UC Santa Barbara Researchers Harness Surface Acoustic Waves for Quantum Optomechanics Advancements

Researchers from the University of California, Santa Barbara, have demonstrated the use of Surface Acoustic Waves (SAWs) in quantum systems, specifically with quantum emitters in two-dimensional (2D) materials. The study showed energy-level splitting consistent with deformation potential coupling, which could be used for on-demand entangled-photon-pair generation from 2D materials.

The research also explored the coupling of solid-state artificial atoms with confined acoustic modes in optomechanical resonators, and the integration of two-dimensional single-photon emitters with SAW resonators. The findings could lead to the development of more compact sensors and quantum electro-optomechanics, and new technologies in quantum computing and communication.

What is the Significance of Surface Acoustic Waves in Quantum Systems?

Surface Acoustic Waves (SAWs) are a versatile tool for coherently interfacing with a variety of solid-state quantum systems, including superconducting qubits, spins, and quantum emitters. They span microwave to optical frequencies and have been used in a variety of applications, including the development of compact sensors and quantum electro-optomechanics.

In a recent study conducted by researchers from the Electrical and Computer Engineering Department at the University of California, Santa Barbara, SAW cavity optomechanics were demonstrated with quantum emitters in two-dimensional (2D) materials, specifically monolayer WSe2 and hBN on a planar lithium niobate SAW resonator driven by superconducting electronics. The researchers used steady-state photoluminescence spectroscopy and time-resolved single-photon counting to map the temporal dynamics of modulated 2D emitters under coupling to different SAW cavity modes.

The results showed energy-level splitting consistent with deformation potential coupling of 35 meV for WSe2 and 125 meV for hBN visible-light emitters. The researchers leveraged the large anisotropic strain from the SAW to modulate the excitonic fine-structure splitting in WSe2 on a nanosecond timescale, which may find applications for on-demand entangled-photon-pair generation from 2D materials.

How Does the Coupling of Solid-State Artificial Atoms Work?

The coupling of solid-state artificial atoms, such as optically active defects and color centers with confined acoustic modes in optomechanical resonators, is an elegant approach for coherently controlling, transferring, and entangling a variety of quantum degrees of freedom, including photons, phonons, and spins.

In solids, microwave phonons and optical photons have similar wavelengths and can be confined into small mode volume cavities that enable efficient mode overlap and strong interactions. Many platforms have been developed to mediate these interactions, including mechanical membranes, hybrid photonic-phononic crystals, and surface acoustic wave (SAW) resonators.

Single-photon emitters (SPEs) embedded within optomechanical cavities are remarkably sensitive to local strain, exhibiting frequency shifts nearly 2 orders-of-magnitude larger (about 10 GHz/pm) than microscale optical resonators (about 100 MHz/pm). The use of SPEs provides a strong optical nonlinearity that ensures only individual photons are emitted, typically with sub-nanowatt optical power requirements.

What are the Advantages of Two-Dimensional Single-Photon Emitters?

The recent discovery of single-photon emitters (SPEs) in two-dimensional (2D) materials, such as WSe2 and hexagonal boron nitride (hBN), provides an opportunity to further enhance the coupling while simplifying the device and fabrication complexity.

Two-dimensional SPEs, which originate from crystalline defects in the host material, exhibit high optical extraction efficiency and brightness with detection rates up to 25 MHz. They also have indistinguishable and near transform-limited line widths, high single-photon purity, unique spin-valley phenomena, high working temperatures, and site-selective engineering.

The layered structure of 2D materials arising from van der Waals forces ensures that the defects are two-dimensional and are able to function at surfaces devoid of any surface states, allowing for strong proximity interaction with their surrounding environment.

How Does the Integration of Two-Dimensional SPEs with SAW Resonators Work?

The strong proximity interaction in addition to the site-specific fabrication and relaxed lattice-matching requirements makes 2D SPEs an ideal two-level system to be integrated with optomechanical resonators. Proximity effects allow for efficient deformation potential coupling, while the ability to deterministically transfer 2D monolayers onto nearly any surface allows for nanoscale precision in positioning of a single SPE within optomechanical resonators.

In the study conducted by researchers from the University of California, Santa Barbara, they parametrically modulated the resonance frequency of SPEs in monolayer WSe2 and multilayer hBN integrated with a LiNbO3 SAW resonator driven by superconducting electronics and studied the coupling mechanisms, strain susceptibility, and potential for acoustic quantum-regime operation.

They demonstrated cavity phonon-SPE coupling with deformation potential coupling of at least 35 meV for WSe2 and 125 meV for hBN, which is larger than or comparable to alternative SPE host materials. The dynamics of the SPE-SAW cavity system were measured through time-resolved stroboscopic and steady-state photoluminescence spectroscopy.

What are the Potential Applications of This Research?

The research conducted by the team at the University of California, Santa Barbara, has significant implications for the field of quantum optomechanics. By leveraging the large anisotropic strain from the SAW to modulate the excitonic fine-structure splitting in WSe2 on a nanosecond timescale, it may be possible to develop applications for on-demand entangled-photon-pair generation from 2D materials.

This could lead to the development of more compact sensors and quantum electro-optomechanics in a multifunctional integrated platform that combines phononic, optical, and superconducting electronic quantum systems.

Furthermore, the ability to efficiently couple 2D SPEs with high-quality SAW resonators could open up new avenues for exploring the potential of 2D materials for quantum optomechanics. This could lead to the development of new technologies and applications in the field of quantum computing and communication.