Electronic decoherence of two-level systems in a Josephson junction

Abstract

The sensitivity of superconducting qubits allows for spectroscopy and coherence measurements on individual two-level systems present in the disordered tunnel barrier of an Al/AlOx/Al Josephson junction. We report experimental evidence for the decoherence of two-level systems by Bogoliubov quasiparticles leaking into the insulating AlOx barrier. We control the density of quasiparticles in the junction electrodes either by the sample temperature or by injecting them using an on-chip dc-SQUID driven to its resistive state. The decoherence rates were measured by observing the two-level system's quantum state evolving under application of resonant microwave pulses and were found to increase linearly with quasiparticle density, in agreement with theory. This interaction with electronic states provides a noise and decoherence mechanism that is relevant for various microfabricated devices such as qubits, single-electron transistors, and field-effect transistors. The presented experiments also offer a possibility to determine the location of the probed two-level systems across the tunnel barrier, providing clues about the fabrication step in which they emerge. I: INTRODUCTIONWhile superconducting circuits based on Josephson junctions (JJs) rapidly mature towards favorable and applicable qubits for quantum computers [1-3], a major source of their decoherence traces back to spurious material defects that give rise to the formation of low-energy two-level systems (TLSs). On the other hand, sensitivity to tiny perturbations turns JJ qubits into ideal tools to study the properties of TLSs. For example, microwave spectroscopy of JJ phase qubits shows avoided level crossings revealing the TLSs' quantum character as well as their coherent interaction with the qubit [4]. Various microscopic models including dangling bonds, Andreev bound states [5], and Kondo fluctuators [6] have been suggested to explain the origin of TLSs. There is growing evidence [7,8], however, that they are formed by small groups of atoms that are able to tunnel between two energetically almost equivalent configurations. This is most strongly supported by recent experiments where the TLSs' energy splittings were tuned by applying external static strain [9]. TLSs are the source of lowenergy excitations, which are also responsible for the thermal, acoustic, and dielectric properties of glasses at temperatures below 1 K [10,11], which are well studied in bulk materials. Inherent to disordered solids, they are present in surface oxides and insulating layers of any microfabricated device as well as in the tunnel barriers of Josephson junctions.In contrast to traditional measurements performed on glasses that probe huge ensembles of TLSs, the sensitivity of JJ-based qubits allows one to address single TLSs and determine their individual properties. Strain-tuning experiments, e.g., measure a TLS's deformation potential [9] and allow for a detailed analysis of the coherent interaction between two TLSs brought into resonance [12]. In another experiment, the temperatu...

Loss Mechanisms and Quasiparticle Dynamics in Superconducting Microwave Resonators Made of Thin-Film Granular Aluminum

“…We measure single photon Q i on the order of 10 5 [see Fig. 2(b)], comparable with other realizations of high kinetic inductance materials [38,40,44,58], which could be explained by a residual excess quasiparticle density x QP ¼ 5 × 10 −6 , in the range of previously reported values [2,[19][20][21][22][23][24]. Figure 2(b) also shows the Q i dependence on the average circulating photon numbern…”

“…In contrast, for quantum information, circuits are heavily shielded, in an effort to minimize the generation of excess QPs, due to photons, phonons, or other particles with energies larger than twice the superconducting gap. Even residual QP densities as low as 10 −6 , normalized to the density of Cooper pairs, can be responsible for excess decoherence in superconducting quantum circuits [2,[19][20][21][22][23][24]. For temperatures much lower than the critical temperature, in the limit of weak microwave drive, the origin and dynamics of excess QPs is an active field of research [22,23,[25][26][27][28][29], with direct implications for quantum computation with Majorana modes [30,31].…”

Transmission-line resonators for the study of individual two-level tunneling systems

“…7,8 In such experiments, the ability to tune TLS via an applied mechanical strain 9 has proven to be particularly useful for studying TLS interactions 10 and decoherence. 11,12 Employing qubits to study TLS is so far limited to addressing TLS in tunnel junction barriers, which for technical reasons are almost exclusively fabricated from thin (%2 nm) layers of amorphous aluminum oxide, precluding studies on TLS in different materials. Only recently, strong coupling to individual TLS was observed using a superconducting lumped element LC-resonator which featured electric field tuning of TLS.…”

On-chip Thermometry for Microwave Optomechanics Implemented in a Nuclear Demagnetization Cryostat

“…It is thus natural to consider strongly coupled individual TLSs as the most probable source of our problems. Besides, while the actual nature of these microscopic defects remains elusive in most systems, they could be generated in many ways beyond the standard atomic configuration argument [54]; an electron tunneling between nearby traps would be a TLS strongly coupled to its electromagnetic environment, among other possibilities [48]. For Al-based NEMS, these would create (only a few) defects present in (or on) the Al layer; they should carry a dipole moment, which couples them to the microwave drive as well as to the electric field generated by the applied DC voltage.…”