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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 3 - JSPEC (Josephson Junction Spectroscopy of Mesoscopic Systems)

Teaser

Summary

Mesoscopic physics offers an opportunity to study quantum mechanics in a controlled setting. At the atomic scale, systems cannot be easily engineered, whereas mesoscopic systems--such as graphene devices, single-molecule magnets, nanowire or carbon nanotube quantum dots, and superconducting weak links--can be fabricated to have well-defined quantum states upon cooling to sub-Kelvin temperatures. Despite a wide range of relevant transition energies in mesoscopic systems, the experimentalist is often restricted to probing them at frequencies below 20 GHz. At higher frequencies, it becomes exceedingly difficult and costly to propagate and detect microwaves in a cryostat. From the far infrared down to the sub-THz range, free space coupling is difficult because of the mismatch between photon wavelength and the size of single nano- or micro-structures. An alternative strategy is needed to characterize the high-frequency electronic properties of such structures.

This project will develop an on-chip Josephson-junction (JJ) based spectrometer which allows investigation of the electronic properties of mesoscopic systems between 2 GHz and 2 THz. Not only does the technique provide access to a frequency range outside the reach of conventional microwave and optical methods, but the spectrometer is expected to have a narrow emission linewidth comparable to that of the best sources, a high sensitivity comparable to that of the best detectors, and the ability to couple on-chip to mesoscopic systems uniformly over the entire bandwidth. The large bandwidth and on-chip coupling allows following transitions tuned by an external parameter, such as the electric field in graphene or the magnetic flux in superconducting circuits.

This spectrometer will address several outstanding questions on the nature of elementary excitations in different mesoscopic systems. The experiments proposed are a direct measurement of the hybridization of Andreev states in closely spaced superconducting weak links; spectroscopy of diabolic points in topologically non-trivial superconducting circuits; and spectroscopy of zero-crossing Andreev states (Weyl nodes) in multijunction weak links. These experiments will further our understanding of mesoscopic superconductivity, elucidating the link between physical parameters of a superconducting circuit and topological features in its energy spectrum.

Such advances in understanding could lead to device applications in electronics, specifically metrology (quantized current sources, improvements in Josephson junction based voltage standards) and quantum computing (topological quantum bits and gates). These advances are important for a society in which technology plays a more and more important role. Progress in metrology will lead to better detectors, clocks, digitizers, sources, and diverse other components which will find applications in equipment from phones to satellites. Progress in quantum information will lead to computers which can solve certain problems, such as predicting protein-folding or simulating chemical reactions, which are not tractable for classical computers.

Work performed

All work packages are on schedule. Development of the second generation spectrometer (WP 0.1) is complete, although some optimization remains to be done. This result has taken a tremendous amount of work, largely involving setting up the measurement apparatus and installing the cleanroom for fabrication of devices. One doctoral student, one postdoc and the PI are implicated in WP0. A technical publication on properties of the spectrometer is forthcoming. It is now possible to start coupling the second generation spectrometer to mesoscopic systems. This is the major goal for the following reporting period. Work on WP0.2, the third generation spectrometer, has started but is still at the level of designing the circuit and simulating electromagnetic modes--fabrication has not begun.

Excellent progress has been made on WP1, Andreev states, after a change in strategy to overcome technical difficulties. Devices have already been fabricated and measured. The preliminary data is promising. The PI, one postdoc, and one doctoral student who has been hired at the beginning of the second reporting period are working on WP1. Work Package 2: Gapped Graphene has been suspended due to difficulties in fabricating samples. WP3: Majorana Spectroscopy is still at the design stage.

Final results

Various applications of the superconducting circuits developed for the spectrometer include a Josephson vector network analyzer, a cryogenic mixer, a THz camera, a detector for radioastronomy, and a scanning microwave impedance microscope. In itself the proposed Josephson junction spectrometer is a general purpose tool that will benefit researchers studying mesoscopic systems. Ultimately, Josephson junction spectroscopy should not only be useful to detect existing elementary excitations but also to discover new ones.

Beyond high-frequency spectroscopy of mesoscopic systems, there are numerous potential applications of these Josephson junction circuits:
* Cryogenic mixers or other components to facilitate the scaling and integration of superconducting circuit based quantum computers.
* Sources and detectors for the terahertz range, useful for communication, science, manufacturing, and medicine.
* Combining an array of terahertz detectors a highly-sensitive camera could be developed, with applications for security imaging and sensing.
* Coherent detectors of radiation from outer space, a tool for studies of the cosmic microwave background, the composition of the early universe, or other cosmological phenomenon.
* Scanning microwave impedance microscopy
* Topologically protected superconducting quantum bits.
* Improvements to Josephson voltage standards for metrology.
* Improvements to current standards for metrology.

These advances are important for a society in which technology plays a more and more important role.