Quantum coherent materials have become important in magnetic sensors as well as information processing technology. For magnetic sensors, spin-based transport in metals and across tunnel barriers is important because it can increase the signal to noise and reduce the power. These devices have applications in a wide variety of fields, ranging from sensing to imaging. We have developed large (256 element) arrays and systems to make forensic copies of magnetic tapes for data migration and authenticity analysis. In the area of quantum computing, we are developing crystalline tunnel junctions that will improve quantum state visibility and measurements. We are also working to improve the coherence of these devices by developing low-loss dielectrics.
NIST has become a world leader in the area of high sensitivity magnetic field sensors. To achieve this, we have explored spintronic devices, including anisotropic magneto-resistive (AMR) and tunneling magneto-resistive (TMR) devices. With the AMR devices, we have developed tape read heads that have many sensing elements. These devices are designed to operate at DC, and are used to image magnetic tapes. The system that we built was delivered to the FBI and is currently in the process of being validated for use on analog audio tape evidence. The TMR devices are targeted towards low noise field sensors and are integrated with a flux concentrator. We have reached noise levels better than one millionth of the Earth’s magnetic field, with a dynamic range allowing us to run them in an un-shielded environment. These devices are very low power and are targeted towards geological and exploratory work.
For the quantum computing work, we have been developing superconducting devices using a novel approach to making a tunnel barrier. Traditionally these tunnel junctions use convenient amorphous, thermal oxide barriers because the thickness is easily controllable and they tend to be pin-hole free. However, it has been shown that they have energy-absorbing defects at random frequencies due to their amorphous nature. Therefore, we developed a process to grow epitaxial tunnel junctions. This is challenging because the tunnel current depends exponentially on the barrier thickness, and epitaxial barriers require high temperatures to crystallize. We worked around these issues using precision, ultra-high vacuum molecular beam epitaxy (MBE) growth and lattice matched, superconducting rhenium films on sapphire substrates. We are the first to be able to fabricate, characterize, and successfully integrate them into devices. In addition, the performance of the devices improved significantly, enabling us to use optical lithography rather than e-beam.
1) Superconducting qubit development and spectroscopy – Superconducting qubits and their applications is one of our main focusses. In this area, we have developed high quality factor materials and radiation suppression techniques that allow for reliable T1 times on the order of 30 – 40 us. We have developed measurement infrastructure of large arrays of qubit and are currently testing adiabatic state transfer, dark state transfer, and novel gates. These devices are designed using combinations of readout-resonating circuits, capacitors, inductors, and superconducting Josephson junctions. The interactions between these elements is important because it allows us to extract the quantum information from the qubits. Below is an image of the susceptibility of a qubit that is strongly coupled to a resonant cavity. From these spectra we were able to completely describe the system in terms of it’s total quantum state, successfully predicting the transition frequencies. For example the energy difference associated with the transitions 4-7, 1-3, 2-4, and 0-1 are indicated in the figure.
Radiation-suppressed superconducting quantum bit in a planar geometry, M. Sandberg, M. R. Vissers, T. A. Ohki, Jiansong Gao, J. Aumentado, M. Weides, D. P. Pappas, Appl. Phys. Lett. 102, 072601 (2013).
Detailed modelling of the susceptibility of a thermally populated, strongly driven circuit-QED system, Anton Frisk Kockum, Martin Sandberg, Michael R Vissers, Jiansong Gao, Goran Johansson and David P Pappas, J. Phys. B: At. Mol. Opt. Phys. 46, 224014 (2013), article featured on the cover.
2) Development of superconducting, broad-band microwave frequency combs - Coherent optical frequency combs are a critical component in dividing down and translating frequency standards across multiple octaves. Typically these devices are made in the optical regime using high quality factor, nonlinear optical elements. In seminal work in the Quantum Materials and Devices Project, we have succeeded in generating coherent frequency combs at microwave frequencies by exploiting the nonlinearity of the superconducting kinetic inductance. From a single pump we were able to generate multiple octaves of phase-locked frequencies in long coplanar waveguides. The waveguides were patterned as spirals to fit on a 2x2 cm chip, with lengths up to 5 meters long.
Frequency comb generation in superconducting resonators, R.P. Erickson, M.R. Vissers, M. Sandberg, S.R. Jefferts, D.P. Pappas, et al., PRL 113, 187002 (2014) PRL editor’s suggestion award and cover article.
3) Reduction of ion trap heating rates by two orders of magnitude – Quantum Information processing based on trapped ions is one of the most promising approaches to this emerging technology. Keeping an ion in its ground state long enough to run computations is essential to making this technology viable. Anomalous heating of the ions due to fluctuating electric fields is one of the main obstacles to achieving this. In collaboration with the Wineland ion trap group, we are working to understand and reduce this anomalous heating. By carefully cleaning the electrodes of the ion traps in-situ, we have been able to demonstrate two orders of magnitude reduction of the ion heating rate, thereby allowing ion trap researchers to reduce the ion-surface distance and speed up quantum gates. The image above shows the ion mill cleaning of an ion trap from the side. We currently have devised a stylus ion trap that suspends an ion relatively high above the surface, allowing us to bring other surfaces into close proximity with the ion to test cleaning procedures and evaluate their efficacy for future development into ion trap electrodes and preparation. (photo courtesy of D. Hite & Y. Coulomb).
Surface science for improved ion traps, D. A. Hite , Y. Colombe, A. C. Wilson, D. T. C. Allcock, D. Leibfried, D. J. Wineland, and D. P. Pappas, MRS Bulletin 38 , 826 (2013)
One second of audio in a cassette tape imaged using the 256-sensor array.
Lead Organizational Unit:pml
David Pappas, Project Leader
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