(a) Schematic representation of the FET device used in this work. (b) Schematic diagram of the interaction between the trapped electron and the percolation pathways mediated by the MW field (top). Multilevel RTN events recorded in the FET current measured at 80 K (bottom). (c) Wideband CW microwave spectroscopy of the FET channel current performed at 4.2 K. Each narrow spike is a separate resonance that is resolved into a Fano or Lorentzian shape at higher resolution (inset). (d) Density of states (red), amplitude change (blue) and coherence times (inset) histograms. Credit: Tokyo Tech
Scientists from the Tokyo Institute of Technology collaborated with the University of Cambridge to explore the interaction between electronic defect states and microwave fields within the field-effect transistors’ oxide layer at cryogenic temperatures. It has been discovered that the physics of such defect states are in sync with driven two-level systems that possess long coherence times.
The induced dynamics of these defect states can be independently and coherently controlled. It is hoped that the outcomes will add to the correlated electronic glassy dynamics field in condensed matter physics because of the nature of this project.
It is also expected that the results will provide an insight into the charge noise effects in mesoscopic devices, while also enabling new research for the development of novel technologies in the significant area of semiconductor-based quantum information processing.
Defect states that generally function as electron traps in oxide-semiconductor interfaces are often noisy and tend to minimize the performance of nanoscale devices. The electrostatic environment that the conducting electrons experience is modified by such defect states, which force the electrons to infiltrate through nanowire-like pathways at adequately low temperatures.
This enables a detection mechanism to occupy such trap locations by the current calculated in the conduction channel. This effect usually reflects as random telegraph noise (RTN), which matches the incoherent emission, capturing electrons in trap states, and is mediated by the thermal background.
Researchers from the Quantum Nanoelectronics Research Center, the Center for Advanced Photonics and Electronics (University of Cambridge), Cavendish Laboratory (University of Cambridge) and Institute of Innovative Research (Tokyo Tech) were encouraged by the drastic changes in the conductivity triggered by RTN present in field-effect transistors, and explored the potential mechanisms where the defects states occupation can be monitored and dynamically mediated using clear microwave fields.
It was discovered that the dynamics of such trap states match with two-level systems (TLS), the energy levels of which are discrete and just the lowest two are available inside the energy of the excitation signal. The foundation for a quantum bit implementation can be represented by a TLS.
Starting with the microwave spectroscopic signature obtained from the response of the FET used in this research and then displaying several superior quality factor resonances (Q > 104), the extracted coherence times observed in this research are significantly longer, by approximately three orders of magnitude, than any other defect-based implementations of TLS.
The dynamics of the trapped electrons, which are known to be independent of the dielectric’s chemistry, can be explored by conducting single-pulse experiments. Coherent control was attained by following a standard Ramsey protocol.
The experimental behavior found in these experiments was replicated using an optical master equation which explains the dynamics of the trapped electrons and a physical model that is based on linear response theory.
It was also discovered that the defect states are under protection against phonons, which explains the long decoherence times in the experiment. The long-range Coulombic communications with other charges were found to be the main reason for back-action.
As it is possible to independently address every resonance in the frequency space, measure the quasi-uniform density of states and observe the distribution of long coherence times, it is speculated that this study can encourage the possibility of employing systems such as quantum bits or quantum memories in the quantum information processing implementations of the future.