Discrete charge fluctuations, routinely observed in semiconductor quantum dot devices, may contribute significantly to device drift and errors resulting from qubit miscalibration. Understanding the nature and origins of these discrete charge fluctuations may provide insights into material improvements or means of mitigating charge noise in semiconductor quantum dot devices. In this work, we measure multi-level charge fluctuations present in a Si/SiGe double quantum dot device over a range of device operating voltages and temperatures. To characterize the parameter-dependent dynamics of the underlying fluctuating degrees of freedom, we perform a detailed analysis of the measured noise timeseries. We perform algorithmically assisted drift detection and change point detection to detrend the data and remove a slow fluctuator component, as a preprocessing step. We perform model comparison on the post-processed time series between different
n-level fluctuator (
nLF) factorial hidden Markov models (FHMMs), finding that although at most sweep values the independent pair of 2LFs model would be preferred, in a particular region of voltage space the 4LF model outperforms the other models, indicating a conditional rate dependence between the two fluctuators. By tracking fluctuator transition rates, biases, and weights over a range of different device configurations, we estimate gate voltage and conductivity sensitivity. In particular, we fit a phenomenological, detailed balance model to the extracted independent 2LFs rate data, yielding lever arm estimates in the range of
−2μeV/mV up to
4μeV/mV between the two 2LFs and nearby gate electrodes. We expect that these characterization results may aid in subsequent spatial triangulation of the charge fluctuators.
Dispersive readout is a promising technique for enabling scalable measurements of gate-defined semiconductor qubits, but the repeated observation of anomalous signals when using this technique has not yet been explained. The authors study these anomalies and propose that they are caused by charge pockets that appear as gates are depleted, in close proximity to the intentionally formed quantum dots. Understanding these signals gives us a path toward eliminating them in the next generation of devices, and may help in identifying sources of charge noise, the origin of which is not fully understood for these systems.
Cross-correlated charge noise in silicon quantum dots has emerged as an important issue for fault-tolerant quantum computation, as it has been suggested to both limit quantum error correction performance and reveal unique information about noise sources. We develop a method to characterize charge noise cross-correlations in a semiconductor qubit device based solely on quantum-dot transport current. By dynamically switching the site-selective sensitivity of transport current through a double quantum dot to charge noise, we extract potential fluctuations at individual dots without requiring additional sensing devices or spin operations. Based on the obtained power spectral densities, we discuss the coupling of a dominant two-level fluctuator to individual dots. We furthermore verify the consistency of the measurement protocol by applying artificial noise via gate electrodes and investigate the temperature dependence of charge noise correlations. The method offers a convenient and versatile alternative approach that substantially facilitates the assessment of charge noise correlations in diverse semiconductor qubit devices.
Correlated noise across multiqubit architectures is known to be highly detrimental to the operation of error-correcting codes and the long-term feasibility of quantum processors. The recent discovery of spatially dependent correlated noise in multiqubit architectures of superconducting qubits arising from the impact of cosmic radiation and high-energy particles giving rise to quasiparticle poisoning within the substrate has led to intense investigations of mitigation strategies to address this. Correlated noise in semiconductor spin qubits is also now receiving attention, as this platform begins to scale. Here we report the magnitude, frequency, and spatial dependence of noise correlations between four silicon quantum dot pairs as a function of interdot distance at frequencies from 0.3 to 1 mHz. We find the magnitude of charge-noise correlations, quantified by the magnitude square coherence C x y , are significantly suppressed from > 0.5 to < 0.1 as the interdot distance increases from 75 to 300 nm. Using an analytical model we confirm that, in contrast to superconducting qubits, the dominant source of correlated noise arises from low-frequency charge noise from the presence of two level fluctuators (TLFs) at the native silicon-silicon dioxide surface. Knowing this, we conclude with a timely discussion of charge-noise mitigation strategies.
Semiconductor quantum dot qubits often require very different voltages on each gate to bring them to a correct operating point. Here, we present a method by which one can controllably and repeatably alter the nanoscale trapped charge distribution at an oxide-semiconductor interface. We demonstrate this method on a Si/SiGe quantum dot device, and we find that the operating voltages can be controlled and made much more uniform. The method relies on illumination with near-infrared light in the presence of applied gate voltages, and it enables the tuning of the device operating point on a gate-by-gate basis. We present an explanation of the underlying physics using self-consistent Schrödinger-Poisson simulations. As an application of this method, we tune a triple quantum dot to have uniform and small operating voltages in the (1,1,1) charge configuration. Importantly, we show that shifting the operating voltages in this way does not change the measured charge noise.
Controlled adiabatic transfer of a single electron through a chain of quantum dots has been recently achieved in GaAs and Si/SiGe based quantum dots, opening prospects for turning stationary spin qubits into mobile ones and solving in this way the problem of long-distance communication between quantum registers in a scalable quantum computing architecture based on quantum dots. We consider theoretically the process of such an electron transfer between two tunnel-coupled quantum dots, focusing on control by slowly varying the detuning of energy levels in the dots. We take into account the fluctuations in detuning caused by
1/f-type noise that is ubiquitous in semiconductor nanostructures and analyze their influence on probability of successful transfer of an electron in a spin eigenstate. With numerical and analytical calculations we show that probability of electron not being transferred due to $1/{f}^{\ensuremath{\beta}}$ noise in detuning is $\ensuremath{\propto}\phantom{\rule{0.16em}{0ex}}{\ensuremath{\sigma}}^{2}{t}^{\ensuremath{\beta}\ensuremath{-}1}/v$, where $\ensuremath{\sigma}$ characterizes the noise amplitude,
t is the interdot tunnel coupling, and
v is the detuning sweep rate. Interestingly, this means that the noise-induced errors in charge transfer are independent of
t for
1/f noise. For realistic parameters taken from experiments on silicon-based quantum dots, we obtain the minimal probability of charge transfer failure between a pair of dots is limited by
1/f noise in detuning to be the on the order of 0.01. This means that in order to reliably transfer charges across many quantum dots, charge noise in the devices should be further suppressed, or tunnel couplings should be increased, in order to allow for faster transfer (and less exposure to noise), while not triggering the deterministic Landau-Zener excitation.