We introduce a model describing the competition of interactions between $N$ two-level systems (TLSs) against decoherence. We apply it to analyze dye molecules in an optical microcavity, where molecular vibrations provide a local source for decoherence. The most interesting case is when decoherence strongly affects each individual TLS, e.g., via broadening of emission lines as well as vibrational satellites; however, its influence is strongly suppressed for large $N$ due to the interactions between TLSs. In this interaction-dominated regime we find unique signatures in the emission spectrum, including strong $O(\sqrt{N})$ level shifts, as well as $1/N$ suppression of both the decoherence width and the vibrational satellites. These effects are most pronounced in the unexplored regime near $50%$ polarization of the TLSs.We introduce a model describing the competition of interactions between $N$ two-level systems (TLSs) against decoherence. We apply it to analyze dye molecules in an optical microcavity, where molecular vibrations provide a local source for decoherence. The most interesting case is when decoherence strongly affects each individual TLS, e.g., via broadening of emission lines as well as vibrational satellites; however, its influence is strongly suppressed for large $N$ due to the interactions between TLSs. In this interaction-dominated regime we find unique signatures in the emission spectrum, including strong $O(\sqrt{N})$ level shifts, as well as $1/N$ suppression of both the decoherence width and the vibrational satellites. These effects are most pronounced in the unexplored regime near $50%$ polarization of the TLSs.

arXiv:1604.08637Physical Review ASep 26, 201611 citations

Decoherence in n-level quantum systems

A. R. P. Rau, Weichang Zhao

APSMay 1, 20030 citations

Localized Josephson hot spots due to two-level systems

Joshuah T. Heath, Alexander Conkey Tyner, Thue Christian Thann +4 more

Superconducting qubits are often adversely affected by two-level systems (TLSs) within the Josephson junction, which contribute to decoherence and subsequently limit the performance of the qubit. By treating the TLS as a soft (i.e., low-frequency) bosonic mode localized in real space, we find that a single TLS in either the amorphous-oxide surface or the superconducting bulk may result in a localized “hot spot” of amplified Josephson energy. Such amplification is shown to have a non-negligible effect on the T 1 time of a simple phase qubit, regardless of whether or not the TLS is on resonance with the qubit frequency. With this study, we identify unique fingerprints of TLS defects in the Josephson current and qubit decoherence time.Superconducting qubits are often adversely affected by two-level systems (TLSs) within the Josephson junction, which contribute to decoherence and subsequently limit the performance of the qubit. By treating the TLS as a soft (i.e., low-frequency) bosonic mode localized in real space, we find that a single TLS in either the amorphous-oxide surface or the superconducting bulk may result in a localized “hot spot” of amplified Josephson energy. Such amplification is shown to have a non-negligible effect on the T 1 time of a simple phase qubit, regardless of whether or not the TLS is on resonance with the qubit frequency. With this study, we identify unique fingerprints of TLS defects in the Josephson current and qubit decoherence time.

arXiv:2503.08767Physical Review AppliedNov 20, 20251 citation

Decoherence of a Josephson qubit due to coupling to two-level systems

Li-Chung Ku, Clare C. Yu

Noise and decoherence are major obstacles to the implementation of Josephson junction qubits in quantum computing. Recent experiments suggest that two-level systems (TLS) in the oxide tunnel barrier are a source of decoherence. We explore two decoherence mechanisms in which these two-level systems lead to the decay of Rabi oscillations that result when Josephson junction qubits are subjected to strong microwave driving. (A) We consider a Josephson qubit coupled resonantly to a two-level system, i.e., the qubit and TLS have equal energy splittings. As a result of this resonant interaction, the occupation probability of the excited state of the qubit exhibits beating. Decoherence of the qubit results when the two-level system decays from its excited state by emitting a phonon. (B) Fluctuations of the two-level systems in the oxide barrier produce fluctuations and $1∕f$ noise in the Josephson junction critical current ${I}_{0}$ . This in turn leads to fluctuations in the qubit energy splitting that degrade the qubit coherence. We compare our results with experiments on Josephson junction phase qubits.Noise and decoherence are major obstacles to the implementation of Josephson junction qubits in quantum computing. Recent experiments suggest that two-level systems (TLS) in the oxide tunnel barrier are a source of decoherence. We explore two decoherence mechanisms in which these two-level systems lead to the decay of Rabi oscillations that result when Josephson junction qubits are subjected to strong microwave driving. (A) We consider a Josephson qubit coupled resonantly to a two-level system, i.e., the qubit and TLS have equal energy splittings. As a result of this resonant interaction, the occupation probability of the excited state of the qubit exhibits beating. Decoherence of the qubit results when the two-level system decays from its excited state by emitting a phonon. (B) Fluctuations of the two-level systems in the oxide barrier produce fluctuations and $1∕f$ noise in the Josephson junction critical current ${I}_{0}$. This in turn leads to fluctuations in the qubit energy splitting that degrade the qubit coherence. We compare our results with experiments on Josephson junction phase qubits.

arXiv:cond-mat/0409006Physical Review BJul 18, 200555 citations

We introduce a model describing the competition of interactions between

N

two-level systems (TLSs) against decoherence. We apply it to analyze dye molecules in an optical microcavity, where molecular vibrations provide a local source for decoherence. The most interesting case is when decoherence strongly affects each individual TLS, e.g., via broadening of emission lines as well as vibrational satellites; however, its influence is strongly suppressed for large

N

due to the interactions between TLSs. In this interaction-dominated regime we find unique signatures in the emission spectrum, including strong

O(\sqrt{N})

level shifts, as well as

1/N

suppression of both the decoherence width and the vibrational satellites. These effects are most pronounced in the unexplored regime near

50%

polarization of the TLSs.

1∕f

noise in the Josephson junction critical current

{I}_{0}

. This in turn leads to fluctuations in the qubit energy splitting that degrade the qubit coherence. We compare our results with experiments on Josephson junction phase qubits.