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Quantum states of mechanical motion can be important resources for quantum information, metrology and studies of fundamental physics. Recent demonstrations of superconducting qubits coupled to acoustic resonators have opened up the possibility of performing quantum operations on macroscale motional modes 1 – 3 , which can act as long-lived quantum memories or transducers. In addition, they can potentially be used to test decoherence mechanisms in macroscale objects and other modifications to standard quantum theory 4 , 5 . Many of these applications call for the ability to create and characterize complex quantum states, such as states with a well defined phonon number, also known as phonon Fock states. Such capabilities require fast quantum operations and long coherence times of the mechanical mode. Here we demonstrate the controlled generation of multi-phonon Fock states in a macroscale bulk acoustic-wave resonator. We also perform Wigner tomography and state reconstruction to highlight the quantum nature of the prepared states 6 . These demonstrations are made possible by the long coherence times of our acoustic resonator and our ability to selectively couple a superconducting qubit to individual phonon modes. Our work shows that circuit quantum acoustodynamics 7 enables sophisticated quantum control of macroscale mechanical objects and opens up the possibility of using acoustic modes as quantum resources. Circuit quantum acoustodynamics is used to achieve controlled generation of multi-phonon Fock states in a bulk acoustic-wave resonator, which are demonstrated to have a quantum nature.

Net Brillouin amplification is demonstrated in silicon with just 5 mW pumping. Greater than 5 dB amplification is achieved. Both Kerr and Raman nonlinearities are radically enhanced by tight optical-mode confinement in nanoscale silicon waveguides 1 , 2 , 3 , 4 . Counterintuitively, Brillouin nonlinearities—originating from coupling between photons and acoustic phonons—are exceedingly weak in these same nonlinear waveguides 5 . Strong Brillouin interactions have only recently been realized in a new class of optomechanical structures that control the interaction between guided photons and phonons 5 , 6 , 7 . Despite these major advances, appreciable Brillouin-based optical amplification has yet to be observed in silicon. Using a membrane-suspended waveguide, we report large Brillouin amplification in silicon for the first time, reaching levels greater than 5 dB for modest pump powers, and demonstrate a record low (5 mW) threshold for net amplification. This work represents an important step towards the realization of high-performance Brillouin lasers and amplifiers in silicon.

Non-reciprocal light propagation is essential to control optical crosstalk and back-scatter in photonic systems. However, realizing high-fidelity non-reciprocity in low-loss integrated photonic circuits remains challenging. Here, we experimentally demonstrate a form of non-local acousto-optic light scattering to produce non-reciprocal single-sideband modulation and mode conversion in an integrated silicon photonic platform. In this system, a travelling-wave acoustic phonon driven by optical forces in a silicon waveguide spatiotemporally modulates light in a separate waveguide through linear interband Brillouin scattering. This process extends narrowband optomechanics-based schemes for non-reciprocity to travelling-wave physics, enabling large operation bandwidths of more than 125 GHz and up to 38 dB of non-reciprocal contrast between forward- and backward-propagating optical waves. The modulator operation wavelength is tunable over a 35-nm range by varying the optical drive wavelength. Such travelling-wave acousto-optic interactions provide a promising path toward the realization of broadband, low-loss isolators and circulators within integrated photonics.

Brillouin nonlinearities—which result from coupling between photons and acoustic phonons—are exceedingly weak in conventional nanophotonic silicon waveguides. Only recently have Brillouin interactions been transformed into the strongest and most tailorable nonlinear interactions in silicon using a new class of optomechanical waveguides that control both light and sound. In this paper, we use a multi-mode optomechanical waveguide to create stimulated Brillouin scattering between light-fields guided in distinct spatial modes of an integrated waveguide for the first time. This interaction, termed stimulated inter-modal Brillouin scattering, decouples Stokes and anti-Stokes processes to enable single-sideband amplification and dynamics that permit near-unity power conversion. Using integrated mode multiplexers to address separate optical modes, we show that circulators and narrowband filters are not necessary to separate pump and signal waves. We also demonstrate net optical amplification and Brillouin energy transfer as the basis for flexible on-chip light sources, amplifiers, nonreciprocal devices and signal-processing technologies. Here, Kittlaus et al . demonstrate stimulated inter-modal Brillouin scattering on-chip. Through this process, a Brillouin interaction couples light fields that propagate in distinct spatial modes of a Brillouin-active silicon waveguide, which may allow a variety of new processes in silicon photonics.

Emerging technologies based on tailorable photon–phonon interactions promise new capabilities ranging from high-fidelity information processing to non-reciprocal optics and quantum state control. However, many existing realizations of such light–sound couplings involve unconventional materials and fabrication schemes challenging to co-implement with scalable integrated photonic circuitry. Here, we demonstrate direct acousto-optic modulation within silicon waveguides using electrically driven surface acoustic waves (SAWs). By co-integrating electromechanical SAW transducers with a standard silicon-on-insulator photonic platform, we harness silicon’s strong elasto-optic effect to create travelling-wave phase and single-sideband amplitude modulators from 1 to 5 GHz, with index modulation strengths comparable to electro-optic technologies. Extending this non-local interaction to centimetre scales, we demonstrate non-reciprocal modulation with operation bandwidths of >100 GHz and insertion losses of <0.6 dB. This acousto-optic platform is compatible with complementary metal–oxide–semiconductor fabrication processes and existing silicon photonic device architectures, opening the door to flexible, low-loss modulators and non-magnetic optical isolators and circulators in integrated photonic circuits.Direct acousto-optic modulation within complementary metal–oxide–semiconductor compatible silicon photonic waveguides using electrically driven surface acoustic waves is demonstrated. Non-reciprocal operation bandwidths of >100 GHz and insertion losses of <0.6 dB are obtained.

High quality-factor (Q) optical resonators are a key component for ultra-narrow linewidth lasers, frequency stabilization, precision spectroscopy and quantum applications. Integration in a photonic waveguide platform is key to reducing cost, size, power and sensitivity to environmental disturbances. However, to date, the Q of all-waveguide resonators has been relegated to below 260 Million. Here, we report a Si 3 N 4 resonator with 422 Million intrinsic and 3.4 Billion absorption-limited Qs. The resonator has 453 kHz intrinsic, 906 kHz loaded, and 57 kHz absorption-limited linewidths and the corresponding 0.060 dB m −1 loss is the lowest reported to date for waveguides with deposited oxide upper cladding. These results are achieved through a careful reduction of scattering and absorption losses that we simulate, quantify and correlate to measurements. This advancement in waveguide resonator technology paves the way to all-waveguide Billion Q cavities for applications including nonlinear optics, atomic clocks, quantum photonics and high-capacity fiber communications. Integrated photonic all-waveguide resonators are a critical component in many future applications. Here the authors develop an optimized photonic all-waveguide resonator with an ultra-high quality factor, Q, of almost half a billion, and a narrow sub-MHz linewidth.

Spectrally pure lasers, the heart of precision high-end scientific and commercial applications, are poised to make the leap from the laboratory to integrated circuits. Translating this performance to integrated photonics will dramatically reduce cost and footprint for applications such as ultrahigh capacity fibre and data centre networks, atomic clocks and sensing. Despite the numerous applications, integrated lasers currently suffer from large linewidth. Brillouin lasers, with their unique properties, offer an intriguing solution, yet bringing their performance to integrated platforms has remained elusive. Here, we demonstrate a sub-hertz (~0.7 Hz) fundamental linewidth Brillouin laser in an integrated Si3N4 waveguide platform that translates advantages of non-integrated designs to the chip scale. This silicon-foundry-compatible design supports low loss from 405 to 2,350 nm and can be integrated with other components. Single- and multiple-frequency output operation provides a versatile low phase-noise solution. We highlight this by demonstrating an optical gyroscope and a low-phase-noise photonic oscillator.

Silicon is the workhorse of the semiconductor electronics industry, but its lack of optical functionality is a barrier to developing a truly integrated silicon-based optoelectronics platform. Although there are several ways of exploiting nonlinear light-matter interactions to coax silicon into optical functionality, the effects tend to be weak. Otterstrom et al. used a suspended silicon waveguide racetrack structure to stimulate the stronger nonlinear effect of Brillouin scattering and achieve lasing from silicon. The ability to engineer the nonlinearity and tune the optical response through the design of the suspended cavity provides a powerful and flexible route for developing silicon-based optoelectronic circuits and devices. Science , this issue p. 1113 Brillouin scattering is exploited to develop a silicon laser. Brillouin laser oscillators offer powerful and flexible dynamics as the basis for mode-locked lasers, microwave oscillators, and optical gyroscopes in a variety of optical systems. However, Brillouin interactions are markedly weak in conventional silicon photonic waveguides, stifling progress toward silicon-based Brillouin lasers. The recent advent of hybrid photonic-phononic waveguides has revealed Brillouin interactions to be one of the strongest and most tailorable nonlinearities in silicon. In this study, we have harnessed these engineered nonlinearities to demonstrate Brillouin lasing in silicon. Moreover, we show that this silicon-based Brillouin laser enters a regime of dynamics in which optical self-oscillation produces phonon linewidth narrowing. Our results provide a platform to develop a range of applications for monolithic integration within silicon photonic circuits.

A recent renaissance in Brillouin scattering research has been driven by the increasing maturity of photonic integration platforms and nanophotonics. The result is a new breed of chip-based devices that exploit acousto-optic interactions to create lasers, amplifiers, filters, delay lines and isolators. Here, we provide a detailed overview of Brillouin scattering in integrated waveguides and resonators, covering key concepts such as the stimulation of the Brillouin process, in which the optical field itself induces acoustic vibrations, the importance of acoustic confinement, methods for calculating and measuring Brillouin gain, and the diversity of materials platforms and geometries. Our Review emphasizes emerging applications in microwave photonics, signal processing and sensing, and concludes with a perspective for future directions.

Narrow linewidth visible light lasers are critical for atomic, molecular and optical (AMO) physics including atomic clocks, quantum computing, atomic and molecular spectroscopy, and sensing. Stimulated Brillouin scattering (SBS) is a promising approach to realize highly coherent on-chip visible light laser emission. Here we report demonstration of a visible light photonic integrated Brillouin laser, with emission at 674 nm, a 14.7 mW optical threshold, corresponding to a threshold density of 4.92 mW μm −2 , and a 269 Hz linewidth. Significant advances in visible light silicon nitride/silica all-waveguide resonators are achieved to overcome barriers to SBS in the visible, including 1 dB/meter waveguide losses, 55.4 million quality factor (Q), and measurement of the 25.110 GHz Stokes frequency shift and 290 MHz gain bandwidth. This advancement in integrated ultra-narrow linewidth visible wavelength SBS lasers opens the door to compact quantum and atomic systems and implementation of increasingly complex AMO based physics and experiments. In this work the authors demonstrate on-chip integration of Brillouin lasing operating at visible wavelengths, with engineered design for stable output. This technical and scientific advance will help develop integrated light sources for quantum computing or atomic and molecular spectroscopy.