Abstract As qubit decoherence times are increased and readout technologies are improved, nonidealities in the drive signals, such as phase noise, are going to represent a crucial limitation to the fidelity achievable at the end of complex control pulse sequences. While prior studies have addressed the impact of reference oscillator phase noise on qubit performance, its interaction with realistic control pulses and its role in fidelity degradation have not been examined in sufficient detail. Furthermore, previous analyses grounded in the filter-function formalism identify high-frequency spectral components as the dominant source of fidelity loss, a conclusion that has also been used as a compelling marketing feature by some hardware vendors. Here we reevaluate this assertion by means of direct time-domain numerical simulations, in which phase noise realizations with a given power spectral density are applied to the carrier of realistic control pulse sequences, and the resulting qubit evolution is computed with Qiskit-Dynamics and averaged over multiple noise realizations. We show that the claimed dominance of high-frequency components originates from an oversight in interpreting the conversion between the phase-noise and the frequency-noise power spectral densities: at equal power, the components close to the Rabi frequency are by far the most detrimental, while those far from the carrier act only marginally, mainly through residual amplitude modulation. We performed an analysis using both two- and three-level models, the latter capturing leakage outside the computational subspace. We further show that, for realistic phase noise spectra of local oscillators, whose power is concentrated in the low-offset region, the fidelity loss is dominated by such high-power low-frequency components.
Abstract As qubit decoherence times are increased and readout technologies are improved, nonidealities in the drive signals, such as phase noise, are going to represent a crucial limitation to the fidelity achievable at the end of complex control pulse sequences. While prior studies have addressed the impact of reference oscillator phase noise on qubit performance, its interaction with realistic control pulses and its role in fidelity degradation have not been examined in sufficient detail. Furthermore, previous analyses grounded in the filter-function formalism identify high-frequency spectral components as the dominant source of fidelity loss, a conclusion that has also been used as a compelling marketing feature by some hardware vendors. Here we reevaluate this assertion by means of direct time-domain numerical simulations, in which phase noise realizations with a given power spectral density are applied to the carrier of realistic control pulse sequences, and the resulting qubit evolution is computed with Qiskit-Dynamics and averaged over multiple noise realizations. We show that the claimed dominance of high-frequency components originates from an oversight in interpreting the conversion between the phase-noise and the frequency-noise power spectral densities: at equal power, the components close to the Rabi frequency are by far the most detrimental, while those far from the carrier act only marginally, mainly through residual amplitude modulation. We performed an analysis using both two- and three-level models, the latter capturing leakage outside the computational subspace. We further show that, for realistic phase noise spectra of local oscillators, whose power is concentrated in the low-offset region, the fidelity loss is dominated by such high-power low-frequency components.
Abstract As qubit decoherence times are increased and readout technologies are improved, nonidealities in the drive signals, such as phase noise, are going to represent a crucial limitation to the fidelity achievable at the end of complex control pulse sequences. Although the effect of phase noise of reference oscillators on qubit performance has been studied previously, its interaction with realistic time-dependent control pulses and its contribution to fidelity degradation has not yet been investigated in sufficient detail, and remains a critical challenge. Here we study the impact on fidelity of phase noise affecting reference oscillators with the help of numerical simulations, which allow us to directly take into account the interaction between the phase fluctuations in the control signals and the evolution of the qubit state, thereby achieving a comprehensive understanding of the actual role played by the different spectral components of phase noise. In particular, we perform an analysis of the effect of the individual noise frequency contributions, providing a clear identification of the spectral regions that most critically impact fidelity and establishing their relative weight in the overall fidelity degradation. Our method is based on the generation of phase noise realizations consistent with a given power spectral density, that are then applied to the pulse carrier in simulations, with Qiskit-Dynamics, of the qubit temporal evolution. By comparing the final state obtained at the end of a noisy pulse sequence with that in the ideal case and averaging over multiple noise realizations, we estimate the resulting degradation in fidelity, and, exploiting an approximate analytical representation of a carrier affected by phase fluctuations, we shed new light on the nature of the different contributions, and provide an intuitive physical picture.
As qubit decoherence times are increased and readout technologies are improved, nonidealities in the drive signals, such as phase noise, are going to represent a crucial limitation to the fidelity achievable at the end of complex control pulse sequences. Although the effect of phase noise of reference oscillators on qubit performance has been studied previously, its interaction with realistic time-dependent control pulses and its contribution to fidelity degradation have not yet been investigated in sufficient detail, and remains a critical challenge. Here we study the impact on fidelity of phase noise affecting reference oscillators with the help of numerical simulations, which allow us to directly take into account the interaction between the phase fluctuations in the control signals and the evolution of the qubit state, thereby achieving a comprehensive understanding of the actual role played by the different spectral components of phase noise. In particular, we perform an analysis of the effect of the individual noise frequency contributions, providing a clear identification of the spectral regions that most critically impact fidelity and establishing their relative weight in the overall fidelity degradation. Our method is based on the generation of phase noise realizations consistent with a given power spectral density, that are then applied to the pulse carrier in simulations, with Qiskit-Dynamics, of the qubit temporal evolution. By comparing the final state obtained at the end of a noisy pulse sequence with that in the ideal case and averaging over multiple noise realizations, we estimate the resulting degradation in fidelity, and, exploiting an approximate analytical representation of a carrier affected by phase fluctuations, we shed new light on the nature of the different contributions, and provide an intuitive physical picture.
Decoherence is a primary barrier to practical quantum computation, causing quantum states to degrade through phase randomization and irreversible energy relaxation. This study presents a simulation-based evaluation of echo-based dynamical decoupling (DD) pulse sequences as a software-level mitigation strategy in single-qubit systems. Using the Qiskit Aer density matrix simulator, four strategies—baseline, Hahn Echo, Carr-Purcell-Meiboom-Gill (CPMG), and Mixed Synchronization—were evaluated across low-frequency phase noise, high-frequency phase noise, and combined T₁ + T₂ noise regimes. Hahn Echo restored mean fidelity to 100.00% under low-frequency noise (baseline: 57.90%), while CPMG achieved 97.62% under high-frequency noise (baseline: 2.45%). Under combined noise, all strategies showed limited improvement, with Mixed-Sync reaching 65.34% versus 60.95%. Coefficient of variation (CV) analysis across 500 Monte Carlo trials showed that CPMG reduced purity-decay variance threefold under high-frequency noise, with bootstrap confidence intervals confirming statistical significance. Time-resolved analysis showed that CPMG suppressed variance growth throughout evolution, reducing variance growth from +267% (baseline) to +89%, while baseline and Hahn Echo exhibited progressive amplification of variance. These results show that dynamical decoupling influences the temporal structure of noise, not just mean fidelity. In T₂-dominated regimes, pulse sequences constrain variability; in mixed regimes, they partially shape it; and in T₁-dominated regimes, both mean performance and variability converge to hardware-limited behavior. DD effectively corrects reversible phase errors but cannot overcome irreversible T₁ loss, with implications for NISQ circuit design.
Experimentalists seeking to improve the coherent lifetimes of quantum bits have generally focused on improvements to qubit designs, materials, and system isolation from environmental perturbations. In the case of the phase degree of freedom in a quantum superposition, however, the coherence that must be preserved is ultimately that of the qubit relative to the system clock, rather than that of the qubit in isolation. In this manuscript we clarify the impact of clock instability on qubit dephasing and provide quantitative estimates of fidelity upper-bounds set by noisy phase fluctuations in the clock. We first indicate analytically that such phase fluctuations in the clock - typically referred to as the local oscillator (LO) - are indistinguishable from a pure dephasing field arising from other environmental mechanisms. Using these results, we apply commonly quoted LO phase-noise specifications to calculate the resultant performance bounds on qubit operational fidelities. We find that laboratory grade LOs contribute error probabilities beyond 10−4 for operation times <1μs, while the use of precision LOs can suppress error rates by 104. We find that in either case phase fluctuations at frequencies far from the carrier dominate operational error rates, and due to their high-frequency spectral content, are difficult to mitigate using dynamic error suppression strategies. Further, we consider the importance of LO noise bandwidth and its impact on the degree to which thermal phase fluctuations in the LO contribute an effective error floor. These observations and analysis of the flow-down effects of such errors on the implementation of benchmarking and quantum error correction protocols highlight challenges and advantages that particular qubit technologies possess in the context of phase-noise-induced errors and motivate enhanced research in precision frequency metrology.
High-fidelity single- and multiqubit operations compose the backbone of quantum information processing. This fidelity is based on the ability to couple single- or two-qubit levels in an extremely coherent and precise manner. A necessary condition for coherent quantum evolution is a highly stable local oscillator driving these transitions. Here we study the effect of fast noise, that is, noise at frequencies much higher than the local oscillator linewidth, on the fidelity of one- and two-qubit gates in a trapped-ion system. We analyze and measure the effect of fast noise on single-qubit operations, including resonant $\ensuremath{\pi}$ rotations and off-resonant sideband transitions. We further numerically analyze the effect of fast phase noise on the M\o{}lmer-S\o{}rensen two-qubit gate. We find a unified and simple way to estimate the performance of all of these operations through a single parameter given by the noise power spectral density at the qubit response frequency. While our analysis focuses on phase noise and on trapped-ion systems, it is relevant for other sources of fast noise as well as for other qubit systems in which spinlike qubits are coupled by a common bosonic field. Our analysis can help in guiding the design of quantum hardware platforms and gates, improving their fidelity towards fault-tolerant quantum computing.
We present novel techniques for modelling and mitigating the effects of time-dependent noise created by interactions between a quantum system and its environment, and by errors in applied controls. Our scope encompasses quantum storage and nontrivial QIP tasks in both single- and multi-qubit systems, with a particular focus on control operations applied to trapped-ion qubits. The work begins with the introduction of a computationally efficient approach to characterizing the impact of time-dependent noise in single-qubit systems, in terms of the spectral characteristics of the noise and a set of generalized control filter functions. In conjunction with brute-force numerical simulation, we show that this novel analytical technique can used to model the evolution of a noisy single-qubit system subject to an arbitrary piecewise-constant control sequence, and to accurately predict the effects of experimentally engineered noise in single-qubit trapped ion systems. We also obtain a number of new and important results that extend the range of utility of state-preserving dynamical decoupling control sequences. These device-independent results provide a means to create a practically useful high-fidelity, long-time quantum memory in both single- and multi-qubit systems. We go on to explain how, given the freedom to arbitrarily and quasi-instantaneously change the phase of the interaction between a system of qubits and a finite collection of oscillators, frequency-specific phase modulation protocols can be implemented that decouple the qubits from targeted oscillator modes at pre-determined times. This technique can be used to generate fast, high-fidelity multi-qubit entangling operations in, for example, ion trap QIP devices.