NTT and Nihon University achieve world's first hybrid state of electrons resonating at an optical communication wavelength and a gigahertz ultrasonic wave

NTT and Nihon University have developed an ultrasonic device doped with the rare earth element erbium (Er), which resonates with light at communication wavelengths, and have successfully created a hybrid state of photoexcited electrons with a lifetime of several milliseconds and gigahertz ultrasound wavelength. This achievement facilitates the control of rare-earth electrons with high coherence through low-voltage ultrasonic excitation and is expected to contribute to developing future applications in energy-saving quantum optical memory devices. The results were published online in the U.S. scientific journal, Physical Review Letters.

Er, a rare earth element, possesses inner-shell electrons that resonate with light at communication wavelengths. Since inner-shell electrons shielded by outer-shell electrons are insensitive to the outside world, quantum optical memory uses Er as an element that provides high quantum coherence. However, the shielding effect of outer-shell electrons has a drawback, making it challenging to externally control the inner-shell electrons. Traditionally, modulating the optical resonance frequency of Er by 1 GHz in crystals using an electric field has required high voltages exceeding 100 V, presenting a challenge of low controllability.

In response to this issue, NTT is conducting research to develop energy-efficient quantum optical memory devices using a mechanical oscillator capable of achieving significant modulation at low voltages. To achieve this, a major challenge has been the creation of a hybrid state of electrons and vibrations that allows for the control of optical responses of electrons through mechanical vibrations.

In this study, NTT and Nihon University developed a device that generates surface acoustic waves, a type of ultrasound, on an Er-doped crystal substrate. This achievement concentrates approximately 2 GHz of vibrational strain on the crystal surface, enabling successful high-speed modulation of the optical resonance frequency of Er. This modulation speed surpasses the lifetime of the excited electrons, modulating the electrons at a frequency beyond the resonance linewidth, thereby creating a hybrid state of electrons resonating at the communication wavelength band and gigahertz ultrasound. Utilizing this state, the optical response of Er-excited electrons with high coherence can be controlled by ultrasound at low voltage, promising future applications in energy-saving quantum optical memory devices.