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Kaikai Liu

Publications and source records attributed to Kaikai Liu.

At least 19 recordsLinked to original sources
arXiv2026

Single- and Two-Qubit Gates Driven by an Integrated Photonic Laser

Trapped ions are a leading technology for quantum computing, but their reliance on bespoke tabletop laser and optical systems remains a major obstacle to scaling and robustness. Integrated silicon nitride lasers, compatible with future monolithic integration with surface electrode ion traps, have recently demonstrated frequency-selective qubit state preparation and measurement as well as interrogation of an optical clock transition. However, coherent qubit gates driven by integrated laser sources have not yet been demonstrated because coherent quantum logic imposes substantially more stringent performance requirements on the laser. Here, we use a visible-wavelength integrated Brillouin laser stabilized to an integrated coil resonator to drive coherent single- and two-qubit gates with $^{88}$Sr$^+$ optical qubits. We measure an average single-qubit fidelity of 99.61% $\pm$ 0.03% per Clifford gate with randomized benchmarking and use a two-qubit Mølmer-Sørensen interaction to generate an entangled Bell state with a fidelity of 92.35% $\pm$ 1.50%. The qubit exhibits a bare Ramsey coherence time of 660 $\pm$ 9 $μ$s, extended to 1.750 $\pm$ 0.033 ms by spin echo. These results demonstrate that integrated visible-wavelength narrow-linewidth photonic lasers can meet the phase-noise requirements for coherent trapped ion quantum logic, providing a path for scalable optical systems integrated within trapped ion quantum processors.

quant-ph↗
arXiv2026

MoRAL: Sensor-Grounded BEV Reasoning for Compact VLMs toward Edge-Oriented Autonomous Driving

Deploying vision-language models (VLMs) for safety-critical spatial reasoning on resource-constrained autonomous driving platforms requires both compact model size and reliable metric grounding. We present MoRAL (Multimodal Reasoning for Autonomous Language Models), a two-stage fine-tuning pipeline that teaches Cosmos-Reason2-2B to first read a physics-encoded Bird's Eye View (BEV) representation and then reason over it for driving decisions. The BEV image encodes LiDAR metric distance as color bands, object class as cluster morphology, and radar Doppler velocity as directional wedge overlays, externalizing spatial perception into the input image so that no learned 3D backbone is required at inference. Stage 1 fine-tunes the vision encoder on 60,000 grounding records; zero-shot baselines produce no parseable BEV outputs, confirming the vocabulary requires explicit training. Stage 2 fine-tunes the full model (52M parameters, 2.4% of total) on 57,696 chain-of-thought records generated by Cosmos-Reason2-8B as teacher, spanning eight driving question types. On 2,304 held-out nuScenes frames evaluated by Gemma 4 (31B) calibrated against human review, MoRAL wins seven of eight question types over a zero-shot 8B baseline despite using four times fewer parameters, with the largest margins on question types requiring structured multi-step physics reasoning. Emergency braking recall improves from 10.8% to 47.8%, output degeneration falls from 94.1% to 20.8%, and the full pipeline fits a consumer 8 GB GPU at 42 tok/s without quantization. These results establish a reproducible foundation for compact, physics-grounded VLM reasoning on mobile edge platforms.

cs.CV↗
arXiv2026

Heterogeneously Integrated Balanced Photodetector on an Ultra-Low Loss Silicon Nitride Delay Line Interferometer

Thin core silicon nitride photonics enables ultra-low loss, CMOS foundry compatible integration that supports wavelengths from the visible to shortwave infrared. Applications that can benefit from the resulting lower cost, improved robustness, and portability include quantum sensing and computing, ultra-low noise microwave generation, optical clocks, optical gyros, coherent fiber communications, and fiber sensing. An important next step is integration of functional circuits and systems on chip with heterogeneous integration of active components such as high-performance photodetection. Yet to date integrated high-performance photodetectors on the thin film silicon nitride platform has remained elusive. In this work, we demonstrate heterogeneous integration of an InGaAs on InP substrate Modified Uni-Traveling Carrier balanced photodetector with a 15-meter-long unbalanced thin core silicon nitride Mach-Zehnder Interferometer with a bandwidth of 0.92 GHz and a responsivity of 0.305 A/W at 1550 nm with a propagation loss as low as 2.5 dB/m at 1600 nm. Using this circuit we demonstrate two functions, a meter-scale differential interferometer laser stabilization circuit achieving a nearly 23 dB noise suppression at 1 kHz offset and an optical frequency discriminator frequency noise measurement with high sensitivity across 6 orders of magnitude from 10 Hz to 10 MHz. These results demonstrate that the high performance of thin core silicon nitride devices can be combined with integrated high-performance photodetection to realize on-chip stabilized lasers and circuits and pave the path towards full systems on chip.

physics.optics↗
arXiv2026

Versatile CMOS modulation-free self-isolating stabilized precision lasers on a chip

Ultra-low-noise stabilized lasers are a fundamental tool for precision quantum technologies, optical clocks, microwave and millimeter-wave generation, and fiber sensing. Existing systems rely on table-top bulk-optic components -- discrete lasers, reference cavities, isolators, modulators and frequency shifters -- limiting portability, scalability, and manufacturability. While these systems offer flexibility in laser design to tailor linewidth, frequency noise, and wavelength to specific applications, fully integrating a stabilized laser onto a chip without sacrificing performance and versatility has remained elusive. Here, we report integration of the precision stabilized laser in the low-loss silicon nitride photonic platform, combining a flexible isolator-free core laser design with a modulation-free stabilization cavity. We demonstrate a stabilized widely tunable self-isolating extended cavity tunable laser monolithically integrated with an on-chip coil-loaded Mach-Zehnder interferometer (CL-MZI). This design yields a fundamental linewidth of 1.7 - 10.5 Hz across a 60 nm tuning range, integral linewidth of 299 - 505 Hz over a 30 nm tuning range, frequency noise reduction of over 5 orders of magnitude, and an Allan-Deviation (ADEV) of 6.5x10-13 at 0.08 ms. We next highlight the versatility of this approach by demonstrating a self-isolating stimulated Brillouin scattering (SBS) laser, that provides nonlinear noise suppression of high frequency noise by multiple orders of magnitude, stabilized to an on-chip CL-MZI. The stabilized SBS laser achieves 4 Hz fundamental linewidth, 74 Hz integral linewidth, and ADEV of 2.8x10-13 at 5 ms. These results bring the performance and versatility of table-top stabilized laser systems to a chip for the first time, providing a path to scalable, low-cost, and manufacturable precision lasers for portable quantum, sensing, and communications applications.

physics.optics↗
arXiv2026

Self-aligned optical microcomb emerging between octave separated lasers

Optical frequency combs (OFCs) are frequency rulers essential for precision metrology, next generation navigation, and testing of fundamental physics. Despite intense efforts, chip-integrated OFCs remain laboratory-bound, unable to fulfill their promise of compact and cost-effective deployment. While improvement in fabrication and integration are important, a conceptual limitation has fundamentally stymied progress: on-chip OFC architectures have aimed to miniaturize their table-top counterparts and relied on cascading outward from (i.e., spectrally broadening) a single pump. In integrated platforms, this approach does not readily allow for the generation of strong and low-noise octave-spaced signals that are crucially needed for robust zero-frequency offset detection. Here, we overcome this limitation via an architectural inversion where an optical microcomb forms by filling the spectrum between two octave-separated pump lasers. The two pumps generate a parametrically driven cavity soliton (PDCS) in an integrated $χ^{(3)}$ resonator, which robustly self-aligns to (i.e., synchronizes with) the pump lasers across multiple foundry-fabricated devices and operating configurations. This produces a single octave-spanning comb extending from telecom to visible wavelengths, whose zero-frequency offset is completely defined by the two harmonically-related pump lasers, and can therefore be reliably detected and stabilized. We showcase our platform's capabilities by executing all of the three core tasks of OFC metrology: optical frequency synthesis, low-noise millimeter-wave generation, and integrated optical clock readout, using the same self-aligned microcomb with only its input locks changed.

physics.optics↗
arXiv2025

Octave Spanning Visible to SWIR Integrated Coil-Stabilized Brillouin Lasers

Narrow linewidth stabilized lasers are central to precision applications that operate across the visible to short-wave infrared wavelengths, including optical clocks, quantum sensing and computing, ultra-low noise microwave generation, and fiber sensing. Today, these spectrally pure sources are realized using multiple external cavity tabletop lasers locked to bulk-optic free-space reference cavities. Integration of this technology will enable portable precision applications with improved reliability and robustness. Here, we report wavelength-flexible design and operation, over more than an octave span, of an integrated coil-resonator-stabilized Brillouin laser architecture. Leveraging a versatile two-stage noise reduction approach, we achieve low linewidths and high stability with chip-scale laser designs based on the ultra-low-loss, CMOS-compatible silicon nitride platform. We report operation at 674 and 698 nm for applications to strontium neutral and trapped-ion clocks, quantum sensing and computing, and at 1550 nm for applications to fiber sensing and ultra-low phase noise microwave generation. Over this range we demonstrate frequency noise reduction from 1 Hz to 10 MHz resulting in 1.0 Hz -17 Hz fundamental and 181 Hz - 630 Hz integral linewidths and an Allan deviation of 6.5 x 10-13 at 1 ms for 674 nm, 6.0 x 10-13 at 15 ms for 698 nm, and 2.6 x10-13 at 15 ms for 1550 nm. This represents the lowest achieved linewidths and highest stability for integrated stabilized Brillouin lasers over an order of magnitude improvement in operating wavelength range. These results unlock the potential of integrated, ultra-low-phase-noise stabilized lasers for precision applications and further integration in systems-on-chip solutions.

physics.optics↗
arXiv2025

Multi-laser stabilization with an atomic-disciplined photonic integrated resonator

Precision atomic and quantum experiments rely on ultra-stable narrow linewidth lasers constructed using table-top ultra-low expansion reference cavities. These experiments often require multiple lasers, operating at different wavelengths, to perform key steps used in state preparation and measurement required in quantum sensing and computing. This is traditionally achieved by disciplining a cavity-stabilized laser to a key atomic transition and then transferring the transition linewidth and stability to other lasers using the same reference cavity in combination with bulk-optic frequency shifting such as acousto-optic modulators. Transitioning such capabilities to a low cost photonic-integrated platform will enable a wide range of portable, low power, scalable quantum experiments and applications. Yet, today's bulk optic approaches pose challenges related to lack of cavity tunability, large free spectral range, and limited photonic integration potential. Here, we address these challenges with demonstration of an agile photonic-integrated 780 nm ultra-high-Q tunable silicon nitride reference cavity that performs multiple critical experimental steps including laser linewidth narrowing, high resolution rubidium spectroscopy, dual-stage stabilization to a rubidium transition, and stability transfer to other lasers. We achieve up to 20 dB of frequency noise reduction at 10 kHz offset, precision spectroscopy over a 250 MHz range, and dual-stage locking to rubidium with an Allan deviation of $8.5 \times 10^{-12}$ at 1 s and up to 40 dB reduction at 100 Hz. We further demonstrate the transfer of this atomic stability to a second laser, via the rubidium-disciplined cavity, and demonstrate multi-wavelength Rydberg electrometry quantum sensing. These results pave the path for integrated, compact, and scalable solutions for quantum sensing, computing and other atomic and trapped ion applications.

physics.optics↗
arXiv2025

Photonic chip-based optical frequency division with PZT-integrated soliton microcombs

Optical frequency division (OFD) produces low-noise microwave and millimeter-wave signals by transferring the exceptional stability of optical references to electronic frequency domains. Recent developments in integrated optical references and soliton microcombs have paved the way for miniaturizing OFD oscillators to chip scale. Critical to this realization is a rapid tunable frequency comb that is stabilized to the optical references, thereby coherently linking optical and electronic frequencies. In this work, we advance the on-chip OFD technology using an integrated high-speed PZT stress-optic actuator on the SiN soliton microcomb resonator. The integrated PZT actuator tunes the resonance frequency of the soliton-generating microresonator with a bandwidth exceeding 10s MHz and independently adjusts the soliton repetition rate without perturbing the frequency comb offset. Optical frequency division and low-noise mmWave generation are demonstrated by feedback control of the soliton repetition rate through the integrated PZT-actuator, and the soliton microcomb is stabilized to a pair of reference lasers that are locked to an integrated 4-meter SiN coil reference cavity. Our approach provides a fast, versatile and integrated control mechanism for OFD oscillators and their applications in advanced communications, sensing, and precise timing.

physics.optics↗
arXiv2025

Integrated Architecture for the Automated Generation and Coil Stabilization of a PZT-Enabled Microcomb

Silicon nitride Dissipative Kerr Soliton (DKS) microcombs have emerged as a future solution to bring metrological optical frequency comb capabilities into a photonic integrated platform with mass-scale fabrication benefits. Precision applications demand low comb line phase noise as well as high repetition rate stability, but current approaches to achieve this involve complex architectures, multiple lasers, and high-power components, which are challenging to integrate to the chip scale. To achieve this goal, new architectures are needed to simplify the comb generation, actuation, and pump laser requirements, while enabling chip-integrated solutions. Here we demonstrate a greatly simplified stabilized DKS comb architecture with a single laser and a single point electronic control of both the microcomb generation and its stabilization to a coil-resonator reference. The silicon nitride microcomb is integrated with a low power, broadband PZT actuator that is driven by a simple electronic control sequence that generates a soliton and stabilizes it to the 16-meter silicon nitride coil resonator. PZT-enabled control brings flexibility and simplicity to the soliton generation and stabilization using a single CW pump laser, resulting in significantly reduced electronic and optical infrastructure. We demonstrate coil-resonator locking which suppresses the 1 kHz frequency noise by 40 dB over the 35 nm wide comb spectrum, with comb line linewidths as low as 66 Hz and 108 GHz soliton repetition rate phase noise equivalent to -118 dBc/Hz when divided down to 10 GHz. The low power PZT actuator consumes nW bias power and the coil resonator allows flexible dual locking using arbitrary comb lines. These results show a clear path towards full chip integration of stabilized soliton microcombs with simplicity and versatility absent in other schemes.

physics.optics↗
arXiv2025

Hybrid integrated ultra-low linewidth coil stabilized isolator-free widely tunable external cavity laser

Precision applications including quantum computing and sensing, mmWave/RF generation, and metrology, demand widely tunable, ultra-low phase noise lasers. Today, these experiments employ table-scale systems with bulk-optics and isolators to achieve requisite noise, stability, and tunability. Photonic integration will enable scalable, reliable and portable solutions. Here we report a hybrid-integrated external cavity widely tunable laser stabilized to a 10-meter-long integrated coil-resonator, achieving record-low 3 - 7 Hz fundamental linewidth across a 60 nm tuning range and 27 - 60 Hz integral linewidth with 1.8E-13 Allan deviation at 6.4 ms across 40 nm, delivering orders of magnitude frequency noise and integral linewidth reduction over state of the art. Stabilization is achieved without an optical isolator, leveraging resilience to optical feedback of 30 dB beyond that of commercial DFB lasers. The laser and reference cavity are fabricated in the same Si3N4 CMOS-compatible process, unlocking a path towards fully integrated visible to ShortWave-IR frequency stabilized lasers.

physics.optics↗
arXiv2025

Large mode volume integrated Brillouin lasers for scalable ultra-Low linewidth and high power

Generating ultra-low linewidths and high output power in an integrated single mode laser remains a critical challenge for future compact, portable, precision applications. Moreso, achieving these characteristics in a laser design that enables scaling to lower linewidths and higher power, and implementation in a wafer-scale integration platform that can operate from visible to near-IR and be integrated with other components. Such an advance could impact a wide array of applications including atomic and quantum sensing and computing, metrology, coherent fiber communications and sensing, and ultra-low-noise mmWave and RF generation. Yet, achieving these goals in an integrated laser has remained elusive. Here, we report a class of integrated laser that overcomes these limitations, with demonstration of a 31 mHz instantaneous linewidth, the lowest linewidth to date to the best of our knowledge, with 41 mW output power and 73 dB sidemode suppression ratio and can be tuned across a 22.5 nm range. This performance is possible due to Brillouin nonlinear dynamics that occurs within a large mode volume, nonlinear photon-phonon, MHz-scale-FSR, ultra-low loss silicon nitride resonator cavity. This laser design can scale to a new operating regime of mHz fundamental linewidth and Watt class lasers. Such lasers hold promise to unlock new sensitivity and fidelity for precision quantum experiments, portable precision applications, and atomic, molecular, and optical physics.

physics.optics↗
arXiv2024

Trapped ion qubit and clock operations with a visible wavelength photonic coil resonator stabilized integrated Brillouin laser

Integrating precise, stable, ultra-low noise visible light lasers into atomic systems is critical for advancing quantum information sciences and improving scalability and portability. Trapped ions are a leading approach for high-fidelity quantum computing, high-accuracy optical clocks, and precision quantum sensors. However, current ion-based systems rely on bulky, lab-scale precision lasers and optical stabilization cavities for optical clock and qubit operations, constraining the size, weight, scalability, and portability of atomic systems. Chip-scale integration of ultra-low noise lasers and reference cavities operating directly at optical clock transitions and capable of qubit and clock operations will represent a major transformation in atom and trapped ion-based quantum technologies. However, this goal has remained elusive. Here we report the first demonstration of chip-scale optical clock and qubit operations on a trapped ion using a photonic integrated direct-drive visible wavelength Brillouin laser stabilized to an integrated 3-meter coil-resonator reference cavity and the optical clock transition of a $^{88}$Sr$^+$ ion trapped on a surface electrode chip. We also demonstrate for the first time, to the best of our knowledge, trapped-ion spectroscopy and qubit operations such as Rabi oscillations and high fidelity (99%) qubit state preparation and measurement (SPAM) using direct drive integrated photonic technologies without bulk optic stabilization cavities or second harmonic generation. Our chip-scale stabilized Brillouin laser exhibits a 6 kHz linewidth with the 0.4 Hz quadrupole transition of $^{88}$Sr$^+$ and a self-consistent coherence time of 60 $μ$s via Ramsey interferometry on the trapped ion qubit. Furthermore, we demonstrate the stability of the locked Brillouin laser to 5$\times10^{-13}/ \sqrtτ$ at 1 second using dual optical clocks.

quant-ph↗
arXiv2024

Kerr optical frequency division with integrated photonics for stable microwave and mmWave generation

Optical frequency division (OFD) has revolutionized microwave and mmWave generation and set spectral purity records owing to its unique capability to transfer high fractional stability from optical to electronic frequencies. Recently, rapid developments in integrated optical reference cavities and microresonator-based optical frequency combs (microcombs) have created a path to transform OFD technology to chip scale. Here, we demonstrate an ultra-low phase noise mmWave oscillator by leveraging integrated photonic components and Kerr optical frequency division. The oscillator derives its stability from an integrated CMOS-compatible SiN coil cavity, and the optical frequency division is achieved spontaneously through Kerr interaction between the injected reference lasers and soliton microcombs in the integrated SiN microresonator. Besides achieving record-low phase noise for integrated mmWave oscillators, our demonstration greatly simplifies the implementation of integrated OFD oscillators and could be useful in applications of Radar, spectroscopy, and astronomy.

physics.optics↗
arXiv2024

Anneal-free ultra-low loss silicon nitride integrated photonics

Heterogeneous and monolithic integration of the versatile low loss silicon nitride platform with low temperature materials such as silicon electronics and photonics, III-V compound semiconductors, lithium niobate, organics, and glasses, has been inhibited by the need for high temperature annealing as well as the need for different process flows for thin and thick waveguides. New techniques are needed to maintain the state-of-the-art losses, nonlinear properties, and CMOS compatible processes while enabling this next generation of 3D silicon nitride integration. We report a significant advance in silicon nitride integrated photonics, demonstrating the lowest losses to date for an anneal-free process at a maximum temperature of 250 C, with the same deuterated silane based fabrication flow, for nitride and oxide, for an order of magnitude range in nitride thickness without requiring stress mitigation or polishing. We report record low losses for anneal-free nitride core and oxide cladding, enabling 1.77 dB/m loss and 14.9 million Q for 80 nm nitride core waveguides, more than half an order magnitude lower loss than previously reported 270 C processes, and 8.66 dB/m loss and 4.03 million Q for 800 nm thick nitride. We demonstrate laser stabilization with over 4 orders of magnitude frequency noise reduction using a thin nitride reference cavity. And using a thick nitride micro-resonator, we demonstrate parametric gain and Optical Parametric Oscillation (OPO) with the lowest reported OPO threshold per unit resonator length for low temperature fabricated nitride, and supercontinuum generation over two octaves. These results represent a significant step towards a uniform ultra-low loss silicon nitride homogeneous and heterogeneous platform for both thin and thick waveguides capable of linear and nonlinear photonic circuits and integration with low temperature materials and processes.

physics.optics↗
arXiv2023

Chip-Scale, Sub-Hz Fundamental Sub-kHz Integral Linewidth 780 nm Laser through Self-Injection-Locking a Fabry-Pérot laser to an Ultra-High Q Integrated Resonator

Today's state of the art precision experiments in quantum, gravimetry, navigation, time keeping, and fundamental science have strict requirements on the level and spectral distribution of laser frequency noise. For example, the laser interaction with atoms and qubits requires ultra-low frequency noise at multiple offset frequencies due to hyperfine atomic transitions, motional sidebands, and fast pulse sequencing. Chip-scale integration of lasers that meet these requirements is essential for reliability, low-cost, and weight. Here, we demonstrate a significant advancement in atomic precision light sources by realizing a chip-scale, low-cost, 780 nm laser for rubidium atom applications with record-low 640 mHz (white noise floor at 0.2 Hz$^2$/Hz) fundamental and 732 Hz integral linewidths and a frequency noise that is multiple orders of magnitude lower than previous hybrid and heterogeneous self-injection locked 780 nm lasers and lower noise than bulk microresonator implementations. The laser is a Fabry-Pérot laser diode self-injection locked to an ultra-high Q photonic integrated silicon nitride resonator. This performance is enabled by a 145 million resonator Q with a 30 dB extinction ratio, the highest Q at 780 nm, to the best of our knowledge. We analyze the impact of our frequency noise on specific atomic applications including atomic frequency references, Rydberg quantum gates, and cold atom gravimeters. The photonic integrated resonator is fabricated using a CMOS foundry-compatible, wafer-scale process, with demonstrated integration of other components showing promise for a full system-on-a-chip. This performance is scalable to other visible atomic wavelengths, opening the door to a variety of transitions across many atomic species and enabling low-power, compact, ultra-low noise lasers impacting applications including quantum sensing, computing, clocks and more.

physics.optics↗
arXiv2023

Integrated optical frequency division for stable microwave and mmWave generation

The generation of ultra-low noise microwave and mmWave in miniaturized, chip-based platforms can transform communication, radar, and sensing systems. Optical frequency division that leverages optical references and optical frequency combs has emerged as a powerful technique to generate microwaves with superior spectral purity than any other approaches. We demonstrate a miniaturized optical frequency division system that can potentially transfer the approach to a CMOS-compatible integrated photonic platform. Phase stability is provided by a large-mode-volume, planar-waveguide-based optical reference coil cavity and is divided down from optical to mmWave frequency by using soliton microcombs generated in a waveguide-coupled microresonator. Besides achieving record-low phase noise for integrated photonic microwave/mmWave oscillators, these devices can be heterogeneously integrated with semiconductor lasers, amplifiers, and photodiodes, holding the potential of large-volume, low-cost manufacturing for fundamental and mass-market applications.

physics.optics↗