition-based approach is shown to be highly dependent upon the dominant instability regime of the jets. Further analysis shows that the integral length scales of the broadband velocity fluctuations normalized by the incompressible momentum thickness in the screeching jets considered agree well with values reported in studies on nonscreeching jets despite the absolute values differing.

Qubit Noise Spectroscopy for Non-Gaussian Dephasing Environments.

We introduce open-loop quantum control protocols for characterizing the spectral properties of non-Gaussian noise, applicable to both classical and quantum dephasing environments. By engineering a multidimensional frequency comb via repetition of suitably designed pulse sequences, the desired high-order spectra may be related to observable properties of the qubit probe. We prove that access to a high time resolution is key to achieving spectral reconstruction over an extended bandwidth, overcoming the limitations of existing schemes. Non-Gaussian spectroscopy is demonstrated for a classical noise model describing quadratic dephasing at an optimal point, as well as a quantum spin-boson model out of equilibrium. In both cases, we obtain spectral reconstructions that accurately predict the qubit dynamics in the non-Gaussian regime.

Classical nature of nuclear spin noise near clock transitions of Bi donors in silicon

Whether a quantum bath can be approximated as classical noise is a fundamental issue in central spin decoherence and also of practical importance in designing noise-resilient quantum control. Spin qubits based on bismuth donors in silicon have tunable interactions with nuclear spin baths and are first-order insensitive to magnetic noise at so-called clock-transitions (CTs). This system is therefore ideal for studying the quantum/classical nature of nuclear spin baths since the qubit-bath interaction strength determines the back-action on the baths and hence the adequacy of a classical noise model. We develop a Gaussian noise model with noise correlations determined by quantum calculations and compare the classical noise approximation to the full quantum bath theory. We experimentally test our model through dynamical decoupling sequence of up to 128 pulses, finding good agreement with simulations and measuring electron spin coherence times approaching one second - notably using natural silicon. Our theoretical and experimental study demonstrates that the noise from a nuclear spin bath is analogous to classical Gaussian noise if the back-action of the qubit on the bath is small compared to the internal bath dynamics, as is the case close to CTs. However, far from the CTs, the back-action of the central spin on the bath is such that the quantum model is required to accurately model spin decoherence.

Low frequency noise behaviors in the partially depleted silicon-on-insulator device

Low frequency noise in the partially depleted silicon-on-insulator (SOI) NMOS device is investigated in this paper. The experimental results show low frequency noise behaviors are in good consistence with classical noise model. Based on McWhorter model, the low frequency noise in the SOI device results from the exchange of carriers between channel and oxide. The densities of trapped charges in the front gate oxide and buried oxide are extracted. Due to the difference between manufacture processes, the extracted density of trapped charges in the buried oxide (Nt=8×1017 eV-1·cm-3) is larger than that in the gate oxide (Nt=2.767×1017 eV-1·cm-3), and the result is in good agreement with testing result of transfer characteristics in part 2. Based on the charge tunneling mechanism, the spatial distribution of trapped charges in the gate oxide and buried oxide are extracted by using the tunneling attenuation coefficient (λ=0.1 nm for SiO2) and time constant (τ0=10-10 s), and the result also proves that the trap in buried oxide is larger than that in gate oxide. In addition, the influence of channel length on the low frequency noise in the SOI device is discussed. The variations of normalized channel current noise power spectral density with channel length are investigated at four frequencies(10 Hz, 25 Hz, 50 Hz, and 100 Hz). The experimental results show that the normalized noise power spectral density decreases linearly with the increase of channel length, which indicates the low frequency noise of SOI device is mainly caused by the flicker noise in the channel, and the contribution of source/drain contact and parasitic resistances could be ignored. Finally, the dependences of back gate voltage on the front gate threshold voltage, front channel current and front channel noise are discussed by considering the charge coupling effect. The experimental results show the measured channel current and channel noise with applying front gate voltage and back gate voltage simultaneously are larger than those with applying the front gate voltage and back gate voltage separately.

Converting a real quantum spin bath to an effective classical noise acting on a central spin

We present a cluster expansion method for approximating quantum spin-bath dynamics in terms of a classical Gaussian stochastic process. The cluster expansion produces the two-point correlation function of the approximate classical bath, permitting rapid evaluation of noise-mitigating quantum control strategies without resorting to computationally intensive dynamical decoupling models. Our approximation is valid for the wide class of models possessing negligible back-action and nearly-Gaussian noise. We study several instances of the central spin decoherence problem in which the central spin and randomly-located bath spins are alike and dipolarly coupled. For various pulse sequences, we compare the coherence echo decay computed explicitly quantum mechanically versus those computed using our approximate classical model, and obtain agreement in most, but not all, cases. We demonstrate the utility of these classical noise models by efficiently searching for the 4-pulse sequences that maximally mitigate decoherence in each of these cases, a computationally expensive task in the explicit quantum model.

Optimal reconstruction for closed-loop ground-layer adaptive optics with elongated spots

The design of the laser-guide-star-based adaptive optics (AO) systems for the Extremely Large Telescopes requires careful study of the issue of elongated spots produced on Shack-Hartmann wavefront sensors. The importance of a correct modeling of the nonuniformity and correlations of the noise induced by this elongation has already been demonstrated for wavefront reconstruction. We report here on the first (to our knowledge) end-to-end simulations of closed-loop ground-layer AO with laser guide stars with such an improved noise model. The results are compared with the level of performance predicted by a classical noise model for the reconstruction. The performance is studied in terms of ensquared energy and confirms that, thanks to the improved noise model, central or side launching of the lasers does not affect the performance with respect to the laser guide stars' flux. These two launching schemes also perform similarly whatever the atmospheric turbulence strength.

Identification of the radiating sources in underwater acoustics

In the context of the narrow-band or wideband array processing problem, in this paper a robust algorithm is developed to improve the accuracy of the estimation of the direction of arrival of the sources. It is well known that when the noise cross-spectral matrix is unknown, these estimates may be grossly inaccurate. The band noise spectral matrix, the classical noise model, is used. By means of a linear operator and an iterative algorithm for estimating the spatial correlation length, a new estimator for the direction of arrival of the sources is developed. The proposed algorithm is based upon a particular partition of the received signal vector which leads one to obtain an approximation of the noise subspace matrix without eigendecomposition. Then using both a propagation operator and noneigenvector algorithm to estimate the projection matrix, a new robust algorithm is developed for the source characterization problem in the presence of noise with an unknown cross-spectral matrix. It will be shown that the performance of bearing estimation algorithms improves substantially when this robust algorithm is used. Experimental data results are presented for the band noise spectral matrix.