Phononic Integrated Circuits (PnICs)

Updated 5 December 2025

Phononic Integrated Circuits are on-chip platforms that guide GHz acoustic phonons using lithographically defined, sub-micron structures to achieve signal control.
They integrate waveguides, directional couplers, resonators, and modulators to facilitate ultra-compact filtering, multiplexing, and phase control.
PnICs enable advanced RF filtering, reconfigurable signal processing, and hybrid quantum operations by precisely managing phonon propagation and interactions.

Phononic Integrated Circuits (PnICs) enable the on-chip manipulation, routing, and processing of coherent acoustic waves—phonons—with lithographically defined structures at GHz frequencies. Analogous to integrated electronic and photonic circuits, PnICs exploit sub-micron wavelength, strong material–wave interactions, and low propagation loss to realize ultra-compact and highly versatile signal processing platforms. Recent developments have established PnICs as a third pillar for information processing, complementing electronics and photonics, and providing new opportunities for RF, optical, and quantum technologies (Xu et al., 30 Oct 2025).

1. Fundamental Principles of Phononic Integration

PnICs operate by confining and guiding quantized elastic vibrations within solid-state waveguides defined by contrasts in acoustic velocity or impedance, analogous to index guiding in optics (Xu et al., 30 Oct 2025, Bicer et al., 2021, Mayor et al., 2020). The key principle is total internal reflection or bandgap engineering to restrict phonon propagation to designed paths. Single- and multi-mode acoustic waveguides in high index-contrast materials such as GaN on sapphire or silicon carbide, LiNbO₃ on sapphire, or SOI with phononic crystals, serve as the platform backbone (Bicer et al., 2021, Safavi-Naeini et al., 2018).

Device-level control is achieved by a complete set of on-chip elements:

Waveguides: GaN (700 nm–1.5 μm) on sapphire/SiC, LiNbO₃ ribs, or silicon phononic crystal line defects, with phase and group velocities vg ≈ 3,900 m/s–4,000 m/s, single-mode operation for w~λ (Xu et al., 30 Oct 2025, Bicer et al., 2021, Patel et al., 2017).
Directional Couplers: Parallel waveguides with sub-micron gaps, evanescent coupling rate κ(g), power transfer length Lc=π/2κ, with Lc≈79 μm at 1.5 GHz (Xu et al., 30 Oct 2025, Zivari et al., 2023).
Splitters and Interferometers: Binary Y-splitters (broadband 50:50, 0.5 dB insertion loss), MMIs (compact mode-mixing), arrays for fanout/multiplexing (Xu et al., 30 Oct 2025).
Polarization Converters: Adiabatic tapers exploiting R–L mode hybridization to transfer between quasi-Rayleigh and quasi-Love modes with >98% efficiency over ~100 μm length, 1.7 GHz bandwidth (Wang et al., 2022).
Resonators: Microring (R~50–115 μm), racetrack, and Fabry–Pérot geometries with Q ranging from 1,000 to >10⁴ at RT and >10⁵ at cryo, FSR = vg/(2πR) ≈ 4–7 MHz (Wang et al., 4 Dec 2025, Xu et al., 2022).
Active Modulators: Thermo-acoustic MZI (phase shift per power: α~4.03 rad/W over 100 μm; π with ~0.8 W), piezo-acoustic phase shifters (±π over tens of microns, <100 ns switching time) (Xu et al., 30 Oct 2025, Taylor et al., 2021, Shao et al., 2021).
Bragg Gratings and Bandgap Structures: Narrowband stop-bands (~33 MHz bandwidth), phononic crystals (PnC) for bandgap engineering, creating reflectors or tight confinement (Xu et al., 30 Oct 2025, Xu et al., 20 Jun 2025).

Phononic guiding is often realized in unreleased (non-suspended) architectures for robust, CMOS-compatible processing, though suspended Si or diamond platforms allow for ultra-high-Q and hybrid quantum functionality (Xu et al., 30 Oct 2025, Taylor et al., 2021, Ding et al., 2023).

2. Architectures, Scaling, and Device Performance

PnIC architectures integrate fundamental building blocks into large-scale layouts for fanout, filtering, frequency routing, and multiplexing. Canonical circuits demonstrated include:

High-Density Splitter Networks: 1×128 cascaded Y-splitter tree with ~200×200 μm² area, density >3,000/cm², output amplitude σ = 1 pm (7.6% splitting error), insertion loss 0.5±0.37 dB per stage (Xu et al., 30 Oct 2025).
Arrayed Acoustic Waveguide Gratings (AAWG): 21-port, FSR = 81 MHz, Δf = 3.8 MHz channel spacing, Q~400, >10 dB channel isolation, for frequency demultiplexing (Xu et al., 30 Oct 2025).
Reconfigurable Frequency Synthesizer: Four-channel AAWG+MZIs, on/off ratio up to 29 dB, continuous tuning of output power ratio over 60 dB (Xu et al., 30 Oct 2025).

Scaling trends and constraints are summarized below.

Device Type	Footprint	Integration Density	Loss (α)	Channel Count
Y-splitter tree	~0.04 mm² (128)	>3,000 /cm²	2.4 dB/mm	(demonstrated 128)
AAWG	~1 mm²	~20–30 /cm²	2.4 dB/mm (GaN)	21
Ring Resonator	R = 50–115 μm	(Q ≈ 10³–10⁴, FSR 4–5 MHz)	1–5 dB/mm	—

Propagating loss varies by material and temperature: α~2.4–4 dB/mm at room temperature (GaN/Sapphire), α < 1 dB/mm achievable via fabrication optimization or at cryogenic temperatures (e.g., Q_0≈3×10⁴ at 7 K, α~1.3 dB/mm) (Xu et al., 2022). Delay lines of up to several mm (for μs delays) have been demonstrated at insertion loss extrapolated from the above α (Bicer et al., 2023).

3. Signal Processing and Hybrid Functionality

PnICs support a spectrum of classical and quantum signal processing functions:

Ultra-Compact RF Filtering and Multiplexing: Multiport splitters, high-Q resonators, and programmable filters for GHz RF signals at <0.1 mm² footprint, supporting applications such as 6G front-end integration (Bicer et al., 2021, Bicer et al., 2023).
Delay Lines and Buffers: On-chip spiral or meandered waveguides (lengths >8 mm, delays >2.5 μs) with massive time–bandwidth compression relative to electromagnetic lines (Bicer et al., 2023, Malik et al., 9 Apr 2025).
Programmable Linear Operations: MZI meshes with thermo- or piezo-acoustic phase shifters, arbitrary SU(N) operations over phonon channels, reconfigurable in real time (Xu et al., 30 Oct 2025, Taylor et al., 2021).
Brillouin and Optomechanical Coupling: Phonon-photon interaction via co-guided structures in GaN/sapphire, LiNbO₃, or Si (g₀ ≃ 120 (mm W^½)⁻¹ in GaN), enabling frequency conversion, optical signal processing, and quantum transduction (Zhang et al., 2 Mar 2025, Safavi-Naeini et al., 2018).
Hybrid Quantum Acoustodynamics (cQAD): Cohesive integration with superconducting qubits—using monolithic or flip-chip approaches—achieves strong Purcell enhancement (F_P up to 19, single-phonon emission probability ≳94%) in high-Q phononic cavities for quantum information processing (Wang et al., 4 Dec 2025).

4. Materials Platforms and Engineering Considerations

Several material stacks have been deployed, each with distinct acoustic, optomechanical, and integration properties:

Material Stack	Key Properties/Advantages	Loss (α), Q	Integration Notes
GaN/Sapphire, GaN/SiC	High index contrast, piezoelectric	2.4–4 dB/mm @RT;<1 dB/mm @7 K	No suspension, robust, SiC–HEMT integration (Xu et al., 30 Oct 2025, Bicer et al., 2021)
LiNbO₃/Sapphire	Strong piezoelectric effect, low loss	4 dB/mm @RT; 0.7 dB/mm @4 K; Q~50,000	~1 μm-wide guides, quantum-compatible (Mayor et al., 2020, Wang et al., 4 Dec 2025)
Si/Phononic Crystal	Full phononic bandgap, ultra-high Q	<0.01 dB/100 μm; Q>10⁶	Suspended, strong optomechanical coupling (Patel et al., 2017, Safavi-Naeini et al., 2018)
LiNbO₃ Thin Film (PnC)	Quasi-BIC modes with Q=6×10⁴, room-T	6×10¹³ Hz f·Q	Electrically tunable, planar fabrication, high modulation depth (Xu et al., 20 Jun 2025)

Substrate selection impacts acoustic mode confinement, loss mechanisms, piezoelectric and optomechanical coupling, and potential for hybrid classical–quantum circuits.

Key loss mechanisms—Akhiezer damping, surface roughness scattering, and thermoelastic damping—can be mitigated by material purification, atomic-layer etches, cryogenic operation, and optimized lithography (Bicer et al., 2023, Xu et al., 2022). Q factors exceeding f·Q ~ 1.5×10¹⁴ Hz are possible at cryogenic temperatures with further advances (Xu et al., 2022).

5. Quantum Information and Programmability

PnICs offer programmable control for quantum-level operations, including:

Cavity QED with Phonons: Superconducting qubits coupled to monolithic or flip-chip phononic cavities demonstrate strong Purcell factors (F_P ~14–19), ~1 MHz coupling, and single-phonon emission with >92% efficiency (Wang et al., 4 Dec 2025).
Reconfigurable Quantum Linear Networks: Piezo-acoustic phase shifters (±π-phase with 10s of μm length under ±50 V bias) and directional couplers (evanescent coupling set by waveguide spacing and length) enable arbitrary linear transformations for SU(N) quantum processors (Taylor et al., 2021, Zivari et al., 2023).
Dynamically-Switched Phononic Memory: Interferometrically-tunable coupling between high-Q localized cavity and bus waveguide allows quantum state write/read with ≳90% fidelity, exploiting time-dependent control over coupling rates via phase shifters (Taylor et al., 2021).
Elementary Quantum Logic and Routing: On-chip directional couplers, MMIs, and programmable interferometric networks for Fock-state interference and bosonic error-correcting code distribution (Zivari et al., 2023, Wang et al., 4 Dec 2025).

Quantum functionalities leverage large zero-point strains, slow group velocities, and acoustic mode confinement, with projected integration alongside superconducting electronics and photonic links (Wang et al., 4 Dec 2025).

6. Future Prospects, Challenges, and Applications

The demonstrated PnIC toolkit supports the emergence of hybrid chips where phonons, photons, and electrons are processed and interconverted on a common platform. Key application domains and forward-looking challenges include:

RF Front-End Miniaturization: Footprint reduction (>100×) versus bulk acoustic or SAW devices, direct monolithic integration with RF amplifiers and switches, and true-time-delay beamforming for communications (Bicer et al., 2021).
Programmable and Reconfigurable Signal Processing: Dynamic control of amplitude, phase, and frequency via thermoelectric or piezo-acoustic elements, integrated isolators/circulators without magnets, and arbitrary channel mapping for photonic–phononic–RF links (Xu et al., 30 Oct 2025, Shao et al., 2021, Zhang et al., 2 Mar 2025).
Quantum Networks: Phononic quantum memories, high-fidelity interconnects, deterministic single-phonon sources, and on-chip quantum logic leveraging strong-coupling, topological phononic networks, and hybrid photonic–phononic links (Wang et al., 4 Dec 2025, Taylor et al., 2021, Xu et al., 20 Jun 2025).
Material and System-Level Improvements:
- Reduction of propagation loss to ≪1 dB/mm via fabrication and material control.
- Integration of non-reciprocal and topological elements for robust routing.
- Extension of dynamic tuning bandwidth to >100 MHz or GHz-level, with advances in piezoelectric modulation (Xu et al., 30 Oct 2025).
- Hybrid wafer-scale integration combining electronics, photonics, and PnICs for versatile, scalable architectures.

PnICs thus represent a foundational technology for future classical and quantum information processing, closing the gap between photon, electron, and phonon domains with lithographically-defined, large-scale, programmable acoustic networks (Xu et al., 30 Oct 2025, Wang et al., 4 Dec 2025).