Physicists have long known that, when cooled to very low temperatures, electrons can be placed on the surface of liquefied helium. Now, researchers from the RIKEN Discovery Research Institute, Wako, in collaboration with colleagues at Japan’s Keio University, Yokohama, have discovered they can effectively excite these electrons using microwave radiation.
At these extremely low temperatures—just above absolute zero—the electrons are stabilized by tiny electrical fields at the surface of the liquid helium, leading to an arrangement of electrons into regular two-dimensional patterns. Recently, scientists in the US proposed that these ordered charges could be used for quantum computing applications, which triggered intense studies in laboratories around the world. To achieve the necessary control of the electron states, the use of microwave radiation was suggested as a way to excite the electrons from one energy state to the other and thereby modify their quantum computing behavior.
Reporting in the journal Physical Review Letters (1), the researchers have studied the influence of microwave radiation on the electrons by measuring the electrical resistance across the floating electrons (Fig. 1). Increased resistivity is a sign of excited and heated electrons. Such measurements provide a relatively accurate picture of the state of the electrons, as “this system is an extremely clean and transparent electron system that provides a direct relationship between experiment and theory,” comments Kimitoshi Kono from the RIKEN team.
Surprisingly, the researchers found that as the temperature is lowered the change in resistivity of the electrons increases significantly through microwave irradiation. The measured values are much higher than estimated in previous theoretical studies. In fact, the sensitivity of the system’s electric current to the microwave radiation is so high that it could even be used as an efficient microwave detector. Regardless, this change in resistivity is a clear sign of an increased electron temperature and therefore of excitation of electrons into higher energy states. In particular, the researchers found that electrons are scattered away from the surface, which significantly reduces the suitability of such microwave-controlled electrons for quantum computing.
Unfortunately, “this is an important limitation towards the realization of quantum computers,” says Kono. To further pursue quantum computing applications, physicists must confine the lateral motion of electrons. For example, nanostructures around the surface could create suitable electric potentials to suppress increases in electron temperature. Therefore, Kono remains convinced that “quantum computing is still possible in this system.”
1. Konstantinov, D., Isshiki, H., Monarkha, Y., Akimoto, H., Shirahama, K. & Kono, K. Microwave-resonance-induced resistivity: evidence of ultrahot surface-state electrons. Physical Review Letters 98, 235302 (2007).
