Verhandlungen der Deutschen Physikalischen Gesellschaft

Two level fluctuators in Josephson qubits

Daniel Mischek, Clemens Müller, Alexander Shnirman, DFG-Center for Functional Nanostructures

Published Jul 1, 2011• 0 citations

Abstract

Spectroscopic analysis of superconducting qubits often shows clear signatures of avoided level crossings, indicating the presence of microscopic two-level fluctuators (TLFs) coupling to the qubit. Within ''direct coherent control'' method it was possible for the first time to investigate coherence times and temperatures properties of single TLFs. Measurements of relaxation time shows unusual behaviour of temperature dependence. Within Bloch Redfield formalism we analyse the influence of higher qubit levels on the resonance shape of relaxation time.

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Verhandlungen der Deutschen Physikalischen Gesellschaft

Two level fluctuators in Josephson qubits

Daniel Mischek, Clemens Müller, Alexander Shnirman, DFG-Center for Functional Nanostructures

Published Jul 1, 2011• 0 citations

OpenAlex

Abstract

Related papers

Rabi spectroscopy of a qubit-fluctuator system

Jürgen Lisenfeld, Clemens Müller, Jared H. Cole +4 more

Superconducting qubits often show signatures of coherent coupling to microscopic two-level fluctuators (TLFs), which manifest themselves as avoided level crossings in spectroscopic data. In this work we study a phase qubit, in which we induce Rabi oscillations by resonant microwave driving. When the qubit is tuned close to the resonance with an individual TLF and the Rabi driving is strong enough (Rabi frequency of order of the qubit-TLF coupling), interesting four-level dynamics are observed. The experimental data show a clear asymmetry between biasing the qubit above or below the fluctuator's level splitting. Theoretical analysis indicates that this asymmetry is due to an effective coupling of the TLF to the external microwave field induced by the higher qubit levels.Superconducting qubits often show signatures of coherent coupling to microscopic two-level fluctuators (TLFs), which manifest themselves as avoided level crossings in spectroscopic data. In this work we study a phase qubit, in which we induce Rabi oscillations by resonant microwave driving. When the qubit is tuned close to the resonance with an individual TLF and the Rabi driving is strong enough (Rabi frequency of order of the qubit-TLF coupling), interesting four-level dynamics are observed. The experimental data show a clear asymmetry between biasing the qubit above or below the fluctuator's level splitting. Theoretical analysis indicates that this asymmetry is due to an effective coupling of the TLF to the external microwave field induced by the higher qubit levels.

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Effects of external driving on the coherence time of a Josephson junction qubit in a bath of two-level fluctuators

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We study the effect of external driving on the two-level systems (TLSs) assumed to be a major obstacle in increasing the coherence time of solid-state Josephson junction qubits. We find, by use of a Bloch-Redfield approach, that external driving has two major effects on the TLS. The first is increased fluctuations between the two states of the TLS; the significance of this effect compared to thermal fluctuations depends on the energy splitting of the TLS compared to temperature. The second effect is a reduction in the intensity of the noise spectrum at low frequencies, and at the same time an increase in intensity around the renormalized Rabi frequency of the TLS and the driving frequency and at beatings between these two frequencies. Finally, we study the ensemble-averaged noise spectrum for a typical distribution of TLSs known to give origin to $1/f$ noise. We find that strong driving leads to reduced noise at low frequencies, and therefore to an increased dephasing time ${T}_{2}^{Q}$ of the qubit. However, this effect is exponentially suppressed when the driving frequency is large compared to temperature, as we typically find for Josephson qubits. We suggest that external driving at frequencies much lower than the qubit frequency might be used in order to enhance the the qubit coherence time.We study the effect of external driving on the two-level systems (TLSs) assumed to be a major obstacle in increasing the coherence time of solid-state Josephson junction qubits. We find, by use of a Bloch-Redfield approach, that external driving has two major effects on the TLS. The first is increased fluctuations between the two states of the TLS; the significance of this effect compared to thermal fluctuations depends on the energy splitting of the TLS compared to temperature. The second effect is a reduction in the intensity of the noise spectrum at low frequencies, and at the same time an increase in intensity around the renormalized Rabi frequency of the TLS and the driving frequency and at beatings between these two frequencies. Finally, we study the ensemble-averaged noise spectrum for a typical distribution of TLSs known to give origin to $1/f$ noise. We find that strong driving leads to reduced noise at low frequencies, and therefore to an increased dephasing time ${T}_{2}^{Q}$ of the qubit. However, this effect is exponentially suppressed when the driving frequency is large compared to temperature, as we typically find for Josephson qubits. We suggest that external driving at frequencies much lower than the qubit frequency might be used in order to enhance the the qubit coherence time.

arXiv:1109.6753Physical Review BDec 12, 20118 citations

Decoherence induced by a sparse bath of two-level fluctuators: Peculiar features of 1 / f noise in high-quality qubits

Mohammad Mehmandoost, V. V. Dobrovitski

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 ≫ 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 ≪ 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 ) ∼ 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. Published by the American Physical Society 2024Progress 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 ≫ 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 ≪ 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 ) ∼ 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. Published by the American Physical Society 2024

arXiv:2404.18659Physical Review ResearchAug 15, 202415 citations

We investigate the energy relaxation

({T}_{1})

process of a qubit coupled to a bath of dissipative two-level fluctuators (TLFs). We consider the fluctuators strongly coupled to the qubit both in the limit of spectrally sparse single TLFs as well as in the limit of spectrally dense TLFs. We conclude that the avoided level crossings, usually attributed to very strongly coupled single TLFs, could also be caused by many weakly coupled spectrally dense fluctuators.

1/f

noise. We find that strong driving leads to reduced noise at low frequencies, and therefore to an increased dephasing time

{T}_{2}^{Q}

of the qubit. However, this effect is exponentially suppressed when the driving frequency is large compared to temperature, as we typically find for Josephson qubits. We suggest that external driving at frequencies much lower than the qubit frequency might be used in order to enhance the the qubit coherence time.