35 research outputs found
Towards understanding two-level-systems in amorphous solids -- Insights from quantum circuits
Amorphous solids show surprisingly universal behaviour at low temperatures.
The prevailing wisdom is that this can be explained by the existence of
two-state defects within the material. The so-called standard tunneling model
has become the established framework to explain these results, yet it still
leaves the central question essentially unanswered -- what are these two-level
defects? This question has recently taken on a new urgency with the rise of
superconducting circuits in quantum computing, circuit quantum electrodynamics,
magnetometry, electrometry and metrology. Superconducting circuits made from
aluminium or niobium are fundamentally limited by losses due to two-level
defects within the amorphous oxide layers encasing them. On the other hand,
these circuits also provide a novel and effective method for studying the very
defects which limit their operation. We can now go beyond ensemble measurements
and probe individual defects -- observing the quantum nature of their dynamics
and studying their formation, their behaviour as a function of applied field,
strain, temperature and other properties. This article reviews the plethora of
recent experimental results in this area and discusses the various theoretical
models which have been used to describe the observations. In doing so, it
summarises the current approaches to solving this fundamentally important
problem in solid-state physics.Comment: 34 pages, 7 figures, 1 tabl
Enhancing the coherence of superconducting quantum bits with electric fields
In the endeavor to make quantum computers a reality, integrated superconducting circuits have become a promising architecture. A major challenge of this approach is decoherence originating from spurious atomic tunneling defects at the interfaces of qubit electrodes, which may resonantly absorb energy from the qubit’s oscillating electric field and reduce the qubit’s energy relaxation time T. Here, we show that qubit coherence can be improved by tuning dominating defects away from the qubit resonance using an applied DC-electric field. We demonstrate a method that optimizes the applied field bias and enhances the average qubit T time by 23%. We also discuss how local gate electrodes can be implemented in superconducting quantum processors to enable simultaneous in situ coherence optimization of individual qubits
Enhancing the Coherence of Superconducting Quantum Bits with Electric Fields
In the endeavour to make quantum computers a reality, integrated
superconducting circuits have become a promising architecture. A major
challenge of this approach is decoherence originating from spurious atomic
tunneling defects at the interfaces of qubit electrodes, which may resonantly
absorb energy from the qubit's oscillating electric field and reduce the
qubit's energy relaxation time . Here, we show that qubit coherence can be
improved by tuning dominating defects away from the qubit resonance using an
applied DC-electric field. We demonstrate a method that optimizes the applied
field bias and enhances the average qubit time by 23%. We also discuss
how local gate electrodes can be implemented in superconducting quantum
processors to enable simultaneous in-situ coherence optimization of individual
qubits.Comment: 5.5 pages and 4 figures (main Text), plus 6 pages with supplementary
figure
Enhancing the coherence of superconducting quantum bits with electric fields
In the endeavour to make quantum computers a reality, integrated superconducting circuits have become a promising architecture. A major challenge of this approach is decoherence originating from spurious atomic tunneling defects at the interfaces of qubit electrodes, which may resonantly absorb energy from the qubit\u27s oscillating electric field and reduce the qubit\u27s energy relaxation time T. Here, we show that qubit coherence can be improved by tuning dominating defects away from the qubit resonance using an applied DC-electric field. We demonstrate a method that optimizes the applied field bias and enhances the average qubit T time by 23%. We also discuss how local gate electrodes can be implemented in superconducting quantum processors to enable simultaneous in-situ coherence optimization of individual qubits
Observation of directly interacting coherent two-level systems in a solid
Parasitic two-level tunneling systems originating from structural material
defects affect the functionality of various microfabricated devices by acting
as a source of noise. In particular, superconducting quantum bits may be
sensitive to even single defects when these reside in the tunnel barrier of the
qubit's Josephson junctions, and this can be exploited to observe and
manipulate the quantum states of individual tunneling systems.
Here, we detect and fully characterize a system of two strongly interacting
defects using a novel technique for high-resolution spectroscopy. Mutual defect
coupling has been conjectured to explain various anomalies of glasses, and was
recently suggested as the origin of low frequency noise in superconducting
devices. Our study provides conclusive evidence of defect interactions with
full access to the individual constituents, demonstrating the potential of
superconducting qubits for studying material defects. All our observations are
consistent with the assumption that defects are generated by atomic tunneling.Comment: 13 pages, 7 figures. Includes supplementary materia
Multi-photon spectroscopy of a hybrid quantum system
We report on experimental multi-photon spectroscopy of a hybrid quantum
system consisting of a superconducting phase qubit coherently coupled to an
intrinsic two-level defect. We directly probe hybridized states of the combined
qubit-defect system in the strongly interacting regime, where both the
qubit-defect coupling and the driving cannot be considered as weak
perturbations. This regime is described by a theoretical model which
incorporates anharmonic corrections, multi-photon processes and decoherence. We
present a detailed comparison between experiment and theory and find excellent
agreement over a wide range of parameters.Comment: 6 pages, 6 figure
Probing defect densities at the edges and inside Josephson junctions of superconducting qubits
Tunneling defects in disordered materials form spurious two-level systems which are a major source of decoherence for micro-fabricated quantum devices. For superconducting qubits, defects in tunnel barriers of submicrometer-sized Josephson junctions couple strongest to the qubit, which necessitates optimization of the junction fabrication to mitigate defect formation. Here, we investigate whether defects appear predominantly at the edges or deep within the amorphous tunnel barrier of a junction. For this, we compare defect densities in differently shaped Al/AlO/Al Josephson junctions that are part of a Transmon qubit. We observe that the number of detectable junction-defects is proportional to the junction area, and does not significantly scale with the junction’s circumference, which proposes that defects are evenly distributed inside the tunnel barrier. Moreover, we find very similar defect densities in thermally grown tunnel barriers that were formed either directly after the base electrode was deposited, or in a separate deposition step after removal of native oxide by Argon ion milling
Correlating decoherence in transmon qubits: Low frequency noise by single fluctuators
We report on long-term measurements of a highly coherent, non-tunable
superconducting transmon qubit, revealing low-frequency burst noise in
coherence times and qubit transition frequency. We achieve this through a
simultaneous measurement of the qubit's relaxation and dephasing rate as well
as its resonance frequency. The analysis of correlations between these
parameters yields information about the microscopic origin of the intrinsic
decoherence mechanisms in Josephson qubits. Our results are consistent with a
small number of microscopic two-level systems located at the edges of the
superconducting film, which is further confirmed by a spectral noise analysis.Comment: 10 Pages, 6 figure