⚛️ quantum physics

Periodically poled thin-film lithium niobate ring Mach Zehnder coupling interferometer as an efficient quantum source of light

This paper proposes a symmetric ring-Mach-Zehnder interferometer utilizing a periodically poled lithium niobate waveguide that, compatible with current fabrication technology, efficiently generates tunable squeezed light exceeding -12 dB and high-purity single photons with near-unity heralding efficiency using sub-milli-watt pump powers.

Original authors: Mrinmoy Kundu, Bejoy Sikder, Heqing Huang, Mark Earnshaw, A. Sayem

Published 2026-09-29

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CC BY 4.0

Original authors: Mrinmoy Kundu, Bejoy Sikder, Heqing Huang, Mark Earnshaw, A. Sayem

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the quest to build a new kind of computer that operates on the laws of quantum mechanics, scientists are turning to light. Unlike traditional computers that use electrical switches to process information, these future machines rely on individual particles of light, called photons, to carry out calculations and transmit data. For this vision to become reality, engineers need two very specific types of light sources. The first is a machine that can spit out single photons on demand, one by one, with perfect timing and no extra noise. The second is a source that produces "squeezed" light, a special state where the uncertainty of the light's properties is reduced below the normal limit, a feature essential for a different branch of quantum computing and ultra-sensitive sensing. The challenge has been creating a single device that can do both of these things efficiently, using low power, and in a format small enough to fit on a computer chip.

A team of researchers at Nokia Bell Labs has proposed a design that meets these demanding requirements by combining two classic optical concepts into a single, tunable structure. They describe a device built from a thin film of lithium niobate, a crystal known for its ability to manipulate light, which is shaped into a ring resonator. This ring is connected to a Mach-Zehnder interferometer, a setup that splits light into two paths and then recombines them. The key innovation lies in how the researchers use heat to control the device. By placing tiny heaters on the chip, they can precisely adjust the temperature of the light paths. This thermal tuning allows them to change how the light enters and leaves the ring without needing to physically alter the chip's structure. This flexibility is crucial because it lets them optimize the device for two very different tasks: generating squeezed light and creating single photons.

When the researchers simulated the behavior of this device, they found it could act as a highly efficient engine for generating squeezed light. By pumping the device with a laser beam at a specific wavelength, they showed that the ring could produce a state of light where the noise in one property is suppressed. Their calculations indicate that with a very low amount of pump power, less than one milliwatt, the device could achieve a squeezing level of minus 12 decibels. This is a significant result because previous attempts to create such light on a chip often required much higher power or resulted in lower quality. Furthermore, the design allows them to tune the device to reach even higher levels of squeezing, up to minus 19 decibels, if they are willing to use slightly more power. The ability to do this with such low energy consumption is vital, as it means the device could potentially be powered by small, on-chip lasers rather than bulky external equipment.

The same device also proved capable of generating single photons, which are the building blocks for linear optical quantum computing. To do this, the researchers simulated a process where a photon from the pump beam splits into a pair of photons inside the ring. One of these photons acts as a signal, while the other serves as a herald, announcing that the signal photon has been created. The team found that by carefully adjusting the temperature of the interferometer, they could ensure that the heralding efficiency—the probability that a heralded photon actually exists—reached 94 to 99 percent. This high efficiency is necessary to ensure that the quantum computer does not waste time waiting for photons that never arrive. However, a common problem in such systems is that the photons are often "impure," meaning they are entangled with other properties in a way that ruins their usefulness for complex calculations.

To solve the purity problem, the researchers proposed a clever modification to the way they pump the device. Instead of using a single, continuous pulse of light, they simulated the use of two short pulses separated by a tiny fraction of a second. This dual-pulse approach allowed them to break the unwanted connections between the photons, resulting in a purity of up to 99 percent while maintaining the high heralding efficiency. While this method initially appeared to reduce the rate at which photons were generated, the researchers noted that this loss could be easily recovered by simply increasing the peak power of the pump pulses. The simulations suggest that with a standard repetition rate and pulse duration, a modest increase in power could restore the generation rate to levels comparable to less pure methods.

The proposed design relies entirely on materials and fabrication techniques that are already available today. The use of thin-film lithium niobate and standard thermal heaters means that this device could be manufactured using existing industrial processes, making it a practical step forward rather than a theoretical curiosity. By demonstrating that a single, tunable ring resonator can produce both high-quality squeezed light and pure single photons, the researchers have offered a generalized solution for the two most critical needs in photonic quantum computing. Their work suggests that the path to scalable quantum computers may not require entirely new materials or impossible engineering feats, but rather the intelligent and precise control of light within a chip that is already within reach.

Technical Summary: Periodically Poled Thin-Film Lithium Niobate Ring Mach-Zehnder Coupling Interferometer as an Efficient Quantum Source of Light

Problem Statement
Integrated photonic technologies are central to quantum computation and communication, yet generating high-efficiency single photons with high purity and heralding efficiency (for linear optical quantum computing) and high-efficiency, scalable squeezed light (for continuous variable quantum computing) remains challenging. While $\chi^{(2)}$ materials like thin-film lithium niobate (TFLN) offer strong nonlinearity and electro-optic capabilities, existing on-chip squeezed light sources often suffer from low squeezing levels or require ultra-fast pulses outside the telecommunication band, complicating detection. Furthermore, achieving simultaneous high purity and high heralding efficiency for single-photon sources typically demands stringent fabrication tolerances or complex femtosecond pumping schemes. There is a need for a device that can actively tune coupling conditions to satisfy doubly resonant requirements for both squeezed light generation and high-purity single-photon production without relying on extreme fabrication precision.

Methodology
The authors propose a symmetric Ring-Mach-Zehnder Interferometer (RMZI) coupled with a periodically poled lithium niobate (PPLN) waveguide. The device architecture consists of a racetrack resonator where one arm contains the PPLN section to facilitate quasi-phase-matching (QPM) between the fundamental harmonic (FH, $\sim$ 1550 nm) and second harmonic (SH, $\sim$ 775 nm) modes. The ring is coupled to a Mach-Zehnder Interferometer (MZI) modulator.

Key operational mechanisms include:

Thermo-Optic Tuning: Resistive heaters are placed on the MZI arms and the ring waveguide. The MZI is configured in a push-pull arrangement to induce opposite phase changes, allowing for the tuning of the transmission spectrum's amplitude without shifting the resonance wavelength of the ring (for the telecom mode).
Dual-Tuning Control: Two independent temperature knobs ( $\Delta T_{MZI}$ and $\Delta T_{ring}$ ) are used. $\Delta T_{ring}$ aligns the resonant wavelengths of the pump and signal modes to satisfy the doubly resonant condition. $\Delta T_{MZI}$ controls the coupling ratio (over-coupling vs. critical coupling) for the signal and pump modes.
Pumping Schemes: The study numerically simulates two regimes:
1. Squeezed Light Generation: Using a continuous-wave pump in the sub-threshold regime.
2. Single-Photon Generation: Utilizing a dual-pulse pump scheme (characterized by pulse splitting ratio $r_p$ and temporal separation $\Delta \tau$ ) to broaden the effective in-resonator pump spectrum, aiming to decouple the trade-off between spectral purity and heralding efficiency.

Key Contributions and Results
The paper presents numerical simulations demonstrating that the proposed RMZI-PPLN device can simultaneously optimize conditions for two distinct quantum applications:

Efficient Squeezed Light Generation:
- By achieving a highly over-coupled condition for the signal mode ( $\eta_a \approx 0.94$ ) and a critically coupled condition for the pump mode ( $\eta_b = 0.5$ ), the device generates tunable squeezed light.
- Performance: The simulation predicts a squeezing level higher than −12 dB with a sub-milliwatt pump power of approximately 0.61 mW (610 $\mu$ W) and a bandwidth of 1.5 GHz.
- Scalability: The authors note that by increasing the pump power to 15 mW, squeezing levels could potentially reach −19 dB with a bandwidth of 7.86 GHz, though this requires higher power.
- Noise Suppression: The design inherently suppresses sideband noise because neighboring modes exhibit significantly higher threshold powers than the resonant mode.
High-Purity Single-Photon Generation:
- The device is shown to generate single-photon pairs using 20 ps pulses with a purity as high as 99% (95%) and a heralding efficiency of 94% (99%), depending on the specific operating point.
- Dual-Pulse Advantage: The authors demonstrate that using a dual-pulse pump scheme allows the system to achieve near-unity spectral purity while maintaining high heralding efficiency. This overcomes the limitations of single-pulse schemes where purity is bounded by the resonance linewidth.
- Trade-off Management: While the dual-pulse scheme reduces the peak pump intensity and generation rate compared to single-pulse schemes (by roughly three orders of magnitude in the normalized simulation), the authors argue this can be compensated by increasing the input peak pump energy (e.g., using table-top or on-chip mode-locked lasers).

Significance and Claims
The paper claims that the proposed RMZI-PPLN design offers a generalized solution for generating both high-efficiency squeezed light and high-purity single photons on a single chip. The primary significance lies in the ability to actively tune the coupling conditions for both the telecom and near-IR modes using thermal heaters, rather than relying on fixed, stringent fabrication parameters.

The authors state that this device serves as a fundamental building block for both Measurement-Based Quantum Computing (MBQC) and Fusion-Based Quantum Computing (FBQC), as well as Continuous Variable Quantum Computing (CVQC). The design is highlighted as being fully compatible with current fabrication technology, utilizing standard TFLN processing and thermal tuning mechanisms. The work suggests that such a device could enable the realization of error-corrected quantum computation and sensing applications by providing the necessary high-purity, high-efficiency quantum light sources at low pump power thresholds.

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