The family of tri-coordinated iridates have been identified as potential candidates supporting a Kitaev quantum spin liquid, in which spins fractionalize into emergent Majorana fermions and magnetic flux excitations. These quasiparticles acquire long-range topological entanglement and are ideal for fault-tolerant quantum computers. While the presence of additional interactions usually leads to conventional ordering, a dominant Kitaev exchange leads to QSL behavior after the magnetic order is suppressed by temperature or magnetic field, with signatures appearing in thermodynamic measurements and dynamical probes.

β-Li₂IrO₃ is an intriguing example of the complex interplay between Kitaev coupling and magnetic field. At low temperatures, an applied field rapidly suppresses the incommensurate spiral order and drives the system into a uniform field-induced zig-zag state, which are strongly intertwined due to Kitaev interactions. Moreover, a magnetic anomaly has been observed at 100K, with an onset in magnetization and a crossover in the heat capacity without causing true magnetic order. Although these results have been suggested to emerge from thermal fractionalization of the spins, very little is known about the low-energy magnetic excitations.

In was only in the early 90s that the use of hard X-ray emission spectrometers to collect X-ray Absorption spectra was first suggested [1]. X-ray emission spectrometers based on Bragg’s law achieve resolutions below 2 eV, a huge improvement compared to solid-state detectors whose resolution is only 150 – 200 eV. With this technical improvement, the characteristic fluorescence of the excited atoms is collected with a resolution below the core-hole lifetime broadening, resulting in better-resolved XAS spectra [2]. Since then, the use of X-ray Spectroscopies with improved resolution exploded and dedicated synchrotron beamlines multiplied. Nowadays, these techniques are largely exploited in many diverse fields of science.

Lanthanides and actinides are among the elements that profit the most of the improved resolution because of their large core-hole lifetime broadenings. Indeed, the demonstration of principle was done on Dy L₃ edge XANES [1]. For actinides, the resolution at L₃ edge is largely improved, but the biggest boost was given to M_4,5 edges, whose conventional XANES are almost featureless. These edges probe directly the 5f states. With better-resolved spectra, the oxidation state can be easily determined and the spectral features that were invisible before bring information about the local coordination and the charge exchange with ligands [3,4].

The information encrypted in these spectra is enormous. Improved resolution makes it more readily available by disclosing details and allowing smaller differences to be appreciated. However, the interpretation often represent the bottleneck to the extraction of relevant information. In this respect, theoretical simulations are fundamental. Nowadays, we have several user-friendly codes that interprets the spectra starting from different approaches, focusing on the intra-atomic interactions or favouring the multi-atomic picture of the system studied.

In this talk, I will briefly introduce some of the techniques exploiting the improved resolution and then focus on their application to actinide science. I will present few examples illustrating the high potential of these techniques and the approach we use in our group to interpret the data [5–7].

References:

[1] K. Hämäläinen et al., Phys. Rev. Lett. 67, 2850 (1991).

[2] P. Glatzel et al., J. Electron Spectrosc. Relat. Phenom. 188, 17 (2013).

[3] K. O. Kvashnina et al., Phys. Rev. Lett. 111, 253002 (2013).

[4] K. O. Kvashnina et al., J. Electron Spectrosc. Relat. Phenom. 194, 27 (2014).

[5] L. Amidani et al., Phys. Chem. Chem. Phys. 21, 10635 (2019).

[6] K. O. Kvashnina et al., Angew. Chem. Int. Ed. 58, 17558 (2019).

[7] A. S. Kuzenkova et al., Carbon 291 (2020).

Multifunctional material stacks compatible with high coherence superconducting quantum circuits could lead to more efficient circuit implementations. One material with a multitude of applications in optics and electronics is silicon-germanium (SiGe). Here we show that a Si/SiGe heterostructure can be incorporated in superconducting quantum circuits without any coherence degradation [1]. This opens pathways for on-chip optical to microwave transduction among other highly attractive applications.

In addition to the SiGe work we will show the effect of various in-situ surface treatments on flux tunable superconducting qubits in a hermetic package [2], as well as the effect of Two Level Systems (TLSs) in large junction Al/AlOx/Al Merged Element Transmons (MET) [3]. Our studies suggest that surface treatments can reduce the 1/f flux noise without necessarily reducing energy relaxation. For the MET qubits we find that large Al/AlOx/Al junctions contains very strongly coupled TLSs and exhibit large fluctuations in energy relaxation times. Despite the large junction area we still observe energy relaxation times higher than 200 microseconds over several hours of measurements for the best performing devices.

Reference:

[1] Investigating microwave loss of SiGe using superconducting transmon qubits

Martin Sandberg, Vivekananda P. Adiga, Markus Brink, Cihan Kurter, Conal Murray, Marinus Hopstaken, John Bruley, Jason Orcutt, Hanhee Paik

Appl. Phys. Lett. 118, 124001 (2021)

[2] Effects of surface treatments on flux tunable transmon qubits

M. Mergenthaler,C. Müller, M. Ganzhorn, S. Paredes, P. Müller, G. Salis, V. Adiga,M. Brink,2 M. Sandberg, J. Hertzberg, S. Filipp, and A. Fuhrer

arXiv:2103.07970

[3] Merged-Element Transmons: Design and Qubit Performance

H. J. Mamin, E. Huang, S. Carnevale, C. T. Rettner, N. Arellano, M. H. Sherwood, C. Kurter, B. Trimm, M. Sandberg, R. M. Shelby, M. A. Mueed, B. A. Madon, A. Pushp, M. Steffen, D. Rugar.

arXiv:2103.09163

Development of qubits – the fundamental logic element of a quantum processor – is transitioning from a scientific discovery phase to an engineering pursuit. In order to reach the scale needed for a broad range of quantum computing applications, key challenges now lie in engineering these quantum systems at every level from individual qubits to highly complex circuits. In this talk I will describe our quantum engineering of superconducting qubit systems, including our approach to predictive control of individual qubits and our three-tier stack platform for extending high-coherence circuits to increasing scale and complexity.

This research was funded in part by the Office of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA) and the Defense Advanced Research Projects Agency (DARPA) under Air Force Contract No. FA8702-15-D-0001. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the ODNI, IARPA, DARPA, or the U.S. Government.