Decoherence in Josephson-junction qubits due to critical-current fluctuations

Abstract

We compute the decoherence caused by 1∕f fluctuations at low frequency f in the critical current I0 of Josephson junctions incorporated into flux, phase, charge, and hybrid flux-charge superconducting quantum bits (qubits). The dephasing time τϕ scales as I0∕ΩΛSI01∕2(1Hz), where Ω∕2π is the energy-level splitting frequency, SI0(1Hz) is the spectral density of the critical-current noise at 1Hz, and Λ≡∣I0dΩ∕ΩdI0∣ is a parameter computed for given parameters for each type of qubit that specifies the sensitivity of the level splitting to critical-current fluctuations. Computer simulations show that the envelope of the coherent oscillations of any qubit after time t scales as exp(−t2∕2τϕ2) when the dephasing due to critical-current noise dominates the dephasing from all sources of dissipation. We compile published results for fluctuations in the critical current of Josephson tunnel junctions fabricated with different technologies and a wide range in I0 and area A, and show that their values of SI0(1Hz) scale to within a factor of 3 of [144(I0∕μA)2∕(A∕μm2)](pA)2∕Hz at 4.2K. We empirically extrapolate SI01∕2(1Hz) to lower temperatures using a scaling T(K)∕4.2. Using this result, we find that the predicted values of τϕ at 100mK range from 0.8to12μs, and are usually substantially longer than values measured experimentally at lower temperatures.

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