Large fluctuations of qubit decoherence by 1/f noise of a bath of two-level fluctuators*

Decoherence by the 1/f noise is a serious problem for superconductor and quantum dot-based quantum computing platforms. The 1/f noise is produced by a bath of Two-Level Fluctuators (TLFs), each TLF randomly changing its state at a rate γ; an ensemble of TLFs with a logarithmically uniform distribution of rates produces 1/f noise [1]. Within Gaussian approximation, the qubit decoherence is determined by the noise first spectral density [2]. However, the validity region of this approximation is poorly understood, and little is known about decoherence outside of this regime [3]. 

We show that the density of TLFs is a key parameter controlling validity of the standard theory [2], which is valid at large TLF densities. When the TLF density is low (high-fidelity qubits in clean samples), the qubit coherence decay exhibits large sample-to-sample fluctuations, although the 1/f noise spectrum remains almost unaltered. We also show that for low TLF density, both the Ramsey and the echo decay times (T₂^* and T₂) are governed by only 1-3 TLFs: if these few TLFs are removed, the qubit coherence time increases by an order of magnitude. This finding opens a way to greatly improving the coherence properties of qubits.

[1] S. Kogan, "Electronic Noise and Fluctuations in Solids" (1996).

[2] L. Cywinski, et al, Phys. Rev. B 77, 174509 (2008) 

[3] P. Szankowski and L. Cywinski, Phys. Rev. A 97, 032101 (2018).