Multilevel spectroscopy of two-level systems coupled to a dc SQUID phase qubit

Abstract

We report spectroscopic measurements of discrete two-level systems (TLSs) coupled to a dc SQUID phase qubit with a 16 µm 2 area Al/AlO x /Al junction. Applying microwaves in the 10 GHz to 11 GHz range, we found eight avoided level crossings with splitting sizes from 10 MHz to 200 MHz and spectroscopic lifetimes from 4 ns to 160 ns. Assuming the transitions are from the ground state of the composite system to an excited state of the qubit or an excited state of one of the TLS states, we fit the location and spectral width to get the energy levels, splitting sizes and spectroscopic coherence times of the phase qubit and TLSs. The distribution of splittings is consistent with non-interacting individual charged ions tunneling between random locations in the tunnel barrier and the distribution of lifetimes is consistent with the AlO x in the junction barrier having a frequency-independent loss tangent. To check that the charge of each TLS couples independently to the voltage across the junction, we also measured the spectrum in the 20-22 GHz range and found tilted avoided level crossings due to the second excited state of the junction and states in which both the junction and a TLS were excited.

Identification of structural motifs as tunneling two-level systems in amorphous alumina at low temperatures

“…They assumed that this scenario could be modelled by an effective double well potential whose barrier height V 0 and energy difference between the two minima ∆ were broadly distributed across the amorphous sample. The energy splitting between the two lowest vibrational eigenstates should be small (∼ 0.1 meV) to contribute to the properties of glasses at temperatures below 1 K 1,2, 8,9,[13][14][15][16]21 .…”

“…The energy asymmetry between the two wells of a TLS can be also varied using an external strain field as δ∆ = 2γ , where is the dimensionless strain and γ is the strain-asymmetry coupling that was measured to be about 1 eV 1,2,21,30 . The coupling between the TLSs and an external electric field F is of the dipole type δ∆ = 2p · F 8,9,[13][14][15][16]21 with the TLS dipole moment p corresponding to a single effective charge moving by a distance of a single atomic bond 9,31 . These TLSs are sparsely present in the bulk of amorphous materials (∼ 10 21 eV −1 cm −3 according to experiments 1,2,21 ) so measurements on those samples yield averages over many different TLSs.…”

“…The same TLSs are believed to be the main source of decoherence and noise in Josephson junction-based superconducting qubits 8,9 , various nanomechanical 10,11 and optical 12 resonators using amorphous materials. Exacerbating the problem is that each sample has its own set of TLS frequencies that cannot be controlled at will 8,[13][14][15][16] . As often happens, the phenomenon that is parasite for many applications at the same time opens the way to others.…”

Multiphoton spectroscopy of a hybrid quantum system

“…[7][8][9][10][11][12] These anticrossings indicate intrinsic, microscopic TLSs being coupled coherently to the qubit circuit. In general, ensembles of two-level microscopic defects are believed to be responsible for loss in a wide variety of systems, including phase-and flux-based superconducting qubits and even nanomechanical oscillators, 13,14 as well as more general effects in amorphous and spin-glass systems.…”

Rapid characterization of microscopic two-level systems using Landau-Zener transitions in a superconducting qubit

“…20,21 With this traditional method, the coupling between a TLS and a qubit results in an anti-crossing (or energy level splitting) on spectroscopy. 5,6,18,19,22,23 However, the visibility of the anti-crossing depends on the width of the resonant peak as well as the measurement resolution. 24 Timeconsuming high resolution spectroscopy measurement hinders the investigation of the dynamic evolution of TLSs in solid-state devices.…”