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| Format: | Preprint |
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2024
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| Online Access: | https://arxiv.org/abs/2404.18659 |
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| _version_ | 1866914920168161280 |
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| author | Mehmandoost, M. Dobrovitski, V. V. |
| author_facet | Mehmandoost, M. Dobrovitski, V. V. |
| contents | Progress in fabrication of semiconductor and superconductor qubits has greatly diminished the number of decohering defects, thus decreasing the devastating low-frequency $1/f$ noise and extending the qubits' coherence times (dephasing time $T_2^*$ and the echo decay time $T_2$). However, large qubit-to-qubit variation of the coherence properties remains a problem, making it difficult to produce a large-scale register where all qubits have a uniformly high quality. In this work we show that large variability is a characteristic feature of a qubit dephased by a sparse bath made of many ($n\gg 1$) decohering defects, coupled to the qubit with similar strength. We model the defects as two-level fluctuators (TLFs) whose transition rates $γ$ are sampled from a log-uniform distribution over an interval $[γ_{m},γ_M]$, which is a standard model for $1/f$ noise. We investigate decoherence by such a bath in the limit of high-quality qubit, i.e.\ when the TLF density $d$ is small (the limit of sparse bath, with $d=n/w\ll 1$, where $n$ is the number of TLFs and $w=\ln{[γ_M/γ_{m}]}$ is the log-width of the distribution). We show that different realizations of the bath produce very similar noise power spectra $S(f)\sim 1/f$, but lead to drastically different coherence times $T_2^*$ and $T_2$. Thus, the spectral density $S(f)$ does not determine coherence of a qubit coupled to a sparse TLF bath, as opposed to a dense bath; instead, decoherence is controlled by only a few exceptional fluctuators, determined by their value of $γ$. We show that removing only two of these TLFs greatly increases $T_2$ and $T_2^*$ times. Our findings help theoretical understanding and further improvements in the coherence properties of semiconductor and superconductor qubits, battling the $1/f$ noise in these platforms. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2404_18659 |
| institution | arXiv |
| publishDate | 2024 |
| record_format | arxiv |
| spellingShingle | Decoherence induced by a sparse bath of two-level fluctuators: peculiar features of $1/f$ noise in high-quality qubits Mehmandoost, M. Dobrovitski, V. V. Mesoscale and Nanoscale Physics Quantum Physics Progress in fabrication of semiconductor and superconductor qubits has greatly diminished the number of decohering defects, thus decreasing the devastating low-frequency $1/f$ noise and extending the qubits' coherence times (dephasing time $T_2^*$ and the echo decay time $T_2$). However, large qubit-to-qubit variation of the coherence properties remains a problem, making it difficult to produce a large-scale register where all qubits have a uniformly high quality. In this work we show that large variability is a characteristic feature of a qubit dephased by a sparse bath made of many ($n\gg 1$) decohering defects, coupled to the qubit with similar strength. We model the defects as two-level fluctuators (TLFs) whose transition rates $γ$ are sampled from a log-uniform distribution over an interval $[γ_{m},γ_M]$, which is a standard model for $1/f$ noise. We investigate decoherence by such a bath in the limit of high-quality qubit, i.e.\ when the TLF density $d$ is small (the limit of sparse bath, with $d=n/w\ll 1$, where $n$ is the number of TLFs and $w=\ln{[γ_M/γ_{m}]}$ is the log-width of the distribution). We show that different realizations of the bath produce very similar noise power spectra $S(f)\sim 1/f$, but lead to drastically different coherence times $T_2^*$ and $T_2$. Thus, the spectral density $S(f)$ does not determine coherence of a qubit coupled to a sparse TLF bath, as opposed to a dense bath; instead, decoherence is controlled by only a few exceptional fluctuators, determined by their value of $γ$. We show that removing only two of these TLFs greatly increases $T_2$ and $T_2^*$ times. Our findings help theoretical understanding and further improvements in the coherence properties of semiconductor and superconductor qubits, battling the $1/f$ noise in these platforms. |
| title | Decoherence induced by a sparse bath of two-level fluctuators: peculiar features of $1/f$ noise in high-quality qubits |
| topic | Mesoscale and Nanoscale Physics Quantum Physics |
| url | https://arxiv.org/abs/2404.18659 |