Blue to Near-IR Integrated PZT Silicon Nitride Modulators for Quantum and Atomic Applications

Abstract: Modulation and control of lasers and optical signals is necessary for trapped-ion and cold neutral atom quantum systems. Given the diversity of atomic species, experimental modalities, and architectures, integrated optical modulators designed to operate across the visible to near-infrared spectrum are a key step towards portable, robust, and compact quantum computers, clocks, and sensors. Integrated optical modulators that are wavelength-independent, CMOS-compatible, and capable of maintaining low waveguide losses and a high resonator quality factor, DC-coupled broadband frequency response, and low power consumption, are essential for scalable photonic integration. Yet progress towards these goals has remained limited. Here we demonstrate four types of integrated stress-optic lead zirconate titanate (PZT) silicon nitride modulators: a coil Mach-Zehnder modulator, a coil pure phase modulator, and bus-coupled and add-drop ring resonator modulators, with operation from 493 nm to 780 nm. The coil MZM operates at 532 nm with a V$π$ of 2.8V, a 0.4 MHz 3-dB bandwidth, and an extinction ratio of 21.5dB. The coil phase modulator operates at 493 nm with a V$π$ of 2.8V and low residual amplitude modulation of -34 dB at a 1kHz offset. The bus-coupled ring resonator modulator operates at 493 nm and the add-drop ring resonator modulator operates at 780 nm. The ring-based modulators have an intrinsic quality factor of 3.4 million and 1.9 million, a linear tuning strength of 0.9 GHz/V and 1 GHz/V, and a 3-dB bandwidth of 2.6 MHz and 10 MHz, respectively. All four modulator designs maintain the low optical waveguide loss of SiN, are DC coupled with broadband frequency response, operate independent of wavelength, and consume only tens of nW per actuator. Such solutions unlock the potential for further integration with other precision SiN components to realize chip-scale atomic and quantum systems.

• The paper demonstrates integrated PZT-on-SiN modulators with low Vπ (2.8 V) and high extinction ratios, enabling efficient optical control from blue to NIR wavelengths.
• The design leverages the stress-optic effect in CMOS-compatible SiN waveguides to achieve ultra-low-loss, high-Q performance and broadband DC-coupled modulation.
• Experimental results reveal energy-efficient operation (~20 nW per actuator) and modulation bandwidths up to 10 MHz, ideal for quantum computing and atomic clocks.

Integrated PZT-on-SiN Modulators for Visible to Near-IR Quantum and Atomic Systems

Overview

This work presents the development and characterization of integrated lead zirconate titanate (PZT) actuated silicon nitride (Si$_3$N$_4$) modulators specifically designed for blue to near-infrared (NIR) operation, addressing key requirements for quantum computing, atomic clocks, and quantum sensing. The authors demonstrate four modulator architectures (a Mach-Zehnder modulator, a pure phase modulator, a bus-coupled ring resonator, and an add-drop ring resonator) operational at wavelengths spanning 493 nm, 532 nm, and 780 nm. The technical approach leverages the stress-optic effect in PZT deposited atop ultra-low-loss Si$_3$N$_4$ waveguides, yielding devices that are simultaneously high-Q, low-loss, DC-coupled, wavelength-independent, and with extremely low power consumption.

Technical Contributions

Platform and Device Physics

The work utilizes a CMOS-compatible Si$_3$N$_4$ integration platform, known for low propagation loss across the visible-NIR regime, and monolithically integrates PZT actuators for electrical control. The PZT creates lateral strain in the waveguide under applied voltage, modifying refractive index via the stress-optic effect. This approach preserves native waveguide losses and allows for backend post-processing without the necessity for complicated "pull-back" structures typical in platforms like AlN.

Device cross-sections use thin (20–120 nm) and wide (900 nm–2 µm) Si$_3$N$_4$ waveguides, with the PZT layer and electrodes offset by a controlled gap to maximize modulation efficiency while minimizing optical mode overlap. The measured leakage current per actuator remains under 1 nA, corresponding to power consumption per actuator in the $\sim$20 nW range.

Modulation Schemes and Metrics

Four variants of modulators are fabricated and tested:

• Coil Mach-Zehnder Modulator (MZM): Operates at 532 nm, 5 cm PZT arm, $V_\pi = 2.8$ V, 3-dB bandwidth 0.4 MHz, extinction ratio 21.5 dB, optical loss 0.24 dB/cm, and power dissipation $\sim$5 nW. The performance is capacitance-limited due to the actuator's geometry.
• Coil Phase Modulator: Pure phase device at 493 nm, with $V_\pi = 2.8$ V, 180° phase lag at 1 MHz, and measured residual amplitude modulation (RAM) of $-34$ dB at 1 kHz, a property critical for high-fidelity laser locking and gyroscope applications.
• Bus-Coupled Ring Resonator: At 493 nm, 750 µm radius, $Q_i = 3.4$ million, tuning strength $0.9$ GHz/V, ER 18.7 dB, 3-dB modulation bandwidth up to 2.6 MHz. The device is suitable for fast gating and agile frequency modulation in atomic systems.
• Add-Drop Ring Resonator: At 780 nm, slightly undercoupled, $Q_i = 1.9$ million, tuning strength $1.01$ GHz/V, ER 12.1 dB, and 3-dB modulation bandwidth of 10 MHz, relevant for Rb D2 cooling transition in atomic clocks.

All devices are DC-coupled with broadband response and operate nearly independent of wavelength, a crucial factor for addressing multi-species quantum architectures.

Notable numerical results include:

• $V_\pi$ as low as 2.8 V.
• Quality factors up to 3.4 million at 493 nm.
• On-off extinction ratios up to 21.5 dB.
• Modulation bandwidths up to 10 MHz.
• Actuator power consumption consistently in the tens of nanowatts.

The authors assert that their PZT stress-optic approach provides tuning strengths "orders of magnitude stronger" than prior stress-optic actuators, while maintaining ultra-low waveguide losses and high-Q resonator operation – a combination not previously demonstrated in the visible-NIR regime.

Implications

Practical Relevance

These modulators are directly applicable to ion and neutral atom quantum computation platforms, precision atomic clocks, and quantum sensors, where robust, scalable, and low-power optical control is essential. The demonstrated compatibility with both visible and NIR transitions (i.e., Ba$^+$ at 493 nm, Rb at 780 nm) makes the devices immediately relevant for multiple atomic species. The performance specifications—particularly the low $V_\pi$, high ER, high Q, broadband DC-coupled operation, and extreme energy efficiency—address major integration barriers for chip-scale atomic and quantum systems.

Potential for integration with other Si$_3$N$_4$ components (e.g., narrow linewidth lasers, Brillouin lasers, precision beam delivery) enables complete systems-on-chip, a longstanding objective for deployable quantum and atomic technologies.

Theoretical and Methodological Impact

The approach of integrating backend PZT actuators on Si$_3$N$_4$ platforms extends the functional range of silicon photonics into the visible regime without compromising low-loss performance or Q, which is essential for resonantly enhanced functionalities. The stress-optic mechanism retains active control without introducing significant optical dissipation or residual cross-talk (RAM), which has typically been a limitation in, for example, lithium niobate or AlN-based platforms.

The achieved $Q_i$ values in visible-range ring modulators, together with high ER and fast tuning, open prospects for exploring new regimes in precision laser stabilization, cavity-enhanced quantum operations, and fast qubit manipulation protocols. The ultra-low energy dissipation per actuator also aligns with scaling requirements in multi-qubit (and multi-species) quantum processors and sensor networks.

Future Developments

Ongoing reductions in Si$_3$N$_4$ waveguide loss (down to 0.034 dB/m as reported elsewhere by some of these authors) and optimization of PZT strain engineering suggest that sub-1 V $V_\pi$ operation is feasible. Such improvements will further minimize thermal loads and control circuitry overhead, facilitating large-scale integration.

Broader impacts may include:

• Chip-scale optical lattice/trap arrays with integrated control.
• Fully integrated atomic clocks and inertial sensors.
• Photonic subsystems for non-Abelian anyon braiding and other topologically protected quantum operations.
• On-chip frequency combs and microresonator-based quantum light sources benefiting from integrated, tunable modulators.

Further integration with slow-tuning elements and MEMS for wavelength stabilization, along with advances in backend-foundry compatibility, will drive industrial adoption and enable deployment in non-laboratory environments.

Conclusion

This work demonstrates a suite of high-performance, energy-efficient PZT-on-SiN modulators that address key operational wavelengths and requirements for quantum and atomic systems in the visible to NIR range. By maintaining ultra-low waveguide loss, high-Q operation, broadband DC-coupled modulation, and low RAM across multiple device types, this technology establishes a robust foundation for further photonic integration in chip-scale quantum platforms and precision atomic instruments. The results suggest a near-term pathway toward scalable, deployable quantum-photonic systems with integrated modulation, control, and laser stabilization.