Phonon modes within pristine crystalline resonators now routinely reach the quantum ground state. Such systems are attractive for quantum information science applications, as advanced fabrication and processing can enable relatively long quantum coherence times, and precision control can be realized through optical, electrical, or qubit coupling. In many state-of-the-art systems, the phonon lifetime is limited by disorder. In particular, native oxides or damaged `dead layers' at surfaces can host two-level tunneling states that lead to a particularly problematic form of dissipation that increases at lower temperatures. As mechanical losses are driven down in systems such as micro-fabricated bulk acoustic wave resonators, tunneling states are expected to emerge as the dominant mechanism for phonon decoherence. A quantitative description of these mesoscopic systems therefore requires a framework that captures interactions between a selected phonon mode and a large ensemble of TLS. Here, we derive a quantum master equation for this coupled system, permitting the phonon decoherence produced by two-level tunneling states to be calculated. As an example, we estimate the lifetime of a variety of quantum states within quartz micro-resonators hosting a thin surface layer of tunneling states. We find that the phonon coherence time is maximized at low temperatures, in spite of increased mechanical dissipation, and that phonon-TLS coupling can be reduced for modes with strain nodes at the surfaces.

Fusion-based quantum computing with dual-rail qubits is a leading candidate for scalable quantum computing using linear optics. This paradigm requires single photons which are entangled into small resource states before being fed into a fusion network. The most common sources for single optical photons and for small entangled states are probabilistic and heralded. The realization of a single reliable deterministic source requires many redundant probabilistic sources and a complex optical network for rerouting and retiming probabilistic outputs. In this work, we show how optomechanics enables reliable production of resources for photonic quantum computing without the redundancy of the all-optical approach. This is achieved by using acoustic modes as caches of quantum resources, ranging from single-particle states to small entangled states, with on-demand read-out. The advantages of acoustic modes as optical quantum memories, compared to other technologies, include their intrinsically long lifetimes and that they are solid state, highly tailorable, and insensitive to electromagnetic noise. We show how the resource states can be prepared directly in the acoustic modes using optical controls. This is still probabilistic and heralded, as in the all-optical approach, but the acoustic modes act as a quantum memory which is integrated into the production of the states. The quantum states may be deterministically transferred from acoustic modes to optical modes, on demand, with another optical drive.

We present the first experimental demonstration of free-carrier mediated enhancement of Brillouin optomechanical interactions. An 8 dB enhancement of the Stokes signal is observed in a 750 µ m long aluminum nitride covered silicon waveguide.

We present a quantum field theoretic formulation of acoustoelectric interactions in waveguide-like systems of arbitrary cross-section. Building on an open quantum systems approach, we derive a unified description of plasmon-phonon coupling that incorporates dissipation, noise, and the influence of drift currents. Our analysis captures both bulk and surface plasmon modes, highlighting how drift currents Doppler-shift plasmonic resonances and reshape the phonon noise spectrum. The resulting Heisenberg-Langevin equations yield closed-form expressions for frequency shifts, gain, and noise power spectra, enabling direct evaluation of performance metrics such as the noise factor in acoustoelectric amplifiers and oscillators. In the appropriate limits, this framework reproduces known results while extending them to complex geometries.

High-frequency mechanical oscillators with long coherence times are essential for realizing a variety of high-fidelity quantum sensors, transducers and memories. However, the coherence times needed for quantum applications require advances in probing and mitigating the origins of phonon decoherence in the materials used for mechanical oscillators. Here we identify key sources of phonon decoherence in crystalline media by combining non-invasive laser spectroscopy with materials analysis. Using micro-fabricated high-overtone bulk acoustic-wave resonators as an experimental test bed, we find that phonon-surface interactions are the dominant source of phonon decoherence in crystalline quartz. The probable causes are lattice distortion, subsurface damage and a high concentration of elemental impurities. We use an optimized polishing process to remove the compromised surface layer and produce resonators with quality factors exceeding 240 million at 12 GHz, corresponding to phonon coherence times of over 6 ms. We verify that these mechanical oscillators have negligible dephasing. Building on these results, we propose a path that could reach coherence times beyond 100 ms as the basis for high-frequency quantum memories. These findings demonstrate that enhanced control over surfaces enables a substantial reduction in dissipation and noise.

High-fidelity quantum optomechanical control of a mechanical oscillator requires the ability to perform efficient, low-noise operations on long-lived phononic excitations. Microfabricated high-overtone bulk acoustic wave resonators (mu HBARs) support high-frequency mechanical modes above 10 GHz with coherence times exceeding one millisecond. Here we demonstrate a mu HBAR-based cavity optomechanical system that permits quantum optomechanical control of individual high-coherence phonon modes. We perform laser cooling of the phonon modes from an occupation of approximately 22 phonons to fewer than 0.4, corresponding to laser-based ground-state cooling of a mechanical object with a mass of 7.5 mu g. During the cooling process we do not observe any absorption-induced heating, demonstrating the resilience of the HBAR optomechanical systems against parasitic heating. Our work demonstrates that mu HBARs are promising as the basis for quantum optomechanical systems with robustness to decoherence that is necessary for efficient, low-noise photon-phonon conversion.

We introduce a bulk acoustic resonator based cavity optomechanical system for quantum control of long-lived acoustic phonons and demonstrate laser cooling of individual phonon modes to ground-state $(\langle n\} < 0.4)$.

High-frequency mechanical oscillators with long coherence times are essential to realizing a variety of high-fidelity quantum sensors, transducers, and memories. However, the unprecedented coherence times needed for quantum applications require exquisitely sensitive new techniques to probe the material origins of phonon decoherence and new strategies to mitigate decoherence in mechanical oscillators. Here, we combine non-invasive laser spectroscopy techniques with materials analysis to identify key sources of phonon decoherence in crystalline media. Using micro-fabricated high-overtone bulk acoustic-wave resonators (μHBARs) as an experimental testbed, we identify phonon-surface interactions as the dominant source of phonon decoherence in crystalline quartz; lattice distortion, subsurface damage, and high concentration of elemental impurities near the crystal surface are identified as the likely causes. Removal of this compromised surface layer using an optimized polishing process is seen to greatly enhance coherence times, enabling μHBARs with Q-factors of > 240 million at 12 GHz frequencies, corresponding to > 6 ms phonon coherence times and record-level f-Q products. Complementary phonon linewidth and time-domain ringdown measurements, performed using a new Brillouin-based pump-probe spectroscopy technique, reveal negligible dephasing within these oscillators. Building on these results, we identify a path to > 100 ms coherence times as the basis for high-frequency quantum memories. These findings clearly demonstrate that, with enhanced control over surfaces, dissipation and noise can be significantly reduced in a wide range of quantum systems.

We demonstrate coherent electro-optomechanical control of mechanical motion in an optomechanical resonator with acoustoelectric, voltage-controlled gain for the gigahertz-frequency mechanical modes. We use this scheme to characterize the acoustoelectric enhancement of mechanical and optomechanical resonator properties.

Low-noise and accessible millimeter-wave sources are critical for emergent telecommunications, radar and sensing applications. Current limitations to realizing low-noise, deployable millimeter-wave systems include size, weight, and power (SWaP) requirements, along with complex operating principles. In this paper we provide a compact photonic implementation for generating low phase noise millimeter-waves, which significantly simplifies the architecture and reduces the volume compared to alternative approaches. Two commercial diode lasers are self-injection-locked to a micro-Fabry-Perot cavity, and their heterodyne provides low phase noise millimeter waves reaching -148 dBc/Hz at 1 MHz offset on a 111.45 GHz carrier. Phase noise characterization at such levels and frequencies poses unique challenges, and we further highlight the capabilities of optically-based measurement techniques. Our approach to millimeter-wave generation can leverage advances in photonic integration for further miniaturization and packaging, thus providing a unique source of accessible, compact, and low-noise millimeter waves.

Wideband optical isolators are critical for the robust operation of virtually all photonic systems. However, they have been challenging to realize in the integrated form due to the incompatibility of magnetic media with these circuit technologies. Here we present the first-ever demonstration of an integrated non-magnetic optical isolator with terahertz-level optical bandwidth. The system comprises two acousto-optic beamsplitters that create a non-reciprocal multimode interferometer exhibiting high-contrast, non-reciprocal light transmission. We dramatically enhance the isolation bandwidth of this system by precisely balancing the group delays of the paths of the interferometer. Using this approach, we demonstrate integrated non-magnetic isolators with an optical contrast as high as 24.5 dB, insertion losses as low as -2.16 dB and optical bandwidths as high as 2 THz (16 nm). We also show that the centre frequency and direction of optical isolation are rapidly reconfigurable by tuning the relative phase of the microwave signals used to drive the acousto-optic beamsplitters. With their complementary metal-oxide-semiconductor compatibility, wideband operation, low losses and rapid reconfigurability, such integrated isolators address a key barrier to the integration of a wide range of photonic functionalities on a chip.

Compact photonic systems that offer high frequency stability and low noise are of increasing importance to applications in precision metrology, quantum computing, communication, and advanced sensing technologies. However, on-chip resonators comprised of dielectrics cannot match the frequency stability and noise characteristics of Fabry-Perot cavities, whose electromagnetic modes live almost entirely in vacuum. In this study, we present a novel strategy to interface micro-fabricated Fabry-Perot cavities with photonic integrated circuits to realize compact, high-performance integrated systems. Using this new integration approach, we demonstrate self-injection locking of an on-chip laser to a milimeter-scale vacuum-gap Fabry-Perot using a circuit interface that transforms the reflected cavity response to enable efficient feedback to the laser. This system achieves a phase noise of -97 dBc/Hz at 10 kHz offset frequency, a fractional frequency stability of 5*10-13 at 10 ms, a 150 Hz 1/pi integral linewidth, and a 35 mHz fundamental linewidth. We also present a complementary integration strategy that utilizes a vertical emission grating coupler and a back-reflection cancellation circuit to realize a fully co-integrated module that effectively redirects the reflected signals and isolates back-reflections with a 10 dB suppression ratio, readily adaptable for on-chip PDH locking. Together, these demonstrations significantly enhance the precision and functionality of RF photonic systems, paving the way for continued advancements in photonic applications.

Narrow-linewidth lasers are vital for a broad range of scientific and technological applications, including atomic clocks and precision sensing. Achieving high frequency stability is often as critical as ensuring scalability, portability, and cost-effectiveness in the development of low noise laser systems. Conventional electro-optic stabilization techniques, such as Pound-Drever-Hall locking to ultra-high-finesse resonators held in a vacuum chamber, provide excellent performance but remain challenging to scale. Here, we propose and experimentally demonstrate a cavity-coupled interferometric laser stabilization technique implemented on a silicon photonic chip and integrated with a compact, scalable micro-Fabry-Pérot cavity. The vacuum-gap optical cavity operates in air, achieving a quality factor of approximately 2.0× 10^9 and a fractional frequency instability of 1.45× 10^-12 at one-second averaging time. Integration of the proposed technique with the compact cavity yields more than 38-fold reduction in the laser's integrated linewidth and nearly three orders of magnitude suppression of frequency noise at 10 Hz offset frequency. The hybrid-integration of the proposed photonic chip with the micro-Fabry-Pérot cavity establishes a scalable and portable route toward chip-integrated ultra-stable lasers, paving the way for precision optical systems deployable beyond laboratory environments.

We demonstrate thermal-noise-limited direct locking of a semiconductor distributed feedback (DFB) laser to a sub-1 mL volume, ultrastable optical cavity, enabling extremely compact and simple ultrastable laser systems. Using the optoelectronic laser locking method, we realize over 140 dB suppression of the DFB free-running laser noise at 10 Hz offset, a level we estimate to be 70 dB greater than Pound-Drever-Hall locking can provide, and reach a phase noise level of -120 dBc/Hz at 200 kHz offset. We also demonstrate a new feedforward noise correction method that improves the quality of the heterodyne beat with an optical frequency comb by providing another 60 dB of laser noise rejection - a level that is 15 dB greater than predicted by current models. With feedforward, we transfer the cavity thermal noise limit across the comb spectrum despite the fact that the cavity-locked laser itself is noisy. These results establish a simple, low noise, compact approach to ultrastable laser locking that is compatible with integrated photonics, with applications in low phase noise microwave generation, sensing, and satellite ranging.

We present a wafer-level fabrication method for high-Q, compact vacuum-gap Fabry-Pérot resonators. With quality factors surpassing one billion at 1560 nm, these resonators are well-suited in a range of applications as frequency references.

Vacuum-gap Fabry-Perot cavities are indispensable for the realization of frequency-stable lasers, with applications across a diverse range of scientific and industrial pursuits. However, making these cavity-based laser stabilization systems compact, portable, and rugged enough for use outside of controlled laboratory conditions has proven difficult. Here, we present a fiber-coupled 1396 nm laser stabilization system requiring no free-space optics or alignment, built for a portable strontium optical lattice clock. Based on a 2 mL vacuum-gap Fabry-Perot cavity, this system demonstrates thermal noise-limited performance and 1x10-14 fractional frequency instability. Fiber-integrated optical components have been instrumental in both advancing the field of optics and leveraging those advances across disciplines to facilitate other fields of study. This portable system represents a major step towards making the frequency stability of cavity-based systems broadly accessible.

We demonstrate a vacuum-gap ultrastable optical reference cavity that does not require a vacuum enclosure. Our simple method of optical contact bonding in a vacuum environment allows for cavity operation in air while maintaining vacuum between the cavity mirrors. Vacuum is maintained long term, with no observed degradation in cavity stability for over 1 year after bonding. For a 1550 nm laser stabilized to a 9.7 mL in-vacuum bonded cavity, the measured Allan deviation is 2.4 x 10-14 at 1 s, and its phase noise is thermal-noise-limited from 0.1 Hz to 10 kHz, reaching about -105 dBc/Hz at 10 kHz offset frequency. This represents the highest stability of any oscillator operated without a vacuum enclosure, to our knowledge. Furthermore, we demonstrate a 0.5 mL in-vacuum bonded cavity created using microfabricated mirrors and cavity dicing, with phase noise reaching -95 dBc/Hz at 10 kHz offset frequency. By relieving the need for high-vacuum enclosures, we greatly enhance the portability and utility of low-noise, compact, cavity-stabilized lasers, with applications ranging from environmental sensing to mobile optical clocks to ultralow-noise microwave generation. (c) 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

We present a wafer-level fabrication method for compact vacuum-gap Fabry-Pérot resonators. With fractional frequency stability better than 10 − 13 at 1 second, these resonators are well-suited across a range of applications as low-noise optical frequency references.