Low-frequency noise in Josephson junctions for superconducting qubits

Abstract

The authors have studied low-frequency resistance fluctuations in shadow-evaporated Al/ AlO x /Al tunnel junctions. Between 300 and 5 K the spectral density follows a 1 / f law. Below 5 K, individual defects distort the 1 / f shape of the spectrum. The spectral density decreases linearly with temperature between 150 and 1 K and saturates below 0.8 K. At 4.2 K, it is about two orders of magnitude lower than expected from a recent survey ͓D. J. Van 4 Decoherence due to external sources such as the measurement devices has been studied extensively and is by now well understood, 5 permitting qubit dephasing times of up to several microseconds. 6 Future progress in this field of research depends crucially on understanding and controlling decoherence due to defects in the devices. 7-9 Superconducting qubits contain Josephson junctions, whose Josephson energy, E J = ⌽ 0 I C / ͑2 ͒, determines the potential landscape of the qubit ͑I C is the critical current and ⌽ 0 = h /2e is the superconducting flux quantum͒. Due to imperfections of the tunnel barrier, E J fluctuates in time, leading to fluctuations in the qubit potential. Therefore, the qubit energy splitting is not constant during an experiment, which leads to decoherence.

Spectroscopy of low-frequency noise and its temperature dependence in a superconducting qubit

“…On the other hand, Eroms et al 22 measured resistance fluctuations in aluminum tunnel junctions: they found about 100 times lower noise power at 4.2 K, a linear temperature dependence, and saturation below 0.8 K, i.e., S ic (f ) = (1/100) × A can Ic I −2 c (T /4.2 K)/|f |. With T = 0.8 K, this gives a value about 2.5 times lower than what we observe.…”

“…These expressions reproduce the noise spectrum, with the use of correction factors, as explained in the following section (Eqs. [18][19][20][21][22][23][24].…”

“…This noise may originate in the critical-current fluctuations of the Josephson junctions [19][20][21][22] , and/or fluctuating offset charges, due to, e.g., charge traps located in the oxides of the junction, metal-insulator interfaces, or surfaces. Charge noise can lead to dephasing even in the flux qubit, even though the junctions have a relatively high ratio of Josephson-tunneling to Coulomb-charging energies (E J /E C ≈ 50 in our device).…”

Current fluctuations in rough superconducting tunnel junctions

“…However, the fabrication process is usually not at all epitaxial quasiequilibrium growth, thus one has to expect the oxide layer to be non-crystalline and disordered [20,21]. We will take this into account by investigating rough superconducting tunnel junctions, where we assume that the junction additionally contains a few transport channels with very high transmission eigenvalues.…”

Anomalous decoherence in a dissipative two-level system

“…Compared with the Ohmic case, the QPT of the SBM with sub-Ohmic spectral density, which has been used to model the 1/f noise [11] in quantum dots [12,13] and superconductor qubit systems [14,15], is more involved. Different methods from the sub-Ohmic SBM cannot even lead to a qualitatively consistent result.…”