NTT Corporation (NTT) and Nihon University have succeeded in generating a hybrid state of photoexcited electrons and gigahertz ultrasonics with a long lifetime of a few milliseconds by fabricating ultrasonic elements doped with rare earth elements that resonate with light at communication wavelengths. The ability to control rare earth electrons with high coherence is expected to be applied to energy-saving quantum optical memory devices. This allows for the possibility of creating energy-efficient devices for storing and processing quantum information using light.
This hybrid state enables efficient conversion of energy between light and acoustic waves with a wide range of different frequency bands and the ability to manipulate the number and phases of excited electrons with acoustic waves. In the future, NTT and their collaborators plan to improve the controllability of the hybrid state and apply it to energy-saving quantum optical memory devices by using materials with Erbium (Er) doped only on the topmost surface or by introducing a structure that allows optical access only to Er on the topmost surface.
Er, a rare earth element, has electrons in its core that respond well to light at communication wavelengths. This makes Er useful in quantum optical memory because it provides high quantum coherence. However, the shielding effect of the valence electrons makes the external control of the core electrons difficult. High voltages of tens to hundreds of volts are required to electrically control the optical resonance wavelength of Er, proving challenging for quantum device applications.
NTT found a way to change Er's behavior using low voltages by using the strain caused by mechanical vibration. However, the frequency is limited to about 1 MHz and the modulation speed was slow. NTT and Nihon University found a faster way. They succeeded in high speed modulation of the optical resonance frequency of Er by concentrating a vibrational strain of about 2 GHz on the crystal surface and fabricating an element that generates surface acoustic waves, a type of ultrasonic wave, on Er doped crystal substrate. This creates a hybrid state where electrons and fast sound waves interact since the modulation speed is faster than the lifetime of the excited electrons. By using this hybrid state, it is possible to efficiently convert energy between light and acoustic waves with widely different frequency bands. Therefore, allowing for use in energy-saving quantum optical memory devices.
The ultrasonic device used in this experiment deforms when a voltage is applied to the comb-shaped electrode and thin film formed on an Er doped crystal is placed on top of it. The ultrasonic wave of the wavelength corresponding to the period of the comb electrode is then excited. As a result, when looking at the absorption of light by Er, there is not just one peak but multiple peaks, evenly spaced out. The spaces between these peaks match the frequency of the ultrasonic waves, suggesting that the Er is interacting with these waves. This interaction is strongest near the crystal's top surface. Because of this interaction, both the number and behavior of the excited electrons can be controlled using the ultrasonic waves.
Er typically exists in various forms (isotopes) with slightly different response frequencies. In this experiment an Er with purified isotopes was used. This refinement narrowed down their responses to about 500 million times per second. When sound waves were introduced, Er electrons responded even
faster, approximately 2 billion times per second. Next, a laser beam with exceptionally precise characteristics was needed; NTT and Nihon University used an optical frequency comb to ensure the laser's behavior was precisely calibrated. The crystals employed do not naturally respond to electricity, so a thin film was applied to its surface to induce movement when electricity is applied. NTT created a high-quality film for this experiment. They established an effective setup to study and control the response of electrons in Er crystals.
This development holds immense potential for the field of quantum technology, particularly in the precise control of rare earth electrons, as exemplified by the use of Er. Overcoming challenges associated with the shielding effect of valence electrons, NTT's innovative approach of using low voltages and mechanical vibration has paved the way for a faster modulation speed, reaching frequencies of 2 GHz. This breakthrough enables the efficient conversion of energy between light and acoustic waves across diverse frequency bands, promising applications in energy-saving quantum optical memory devices.