Spatial noise correlations in a Si/SiGe two-qubit device from Bell state coherences

Abstract

We study spatial noise correlations in a Si/SiGe two-qubit device with integrated micromagnets. Our method relies on the concept of decoherence-free subspaces, whereby we measure the coherence time for two different Bell states, designed to be sensitive only to either correlated or anticorrelated noise, respectively. From these measurements we find weak correlations in low-frequency noise acting on the two qubits, while no correlations could be detected in high-frequency noise. We expect nuclear spin noise to have an uncorrelated nature. A theoretical model and numerical simulations give further insight into the additive effect of multiple independent (anti)correlated noise sources with an asymmetric effect on the two qubits as can result from charge noise. Such a scenario in combination with nuclear spins is plausible given the data and the known decoherence mechanisms. This work is highly relevant for the design of optimized quantum error correction codes for spin qubits in quantum dot arrays, as well as for optimizing the design of future quantum dot arrays.

Robust entangling gate for capacitively coupled few-electron singlet-triplet qubits

“…R yields a turning point with respect to ε R , and has a relatively flat dispersion with respect to ε L . Hence, it can be expected that J eff j is insensitive to uncorrelated ε L and ε R noise (the charge noise environment observed in experiments [55]), featuring a simultaneous sweet spot with respected to both ε L and ε R . Fig.…”

“…In our simulation, δε QS j is taken to yield a Gaussian distribution with standard deviation of 8µV × 1eV/9.4V [60], where 9.4V/1eV is the lever arm [60]. δh j and δε j (t) are assumed to be independent from each other [55]. Taken from experimental results, δε…”

Two-Qubit Spectroscopy of Spatiotemporally Correlated Quantum Noise in Superconducting Qubits

“…In addition to different types of superconducting-qubit circuits, the spectroscopy protocol introduced here is applicable to virtually every other qubit platform that can support continuous driving and simultaneous single-qubit readout-in particular, trapped ions, N-V centers, and spin qubits in semiconductors. Indeed, we may expect smaller footprint and higher qubit proximity to naturally expose nearby spin qubits in N-V centers, donor impurities, or quantum dots to shared, highly correlated noise sources (e.g., due to nuclear spins or two-level fluctuators), as indicated by recent experiments [39,40]. In particular, for spin qubits in semiconductors, T 1 is typically many orders of magnitude larger than T 2 (Chan et al [35], for instance, reported T 1 ≈ 1 s and T * 2 = 33 μs), so that the protocol presented in Sec.…”

“…In a multiqubit setting in a weak-coupling regime, complete characterization of dephasing noise necessitates estimation of the full set of spectra {S jk (ω)}, defined by the Fourier transform of the correlation functions of noise operators acting on each possible combination of qubits j and k. While temporal noise correlations that affect qubits individually are now described in terms of self-spectra S jj (ω), coexisting spatial and temporal correlations are captured by the two-qubit cross-spectra {S jk (ω)}, with j = k. Spatial noise correlations have been probed and their strength upper bounded in recent experiments using superconducting fluxonium qubits [38], nitrogen-vacancy (N-V) centers in diamond [39], and spin qubits in semiconductors [40]. However, all the protocols implemented thus far lack the frequency sensitivity needed for full-fledged multiqubit spectroscopy of noise that may be in general spatiotemporally correlated and nonclassical.…”

“…In contrast to existing proposals employing dynamical-decoupling comb-based noise spectroscopy [41][42][43], our approach does not require the design and application of long sequences of nearly instantaneous pulses. As an additional advantage, for instance, with respect to methods exploiting decoherence-free subspaces [40], no entangled states nor two-qubit gates are needed. Instead, our protocol relies on continuous driving of the individual qubits followed by simultaneous single-qubit readout.…”