Efficient bidirectional quantum frequency conversion between telecom and visible bands using adaptively phase-matched III-V nanophotonic waveguides

Domain: neuroscience
Published: 2026-04-08
Editorial outcome: minor_revision

Abstract
Quantum frequency conversion (QFC) is essential for interfacing quantum systems operating at different wavelengths and for realizing scalable quantum networks. Despite extensive progress, achievingQFCwithsimultaneoushighefficiency,lowpumppower,minimalnoise,broadbandwidth,and pump-wavelength flexibility remains challenging. Here, we demonstrate efficient, low-noise, and bidirectional QFC between the telecom (1550-nm) and visible (780-nm) bands using unpoled indium gallium phosphide (InGaP) χ(2) nanophotonic waveguides, eliminating the need for a long-wavelength pump. Leveraging the large nonlinear susceptibility of InGaP together with adaptive phase-matchingcontrol,weobtain27%(55%)internalefficiencyina6-mm(2.5-mm)-longwaveguide with a low pump power of 20 mW (50 mW) and a noise flux spectrSal density of 10−4 counts per second per hertz at signal–pump detuning of 20 nm. These results mark a significant advance in integrated nonlinear photonics for high-performance QFC, facilitating the development of versatile and scalable quantum networks.

Introduction
Future quantum networks will rely on interconnected quantumnodescomposedofmatter-basedqubitsandop-tical fiber links that transmit quantum information via single photons. Quantum frequency conversion (QFC) enablesphotonstoshiftbetweenwavelengths—forexample, bridging the telecom and visible bands—while preserving essential quantum properties such as coherence and entanglement [1–3]. As a result, low-noise, high-efficiencyQFCisindispensableforscalablequantumnetworks and hybrid quantum systems [4–6]. QFC also facilitatessingle-photondetectioniRnotherwiseinaccessible spectral ranges, such as the infrared [7, 8], where direct detection is difficult or requires complex conditions. Beyond quantum technologies, frequency conversion finds wide-ranging applications in telecommunications, spectroscopy, and laser systems.

QFC is typically realized through nonlinear optical processes driven by ancillary pumps, including sum-frequency generation (SFG) [9–13] and four-wave mixing [14–16], using χ(2) or χ(3) nonlinearities. While manydemonstrationshavereliedonbulknonlinearcrystals [17–20] or optical fibers [21], integrated photonic platforms now offer a promising route to efficient QFC throughchip-scaledeviceswithstrongmodeconfinement [14, 22–24]. However, χ(3) nonlinearities are inherently weakanddemandhighpumppowers,whichinturngenerate excessive noise. While optical resonators reduce pump power requirements, they introduce narrow operating bandwidths, limited wavelength tunability, and light-extraction challenges that limit overall end-to-end efficiency [14–16, 22, 25–29]. QFC using χ(2) nonlinearities has relied largely on periodically poled materials to realize quasi-phase matching, but poling disorder introduces statistically white noise from spontaneous parametric down-conversion (SPDC) of the pump [30–32]. Thisnecessitateslong-wavelengthpumps[31,33]andoften multistage conversion schemes [34, 35], leading to degraded performance and increased system complexity. On the other hand, demonstrations of linear frequency conversion in unpoled χ(2) materials have been rare and, to date, limited to the classical regime with modest efficiencies [25, 26, 28]. Thus, achieving QFC with simultaneous high efficiency, low pump power, minimal added noise,broadbandwidth,andpump-wavelengthflexibility remains an open challenge.

Here, wedemonstratehigh-performanceQFCbetween the telecom (1550-nm) and visible (780-nm) bands using unpoled indium gallium phosphide (InGaP) χ(2) nanophotonic waveguides with adaptive phase-matching control, simultaneously addressing multiple critical requirements for QFC. The III-V semiconductor InGaP platform[36–42]offersalargesecond-ordernonlinearsusceptibility (χ(2) = 220 pm/V), low optical loss, and a narrow Raman spectrum [43], while employing noninvasive modal phase matching. We leverage these featurestoreducepumppowerdemands, suppressparasitic noise, and relax constraints on pump-wavelength configuration of QFC. We further implement adaptive phase-matching control to compensate thin-film nonuniformitiesandoptimizebothSFGanddifference-frequencygeneration (DFG) for bidirectional conversion. Our device achieves low pump power for high conversion efficiencies among non-resonant QFC implementations. Importantly, the on-chip noise flux spectral density remains at a low level of 10−4 counts per second per hertz (cps/Hz)—evengiventhespectralproximityofthepump and signal—preserving the coherence and entanglement of input photons in our experiment. By combining efficiency,low power,bidirectionality,andnoisesuppression in a scalable integrated photonic platform, our approach addresses a critical need for advancing the development of versatile and hybrid quantum networks.

Results
Unpoled χ(2) waveguide with adaptive phase matching

The InGaP nanophotonic waveguide is designed to support the modal-phase-matched 1550-nm-band transverse electric (TE00) mode and 780-nm-band transverse magnetic (TM00) mode (Fig. 1). QFC between these modes is realized via SFG or DFG with an ancillary pump satisfying the frequency and phase-matching conditions ω1550 + ωp = ω780 and k1550 + kp = k780, where ω and k denote the angular frequency and wavevector of the 1550-nm-band light, respectively, and similarly for the 780-nm-band light and pump. In the weak-pump limit, the simulated normalized SFG efficiency is ηSFG ≡ P1550P780 / PpL2 = 520,000%/W/cm2 for a lossless, perfectly phase-matched InGaP nanophotonic waveguide.

However, one major challenge in utilizing integrated photonic waveguides for χ(2) nonlinear processes lies in the thickness nonuniformity of the thin-film material [44, 45], which results in accumulated phase mismatch along the waveguide length and thus degraded conversion efficiency. To overcome this, we developed an in situ phase-matching tuning technique, using a nanoheater array integrated adjacent to the waveguide (Fig. 1b) (see the Supplementary Information Note 1 for device fabrication). Bypreciselycontrollingthetemperatureofindividualwaveguidesegmentswithavoltageofonlyuptoa few volts, we can counteract the effect of thickness variations and correct the phase-matching condition along the full waveguide length (Supplementary Note 4). The nanoheater array is programmable through the applied voltage to optimize the phase-matching conditions for different nonlinear processes and different wavelengths.

Foraphase-matchedwaveguide,thephoton-flux-based conversion efficiency of an SFG or DFG process is given by (Supplementary Note 2)

η = ηmax sin2(1/4 \sqrt{8ηSFG Pp} L), (1)

where

ηmax = 8ηSFG Pp e−(α+β)L/2, (2)

L is the waveguide length, P is the pump power, and α and β are the optical losses for the 1550-nm and 780-nm bands, respectively. This model assumes negligible pump loss. From these expressions, the peak conversion efficiencyscalesase−(α+β)L/2,i.e.,thegeometricaverage loss of the transmission efficiency of the two modes, whereas the pump power required to reach it scales as 1/L2 and is inversely proportional to ηSFG.

Optimized SFG and DFG

Figure 2a shows the SFG spectra of a 6-mm-long waveguide under a 1550-nm pump before and after phase-matching optimization, resulting in a near-sinc2 SFG transfer function at 1533 nm with a bandwidth of 31 GHzanda3.2–foldimprovementinconversionefficiency for the spectral peak. The tuning voltages of the array of 15 nanoheaters for the optimization are shown in the inset. The measured SFG conversion efficiency versus pump power after phase-matching optimization with a signal power Ps = 2.3 µW is displayed in Fig. 2b. The internal conversion efficiency is determined by η = PSFG / (Ps η1550 η780), where Ps (PSFG) is the off-chip signal (SFG) power and η1550 (η780) is the fiber coupling efficiency for 1550-nm (780-nm) band light. The uncertainty of the measured conversion efficiency can arise from the fiber coupling efficiency. We use adiabatically tapered fiber couplers with average efficiencies of 80% and 50% for the 1550-nm TE00 and 780-nm TM00 modes, respectively. The measured peak internal (external) conversion efficiency is 27% (11%), limited by the loss of the 780-nm TM00 mode of approximately 20 dB/cm (Supplementary Note 4), whereas the loss of the 1550-nm TE00 mode is approximately 1 dB/cm. This is consistent with the measured quality factor of 780-nm-band microring resonances (Q ∼105) [38] and a simulated group index of ng = 5.7 for the 780-nm TM00 mode (i.e., β = ngω/c). The experimentally inferred ηSFG = 70,000%/W/cm2, by fitting the conversion efficiency curve via Eq. 1, is lowerthanthetheoreticalvalue,leadingtoahigherpump power (∼ 20 mW) to reach the peak efficiency. This discrepancy arises from the residual phase mismatch at length scales shorter than those of individual nanoheaters and can be improved with more discrete nanoheaters (Supplementary Note 4).

Owing to the different waveguide losses for the 1550-nmand780-nmmodesandresidualphasemismatch,the conversion efficiency of DFG with the same pump direction and the SFG must be separated to optimize. In contrast, the conversion efficiencies of DFG and SFG with opposite pump directions are the same due to time reversal (as highlighted in Supplementary Figure 3). Fig. 2c shows the DFG spectra under 1532-nm pump before and after its phase-matching optimization for the same 6-mm waveguideandsamepumpdirectionasSFG,witha2.1–foldimprovementinconversionefficiencyforthespectral peak. Fig. 2d shows the measured DFG conversion efficiency versus pump power after optimization, where the off-chip input signal power is 24 µW. The peak efficiency and the corresponding pump power are both similar to those of the independently optimized SFG process.

To explore scaling behavior, we measured devices of different lengths. A 2.5-mm-long phase-matching-optimized waveguide exhibits approximately double the peak internal (external) conversion efficiency of the 6-mm device to 55% (22%), which is consistent with the exponential scaling with the waveguide loss (Fig. 2e), and a bandwidth of 50 GHz. The pump power (50 mW) required for peak efficiency does not follow the 1/L2 trend though, due to less phase mismatch accumulation in the short 2.5-mm device with inferred ηSFG = 175,000%/W/cm2 (Supplementary Note 6). This deviationisillustratedbythegraybandinFig.2e,whichplots pumppowerversuswaveguidelengthforarangeofηSFG corresponding to various phase mismatches. We also measuredtheon-chipnoiseinthe1550-nmbandofthe6-mm waveguide without signal injection (Supplementary Note 5), which is linear in pump power and reaches approximately 10−4 cps/Hz at a 20-mW pump power and at the wavelength separated from the pump by ±20 nm (∓2.5 THz) (Fig. 2f). The corresponding up-converted noise flux density is approximately 2 × 10−5 cps/Hz according to the calculation (Supplementary Note 5). For comparison, the signal flux spectral density of the entangled-photon source used in the up-conversion experiment is approximately 2×10−3 cps/Hz, which nevertheless is limited by the system efficiency. The linear noise is likely attributed to spontaneous Raman scatteringofthepumpinthewaveguide(seetheSupplementary Note5forthemodeling). Wenoterecentdemonstrations of QFC between 1550 nm and 780 nm wavelengths using bulk poled waveguides and a long-wavelength pump exhibit comparable or lower noise background [46–49] (10−6 − 10−5 cps/Hz), which might be due to different inherentmaterialproperties,largemodesizes,andlarger signal-pump detuning. We also observed that, at wavelengths within approximately 8 nm of the pump, the noise scales quadratically with the pump power, which originates from the degenerate spontaneous four-wave mixing of the pump (Supplementary Note 5). Moreover, the QFCwavelengthcanbetunedbyvaryingthepumpwavelength or device temperature [37], with a simulated SFG phase-matching wavelength range of > 1000 nm for the fundamental mode and > 30 nm for the SFG mode for the fabricated waveguide (Supplementary Note 3).

Single-photon up-conversion

We demonstrate that bidirectional QFC in InGaP nanophotonic waveguides preserves both quantum entanglement and single-photon coherence. First, we verify that the time–energy entanglement of photon pairs generated via SPDC is preserved after one photon is up-converted. The experimental setup is depicted in Fig.3a. AcommercialperiodicallypoledLiNbO3waveguide, pumped by a 780-nm-band continuous-wave laser, generatestime–energyentangledphotonsinthetelecommunications band. Non-degenerate signal and idler photons are spectrally selected via a tunable fiber Fabry–Perot filter with a 2.3-GHz bandwidth. The signal photon is coupled into the InGaP waveguide and up-converted to the 780-nm band via SFG. The signal photonratebeforecouplingintothedeviceisapproximately 4.3 Mcps. Theup-convertedsignalrateandup-converted noise rate in the SFG bandwidth after the device are 151 kcps and 16 kcps, respectively. We perform time-resolved two-photon interference measurements to verify the time–energy entanglement. We first confirm the entanglement of the unconverted SPDC photons via a pair of glass Mach–Zehnder interferometers (MZIs) with a path delay of 1 ns, which is much longer than the coherence time of the SPDC photons. The phase of the MZI is controlled by temperature. Fig. 3b shows the two-photon interference fringe obtained by varying the temperature of one MZI, yielding a visibility of 83%, above the Clauser–Horne limit of 1/\sqrt{2} [50]. After up-conversion, the two-photon interference is measured via one MZI for the idler and one free-space Michelson interferometer for the up-converted signal. Fig. 3c shows the resulting fringe, and time–difference histograms for two interferometer phases are plotted in Fig. 3d. The visibility remains at 76%, confirming that entanglement is preserved after QFC. The two-photon fringe visibility before and after QFC are both limited primarily by the interferometer visibility. Here the inherent time-filtering of the coincidence detection enhances the signal-to-noise ratio of the measurement, which surpasses the signal-to-noise ratio of the up-converted photon flux itself.

Single-photon down-conversion

In the down-conversion direction, we verify the preservation of single-photon coherence for a time-bin qubit. The experimental setup is depicted in Fig. 4a. A 780-nm-band time-bin qubit with a temporal delay τ = 1 ns is created by second-harmonic generation of an attenuated 1550-nm laser pulse (0.43 ns), followed by passing through a phase-tunable unbalanced Michelson interferometer. Each pulse contains approximately one photon before coupling into the device. The qubit is converted to the 1550-nm band via DFG in the InGaP waveguide. After the residual pump is filtered out, quantum state tomography is performed on the down-converted qubit (Supplementary Note 4). Figs. 4b and c show the reconstructed density matrices for the |+⟩- and |R⟩-like states, before and after conversion. Fig. 4d summarizes the fidelity F = ⟨ψi|ρ|ψi⟩ for six basis states—|e⟩, |l⟩, |+⟩, |−⟩, |L⟩, and |R⟩—with an average fidelity of F¯ = 96.2 ± 0.6%. Noise in the bandwidth of the pulse is retained in the calculation of the fidelity. The dominant contribution to infidelity arises from limited interferometer visibility and phase errors, with uncertainty contributed from phase fluctuations and photon shot noise (Supplementary Note 4).

Methods

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors study the use of InGaP waveguides for quantum frequency conversion (QFC) between ~780 nm and ~1550 nm. This configuration can be particularly challenging because in a single stage chi(2) system, the pump and input signal (in the case of upcoversion) or pump and output idler (in the case of downconversion) are very close spectrally, which introduces noise challenges. The authors claim that their platform, which does not rely on poling and the resulting broadband parametric downconversion noise it often induces due to errors, has some advantages with respect to noise. Moreover, its large nonlinearity enables conversion at low pump powers, even in waveguide geometries, which have many advantages with respect to wavelength flexibility and broadband operation.\n\nThere are many things that I like about this paper, in terms of its motivation and characterization of a relatively unstudied platform, its application to an especially challenging but important set of wavelengths for QFC, its identification of using modal phase-matching in a crystalline material as being advantageous from a noise perspective, and the segmented heater approach for overcoming the impact of thickness variation on phase matching ( though to be fair this idea was essentially shown in Ref. 40 already). I also think that there is important progress in the demonstrated results, such as the application of this platform to experiments with quantum light sources.\n\nHowever, I find that the authors have introduced some metrics and terminology that, in my opinion, make it hard for a reader to understand what they accomplished and how it compares with the rest of the field. These issues are significant enough that I believe they must be addressed before the paper can be considered for publication.\n\n1. I think the authors’ use of the ‘loss-inclusive normalized conversion efficiency’ is a problem. I understand that it provides some helpful information about what is possible if propagation losses aren’t an issue (for Figure 2) and overall length of the device, or ability to scale to longer lengths without having issues with phase-matching or loss, are not issues (Figure 5), and in that sense, there is some merit in defining this metric. But it should not replace the standard definition of conversion efficiency for QFC, which is η_int=N_(wg output)⁄N_(wg input) , where N_(wg output) is the photon flux of frequency converted photons at the waveguide output and N_(wg input) is the photon flux of input signal photons at the waveguide input. This is often referred to as an ‘internal efficiency’, while an external efficiency would include, for example, input and output coupling losses (to fiber in most cases). This is the actual conversion efficiency that is useful in experiments, whereas the definitions the authors use are more about projecting what is possible in the future, if advances are made.\n\nThe standard metric is critical in assessing the current work, by allowing a straightforward comparison to other works, and to understand when it may be already useful in experiments. A reader who is interested in the application of this work to their 780 nm source technology, for example, would have an idea of the single photon flux they have at the output of their system, and would simply want to know what the frequency-converted photon flux would be. The current version of the manuscript makes it very hard to do this simple estimate.\n\nTo summarize, I believe that the manuscript has to clearly provide the standard conversion efficiency numbers and can then go into the various other metrics that remove loss and normalize to length.\n\n2. The expression ‘loss-inclusive normalized conversion efficiency’ in the abstract is also misleading. When I first read this, I thought that maybe the authors had included all losses in the system in defining their conversion efficiency (e.g., including coupling loss on and off the chip). It actually seems to me that their conversion efficiency excludes loss and so would be better called a ‘loss-exclusive normalized conversion efficiency’. That being said, I would reiterate my comment from #1, which is that the overall conversion efficiency, including losses, needs to be stated, after which these other conversion efficiencies will make more sense.\n\n3. In my view, Figure 5 is of limited utility. By removing loss and normalizing to waveguide length, the metric is equivalent to an effective nonlinearity, which we already know from earlier works is very high in the authors’ platform. I don’t really see what new information it provides beyond that, and as written the manuscript significantly downplays the current limitations of the platform relative to established platforms, like conventional PPLN waveguides, where wall-to-wall conversion efficiencies (including all losses) of greater than 50 % have been achieved.\n\n4. I also don’t like the authors’ use of ‘programmable’ to describe their system, and don’t quite understand what they mean by saying that they can optimize phase-matching for different nonlinear processes. The nanoheaters compensate for fabrication errors, and the only two processes considered are SFG and DFG, which have to be separately optimized because of the asymmetric loss that is a deficiency of their system, not because inherently SFG and DFG are nonlinear processes that require separate optimization.\n\nI think that if the authors are able to show they can adjust their heaters to address a QFC case that is not possible in a device with perfect thickness control, there may be some merit to calling the device ‘programmable.’ As it stands, I think a better terminology would be ‘adaptive phase-matching’ or something similar, as other authors have used.\n\n5. In my view, one of the more interesting aspects of this work is the noise characterization, but much of the details are left to the supplement…I would rather that characterization be in the main text, perhaps in the place of the current Figure 5.\n\n6. The authors claim that the single-photon limit for noise is 1 cps/Hz, but this actually not low-noise…if you consider photons with a bandwidth of 1 GHz, for example (from resonator SPDC for example), this is 10^9 photons/sec, or >100 pW. This is a pretty huge noise level that would be measurable on a standard optical spectrum analyzer, which is often not the case in low-noise platforms.\n\n7. The authors’ noise result of 10-4 cps/Hz is a bit hard to assess because the noise generation process may also undergo propagation loss (similar to their conversion process). Overall it is still good enough to do the experiments in Figures 3 and 4, which is encouraging, but it would be helpful if the authors can provide additional noise information (see next questions).\n\n8. For Figures 3 and 4, it would be useful if the authors can provide some very basic information on input photon flux into their QFC device, output frequency-converted photon flux, and output noise photon flux (so that a signal-to-noise ratio can be assessed). This would help readers understand how noise might impact interference visibility, for example.\n\n9. Along the lines of 8, it’s not really clear how noise enters into their data. Changing Figure 4(d) to a plot that includes before and after QFC data, so that more quantitative comparisons than those possible from Figure 4(b), might help.\n\n10. The authors claim in the conclusion that their device simultaneously meets all critical requirements for QFC, but I don’t see how this can be justified when internal losses are so high. I think the authors have to make more nuanced claims given the current performance of their devices.", "reviewer_2": "In the manuscript entitled “Efficient bidirectional quantum frequency conversion between telecom and visible bands using programmable III-V nanophotonic waveguides”, the authors demonstrated a quantum frequency conversion process between telecom and visible bands with excellent characterizations (efficiency, noise, etc). The work is implemented on an InGaP waveguide platform with an exceptionally high second-order nonlinearity. Using multiple electrodes for thermal adjustment to achieve programmed mode phase matching control in order to compensate for the thickness inhomogeneity of the film itself, excellent nonlinear frequency conversion performance has been demonstrated. Besides, the authors also demonstrated through experiments that the quantum properties (entanglement and single-photon coherence) are preserved after quantum frequency conversion. The fabrication and the experiments in this work are well-conducted. Some innovative methods are applied in this work. I recommend this work for publication in Nature Communications, and still, I have some questions and suggestions in the following.\n\n- (1) The process of voltage allocation for programmed phase matching remains somewhat unclear. It is difficult to conceive how the optimal voltage distribution is determined simply through control-variable sweeping. Could the authors clarify the method or algorithm used to obtain the optimal voltage configuration?\n(2) Could the authors provide a direct comparison of peak conversion efficiency with and without the programmed phase-matching scheme?\n- It is well established that incorporating resonant structures (e.g., cavities) can significantly enhance conversion efficiency—both relative and absolute—far beyond the statistical results presented in Figure 5 [Sci. Adv. 11, eadu7605 (2025); Optica 7, 1654 (2020); Light Sci Appl 13, 290 (2024)]. Could the authors include a discussion comparing their waveguide-based approach with cavity-enhanced schemes in terms of metrics such as bandwidth and efficiency? Additionally, some commentary on the respective suitability of these platforms for quantum-band applications would be valuable.\n- In Figs. 2(b) and 2(d), the measured conversion efficiency decreases at higher pump powers. What is the underlying mechanism for this roll-off? Given that phase-matching can be actively maintained via electrode tuning, this roll-off is likely not caused by phase mismatch.\n- I understand the present work serves as a demonstration of QFC and a showcase of the InGaP platform's capabilities. As hinted in the introduction, could the authors expand on the potential of this scheme for enabling fully integrated, on-chip quantum applications?", "reviewer_3": "The manuscript by Hu et al. presents a new scheme for quantum frequency conversion using unpoled nanophotonic waveguides fabricated with the InGaP platform. Using local heating, the authors compensate for fluctuations of the phase matching conditions and achieve a conversion working point at quite low pump powers (20 mW). The converter preserves phase and entanglement of photons which is detected in correlation experiments.\n\nQuantum frequency conversion of single and entangled photons assumed a leading role in enabling technologies for quantum networks in the past few years. The technology is quite mature now with highest efficiencies and lowest noise counts typically achieved for sum/difference frequency mixing processes in chi^2 materials (with periodic poling and optical waveguiding for maximum conversion efficiency). A common disadvantage of this approach is the high pump power on the order of few 100 mW required to achieve the optimal working point of maximum conversion. Recent solutions to reduce the pump power are reducing the cross section of optical waveguides in materials like thin film lithium niobate (e.g. Ref. 22; Wang et al., Optica 5, 1438 (2018) – not cited here) or the nanophotonic waveguides presented here. A typical problem in thin film and/or nanophotonic waveguides is the fluctuation of film or waveguide dimensions which modify the phase matching conditions. The novel solution used here is to place nano-heating elements along the waveguide and compensate for local fluctuations by temperature-induced birefringence. Employing a large number of heaters, the overall-phase matching curve of the device can tuned and optimized.\n\nThe novel converter concept results in low pump powers on the order of 20 mW for maximum conversion efficiency which is an important result with view to chip-scale applications. The internal conversion efficiency (~25-30%) is standard for overall QFC devices but good for nanophotonic structures. The noise characteristics, in particular its presentation, requires a separate discussion (see specific comments) as it might be misleading. The original, novel and important results certainly warrant publication. However, I think the presentation of noise performance needs to be improved.\n\nSpecific comments:\n- The authors present internal conversion efficiencies throughout the paper and some information on coupling efficiencies.\nAn important number missing here is the device efficiency, including coupling losses and filtering – this should be mentioned.\n- The loss of the 780 nm TEM_00 mode is given as 20 dB/cm, limiting the internal conversion efficiency. While this might be a good number for nanophotonic waveguides, larger (micrometer-sized) waveguides in PPLN have losses as low as 0.1 dB/cm. Is there a chance to improve these losses or is there a fundamental limit given by the mode size (the field seems to leak out of the waveguide in Fig. 1c – where a scale bar is missing, btw).\n- p.3: the noise rate is given as 10^-4 cts/s/Hz. While this looks like a seemingly small number, it might be misleading: in many recent papers the noise rate is given in cts/s/GHz which would result in 10^5 cts/s/GHz for the converter presented here. This number, however, exceeds many recent realizations – I just pick a few here for conversion between 780 nm and 1550 nm wavelengths (not a full list):\n(i) Optics Express 21, pp. 19473-19487 (2013): efficiency internal (external) = 41% (25%); noise approx. = 10^4/s/GHz\n(ii) Optica Quantum 3, pp. 55-63 (2025); noise approx.= 9000 cts/s/GHz\n(iii) Nature Commun. 9, 1997 (2018) = Ref. 48; efficiency = 33%; noise approx. = 1300 cts/s/GHz\n(iv) PRX Quantum 5, 020307 (2024); efficiency = 48%; noise approx. = 4600 cts/s/GHz\nI would thus propose to compare the noise rates of the manuscript to recent results in literature to put the results achieved here into the proper context.\n- p.3, “The noise is much less than the single-photon limit of ~ 1cps/Hz”. I think this is just wrong in the way it is stated here. Detection of single photons always happens with a certain detection bandwidth – for quantum frequency conversion experiments this is typically given by the filer bandwidth (where the filter is required to suppress Raman or SPDC noise). For the 780/1550nm wavelength pair a typical application might be conversion of Rubidium D2-line photons where a minimum filter bandwidth is on the order of the photon bandwidth (~30 MHz). With the results achieved here (10^-4/s/Hz) this would result in 3000 noise counts /s. In the experiment in Sec. “Single-photon up-conversion” the generated photons have linewidths of 2.3 GHz and noise counts in such a bandwidth rise to 230.000 /s. This is far away from a “single-photon limit”.\n- Given the photon bandwidth in the experiments and the large noise counts within that spectral interval, the experiments demonstrated here only work out as they rely on coincidence measurements, effectively resulting in a time-filtering. This should be mentioned.\n- In fig. 5 the authors present a “loss-inclusive normalized conversion efficiency”. This parameter is highly unusual - as it scales inversely proportional to the square of the device length it seems to have been introduced to produce a number that moves the results presented here to the top of the graph. It is said to be loss-inclusive, but this could only enter indirectly via the dimension (for length-dependent losses) or the pump power. I would consider this parameter only useful for comparison of nano-photonic converters. For applications in quantum communication one should resort to important figures of merit, i.e. efficiency and noise."}, "author_response": "Response to the Reviewers\nWe thank the referees for their helpful comments regarding the science and presentation of\nthe material in our manuscript. In what follows the authors’ responses are in red and the\nrevised manuscript paragraphs are in blue.\nReviewer #1 (Remarks to the Author):\n[...]\nThank you for the support of our work and your helpful comments. Below we provide a\npoint-to-point response.\n1. I think the authors’ use of the ‘loss-inclusive normalized conversion efficiency’ is a\nproblem. [...]\nResponse: We appreciate the reviewer pointing this out. We acknowledge that the\nconversion efficiency is the appropriate metric for benchmarking and experimental\nplanning. Accordingly, we have discarded the \"loss-inclusive normalized efficiency\" metric\nand revised the manuscript and updated the relevant figures to reflect this change. We\nhave added the fiber-to-fiber external efficiency to the main text to offer a realistic\nassessment of the device performance. This number includes both the internal conversion\nand the fiber coupling losses. However, it excludes insertion losses from external\ncommercial components (e.g., filters and WDM), as these are part of the specific\ncharacterization setup rather than the QFC device itself.\nModification:\n• We have included the external conversion efficiency in the section \"Optimized SFG\nand DFG\": 12% for 6-mm device and 22% for 2.5-mm device.\n• We have re-plotted Fig. 5 using the peak internal conversion efficiency.\n2. The expression ‘loss-inclusive normalized conversion efficiency’ in the abstract is also\nmisleading. [...]\nResponse: We thank the reviewer for the suggestion. To avoid any confusion, we have\nremoved the term \"loss-inclusive normalized conversion efficiency\" from the manuscript.\nWe have updated the paper with external conversion efficiencies (including fiber coupling\nloss).\n3. In my view, Figure 5 is of limited utility. [...]\nResponse: We have re-plotted Figure 5 with internal conversion efficiency.\n4. I also don’t like the authors’ use of ‘programmable’ to describe their system, [...]\nResponse: We thank the reviewer for the suggestion and acknowledge that\n\"programmable\" might imply a level of functional reconfiguration beyond our primary\nscope. We have replaced this term with “adaptively phase-matched\" to more accurately\nreflect that the heaters are primarily used to compensate for fabrication errors.\nModification: We replace term “programmable” with “adaptively phase-matched”/\n“adaptive phase-matching” in our main text and SI.\n5. In my view, one of the more interesting aspects of this work is the noise characterization, [...]\nResponse: We appreciate reviewer’s suggestion. However, we feel moving the plot from SI\nto replace Figure 5 would break the flow of the paper, as noise characterization is\ndescribed in Figure 2. The additional noise characterization described in SI is about SFWM\nnoise when signal-pump detuning is less than 10 nm, which is not the case for the\nexperiments in the main text. As a result, we feel leaving it in the SI is more proper.\n6. The authors claim that the single-photon limit for noise is 1 cps/Hz, but this actually not low-noise…if you consider photons with a bandwidth of 1 GHz, for example (from resonator SPDC for example), this is 10^9 photons/sec, or >100 pW. This is a pretty huge noise level that would be measurable on a standard optical spectrum analyzer, which is often not the case in low-noise platforms.\nResponse: We thank the reviewer for this important distinction. We agree that 1 cps/Hz\nresults in a high total count rate when integrated over a broad filter bandwidth (e.g., GHz\nrange). Our point was that 1 cps/Hz represents the critical level where the noise density\nbecomes comparable to an ideal single-photon signal density. We have clarified in the\nrevised text that we use this figure strictly as a metric for ideal single photon source: a\nsingle photon pulse of duration τ and thus bandwidth B=1/ τ corresponds to 1/ τ/B=1\ncps/Hz. We acknowledge that in reality one needs to consider duty cycles and losses, as\nwell as non-ideal single photons, i.e., SPDC photons, where each pulse might contain\nmuch less than one photon in order to suppress multiphoton components. We have refined\nthe language to avoid any implication that this represents a low absolute noise floor for\nbroadband applications.\nModification: We added a sentence in page 3 which reads: “The noise flux spectral density\nis much less than the single-photon flux spectral density ~1 cps/Hz, which corresponds to\nan ideal single-photon pulse of duration τ and thus bandwidth ~1/τ.”\n7. The authors’ noise result of 10-4 cps/Hz is a bit hard to assess because the noise\ngeneration process may also undergo propagation loss (similar to their conversion\nprocess). Overall it is still good enough to do the experiments in Figures 3 and 4, which is\nencouraging, but it would be helpful if the authors can provide additional noise information\n(see next questions).\nResponse: We have now provided more information about noise in the revised manuscript.\nPlease refer to our detailed responses to the specific questions below.\n8. For Figures 3 and 4, it would be useful if the authors can provide some very basic\ninformation on input photon flux into their QFC device, output frequency-converted photon\nflux, and output noise photon flux (so that a signal-to-noise ratio can be assessed). This\nwould help readers understand how noise might impact interference visibility, for example.\nResponse: For Figure 3, we have added the detailed values for the input signal flux, output\nsignal flux, and noise floor as suggested. We have also updated the interference data to\ninclude the background noise contributions. This revision demonstrates that the\nbackground noise leads to a measurable but minor reduction in fringe visibility, helping\nreaders to directly assess the impact of noise on the interference quality. For Figure 4, we\nalso provide additional information.\nModification:\n• We added sentences in the section of “Single-photon up-conversion”: “The signal\nphoton rate before coupling into the device is approximately 4.3 Mcps. The up-\nconverted signal rate and up-converted noise rate in the SFG bandwidth after the\ndevice are 151 kcps and 16 kcps, respectively.”\n• We have also updated the interference data in Figs. 2c and 2d to include the\nbackground noise contributions and added a new section IV-C-2 in SI to explain the\nnoise background.\n• We also added in the section of “Single-photon down-conversion”: “Each pulse\ncontains approximately one photon before coupling into the device.” and “Noise in\nthe bandwidth of the pulse is retained in the calculation of the fidelity.”\n9. Along the lines of 8, it’s not really clear how noise enters into their data. Changing Figure\n4(d) to a plot that includes before and after QFC data, so that more quantitative\ncomparisons than those possible from Figure 4(b), might help.\nResponse: The fidelity of the converted state in Figure 4d is calculated with respect to the\ninitial state and includes the noise in the bandwidth of the converted pulse. The fact that\nthe fidelity is high here is because we used attenuated laser pulse which has approximately\none photon for each pulse, leading to a high signal flux of approximately 3.7x108 cps, while\nthe noise flux in the bandwidth of the pulse is approximately 7.5x103 cps.\nModification: We have added the definition of fidelity used for Figure 4d and stated “Noise\nin the bandwidth of the pulse is retained in the calculation of the fidelity.”\n10. The authors claim in the conclusion that their device simultaneously meets all critical requirements for QFC, but I don’t see how this can be justified when internal losses are so high. I think the authors have to make more nuanced claims given the current performance of their devices.\nModification: We have removed “all critical requirements” in the conclusion.\nReviewer #2 (Remarks to the Author):\n[...]\nThank you for the support of our work and your helpful comments. Below we provide a\npoint-to-point response.\n- (1) The process of voltage allocation for programmed phase matching remains somewhat\nunclear. [...]\nResponse: We clarify that the optimal voltage distribution is not determined by simple\nsweeping, but through a segment-by-segment sequential optimization process.\nSpecifically, we first apply voltage to the initial nano-heater (at the input or output end) to\nspectrally shift the phase-matching peak of the first segment, isolating it from the rest. We\nthen tune the adjacent segment to shift its phase-matching peak to align with the first one’s\nand maximize the combined SFG or DFG signal of the two segments. This procedure is\nrepeated sequentially for all segments one by one to achieve the maximum SFG or DFG\nsignal.\nModification: We have expanded the details regarding the tuning process in Section IV-A of\nthe Supplementary Information.\n(2) Could the authors provide a direct comparison of peak conversion efficiency with and\nwithout the programmed phase-matching scheme?\nResponse: We did not perform QFC for varying pump power to obtain the peak conversion\nefficiency for the phase-matching unoptimized case. However, we show the improvement\nof conversion efficiency at relatively low pump power before and after phase-matching\noptimization, corresponding to Figs. 2a and 2c. For the SFG process in Fig.2a, the peak\nefficiency obtained a 3.2-fold improvement. For the DFG process in Fig.2c, the peak\nefficiency obtained a 2.1-fold increase.\nModification: The improvement factors of the weak pump conversion efficiency\ncorresponding to Fig.2a and Fig.2c are added in section ‘Optimized SFG and DFG’.\n- It is well established that incorporating resonant structures (e.g., cavities) can\nsignificantly enhance conversion efficiency—both relative and absolute—far beyond the\nstatistical results presented in Figure 5 [...]. Could the authors include a discussion comparing\ntheir waveguide-based approach with cavity-enhanced schemes in terms of metrics such\nas bandwidth and efficiency? Additionally, some commentary on the respective suitability\nof these platforms for quantum-band applications would be valuable.\nResponse: Thank you for this suggestion. We want to note that, following other reviewers’\nsuggestions, we have re-plotted Fig. 5 using the peak internal conversion efficiency. We\nhave updated the text to compare the waveguide and cavity approaches in the introduction\nand conclusion. We explicitly mention that cavity schemes are often bandwidth-limited\n(typically hundreds of MHz to GHz), and we discuss specific examples where the\nbroadband nature of the waveguide approach is more suitable for quantum networking\ntasks. We also want to point out that it is actually challenging to realize a high end-to-end\nconversion efficiency using the cavity approach, because of the light extraction issue\nrequiring over-coupled cavities and more intense thermal-optic effect limiting the pump\npower. Below we list the internal conversion efficiency (i.e., w/o fiber coupling loss) of some\ncavity-based QFC papers we cited:\n[...]\nModification:\n• We have added sentences in the introduction and conclusion to compare the\napproaches based on cavities and waveguides. In the introduction: “While optical\nresonators reduce pump power requirements, they introduce narrow operating\nbandwidths, limited wavelength tunability, and light-extraction challenges that limit\noverall end-to-end efficiency.” In the conclusion: “In contrast to QFC in cavities,\nnon-resonant waveguides support broad bandwidths and can be continuously\ntuned over wide wavelength ranges. This broadband capability significantly\nfacilitates matching specific operating wavelengths within quantum networks.”\n• We added Ref[Wang2025] and Ref[Lu2020] as pointed out by reviewer to main text.\n- In Figs. 2(b) and 2(d), the measured conversion efficiency decreases at higher pump\npowers. What is the underlying mechanism for this roll-off? Given that phase-matching can\nbe actively maintained via electrode tuning, this roll-off is likely not caused by phase\nmismatch.\nResponse: Your understanding is correct that the roll-off is not caused by phase mismatch,\nas phase matching is actively maintained. Instead, this saturation and then decrease in\nefficiency is attributed to the oscillatory nature of the energy transfer (often referred to as\nback-conversion) inherent in the parametric optical frequency conversion process. This\nbehavior is explicitly predicted by Eq. 1, which shows that the conversion efficiency follows\na sinusoidal dependence on the pump intensity. Consequently, once the peak of the sine\nfunction is passed, further increasing the pump power leads to a reduction in efficiency.\n- I understand the present work serves as a demonstration of QFC and a showcase of the\nInGaP platform's capabilities. As hinted in the introduction, could the authors expand on\nthe potential of this scheme for enabling fully integrated, on-chip quantum applications?\nModification: We added sentences in the conclusion, which reads: “While it remains a\nsignificant challenge for currently popular nanophotonic platforms, such as silicon nitride\nand LiNbO , to integrate on-chip laser sources, the III--V semiconductor platform offers the\n3\npotential for directly integrating pump lasers with the QFC component toward a turn-key\nsystem and further the integration of quantum dot single-photon sources to serve as a\nmonolithic quantum light source in the telecom band for quantum communication.”\nReviewer #3 (Remarks to the Author):\n[...]\nThank you for the support of our work and your helpful comments. Below we provide a\npoint-to-point response.\nSpecific comments:\n- The authors present internal conversion efficiencies throughout the paper and some\ninformation on coupling efficiencies. An important number missing here is the device\nefficiency, including coupling losses and filtering – this should be mentioned.\nResponse: We have updated the manuscript to report the fiber-to-fiber device efficiency.\nBased on our measured internal efficiency of 30% for the 6-mm waveguide (55% for the\n2.5-mm waveguide) and the coupling efficiencies of our adiabatic tapered couplers (~80%\nfor 1550 nm and ~50% for 780 nm), the overall device efficiency is 12% (22%) for the 6-mm\n(2.5-mm) waveguide. We clarify that this figure represents the performance of the\npackaged device (fiber-chip-fiber). We have used WDM/filters to separate the pump and\nsignal in the experiment, but the loss of these benchtop optical components are excluded\nfrom the conversion efficiency of the QFC device as a standard practice.\nModification: We added external conversion efficiency for the two waveguides in section\n“Optimized SFG and DFG” in main text.\n- The loss of the 780 nm TEM_00 mode is given as 20 dB/cm, limiting the internal\nconversion efficiency. While this might be a good number for nanophotonic waveguides,\nlarger (micrometer-sized) waveguides in PPLN have losses as low as 0.1 dB/cm. Is there a\nchance to improve these losses or is there a fundamental limit given by the mode size (the\nfield seems to leak out of the waveguide in Fig. 1c – where a scale bar is missing, btw).\nResponse: We agree with the reviewer that mode leakage is likely to contribute to the\nobserved loss by increasing scattering at the waveguide boundaries. This is, however, an\nengineering challenge rather than a fundamental limit. Future iterations can mitigate this\nby improve surface quality via material growth and dry etch. We have corrected Fig. 1c to\ninclude the missing scale bar.\n- p.3: the noise rate is given as 10^-4 cts/s/Hz. While this looks like a seemingly small\nnumber, it might be misleading: in many recent papers the noise rate is given in cts/s/GHz\nwhich would result in 10^5 cts/s/GHz for the converter presented here. This number,\nhowever, exceeds many recent realizations – I just pick a few here for conversion between\n780 nm and 1550 nm wavelengths (not a full list):\n[...]\nResponse: We thank the reviewer for providing these valuable references and for the\ninsightful comment regarding noise units. We acknowledge that our noise level is slightly\nhigher than these demonstrations. We have incorporated these references into the revised\nmanuscript to provide a more comprehensive and nuanced comparison.\nHowever, we would like to highlight a critical factor influencing these noise levels: the\nspectral detuning between the pump and the signal. In the references provided, the pump-\nsignal separation is relatively large. For instance:\n[...]\nIn our experimental setup, the pump-signal detuning is significantly smaller (Δλ = 20 nm).\nReducing this spectral gap typically increases background noise contributions (e.g., from\nRaman scattering).\nModification: We added sentences in page 4 regarding noise performance, which reads:\n“We note recent demonstrations of QFC between 1550 nm and 780 nm wavelengths using\nbulk poled waveguides and a long-wavelength pump exhibit comparable or lower noise\nbackground [46-49] (10-6-10-5 cps/Hz), which might be due to different inherent material\nproperties, large mode sizes, and larger signal-pump detuning.”\n- p.3, “The noise is much less than the single-photon limit of ~ 1cps/Hz”. I think this is just\nwrong in the way it is stated here. [...]\nResponse: We acknowledge that the term \"single-photon limit\" could be confusing and\nmight mistakenly imply a \"low-noise standard\" for broadband detection, which was not our\nintent. Our point was that 1 cps/Hz represents the critical level where the noise density\nbecomes comparable to an ideal single-photon signal density. We have clarified in the\nrevised text that we use this figure strictly as a metric for ideal single photon source: a\nsingle photon pulse of duration τ and thus bandwidth B=1/ τ corresponds to 1/ τ/B=1\ncps/Hz. We acknowledge that in reality one needs to consider duty cycles and losses, as\nwell as non-ideal single photons, i.e., SPDC photons, where each pulse might contain\nmuch less than one photon in order to suppress multiphoton components, or when the\nsingle photon bandwidth is smaller than the filter bandwidth. We have refined the language\nto avoid any implication that this represents a low absolute noise floor for broadband\napplications.\nModification: We have removed the term \"single-photon limit\". We used a more accurate\ndescriptor “single-photon flux spectral density” and added a concise explanation to clarify\nits definition in page 3, which reads: “The noise flux spectral density is much less than the\nsingle-photon flux spectral density ~1 cps/Hz, which corresponds to an ideal single-photon\npulse of duration τ and thus bandwidth ~1/τ.”\n- Given the photon bandwidth in the experiments and the large noise counts within that\nspectral interval, the experiments demonstrated here only work out as they rely on\ncoincidence measurements, effectively resulting in a time-filtering. This should be\nmentioned.\nResponse: We appreciate the reviewer's comment. However, we think this should be case-\ndependent. As we pointed out in the comment above, the single-photon flux spectral\ndensity, 1 cps/Hz, represents the critical level where the noise density becomes\ncomparable to a single-photon signal density. It is independent of bandwidth and\nintegration time, and can be used as an absolute benchmark. As the reviewer pointed out,\nin the experiment in Sec. “Single-photon up-conversion”, the generated photons have\nlinewidths of 2.3 GHz and noise counts in such a bandwidth rise to 2.3*105 /s. But a single\nphoton signal in the same bandwidth can have counts up to 2.3*109 /s, which is much\nlarger than the noise counts. We acknowledge that in cases where the single-photon signal\nhas a much smaller bandwidth than the filter bandwidth or the single-photon signal\ncontains much less than one photon per pulse, the noise counts could exceed the signal\ncounts, but this becomes a case-by-case study. Depending on signal photon flux and\nsignal/filter bandwidth, measurement may or may not require time-filtering. For Fig.3, we\nacknowledge that the time-filtering of the coincidence counts indeed enhances the signal-\nto-noise ratio.\nModification: We added a sentence in the section of “Single-photon up-conversion”, which\nreads: “Here the inherent time-filtering of the coincidence detection enhances the signal-\nto-noise ratio of the measurement, which surpasses the signal-to-noise ratio of the up-\nconverted photon flux itself.”\n- In fig. 5 the authors present a “loss-inclusive normalized conversion efficiency”. This\nparameter is highly unusual - as it scales inversely proportional to the square of the device\nlength it seems to have been introduced to produce a number that moves the results\npresented here to the top of the graph. It is said to be loss-inclusive, but this could only\nenter indirectly via the dimension (for length-dependent losses) or the pump power. I would\nconsider this parameter only useful for comparison of nano-photonic converters. For\napplications in quantum communication one should resort to important figures of merit,\ni.e. efficiency and noise.\nResponse: We thank the reviewer for the suggestion. We have re-plotted Figure 5 with\nstandard internal conversion efficiency."}, {"round": 2, "reviewer_comments": {"reviewer_1": "In my first review, I acknowledged the advances and technical quality of this work but expressed concerns about the use of non-standard metrics that obscured high loss numbers, had some questions about noise levels reported, and a few other suggestions.\n\nI find that the authors have largely addressed my questions in their revisions, and I think the paper far clearer. I am basically ready to recommend acceptance, but I have a few additional points for them to consider (I think points #4 and #5 are especially important):\n\n1. I think the authors should include their internal efficiency numbers in the abstract, as quoting pump powers and noise fluxes without efficiency really lacks context. They could even consider quoting efficiency, pump power, and noise for both their 2.5 mm and 6 mm long waveguides, since they present an interesting trade-off.\n2. I think the authors should define cps in the abstract (or just write it out), since it isn’t an especially widely used unit\n3. What is the gray band in Figure 2e? Is this uncertainty associated with the theoretical predictions, and if so, what factors into this uncertainty?\n4. Equation 2 sets a limit on conversion efficiency based on the average loss in the two bands. Since the authors quote 20 dB/cm loss in the 780 nm band and 1 dB/cm loss in the 1550 nm band, I would assume that for a 6 mm long waveguide, the maximum internal efficiency would be (-10.5 dB/cm)*(0.6 cm) = -6.3 dB = 23 %. However, the authors quote 30 %, which is substantially different. Can they comment on this discrepancy? In addition, the external efficiency should be reduced by the insertion losses of 80 % and 50 %, which would suggest an external efficiency of 9.4 % (which differs from the 12 % they quote).\n5. I may have missed it, but I don’t see a place where the authors have explained how they measured their conversion efficiencies experimentally, and what kind of uncertainties there are on them.\n6. Figure 2b suggest a peak internal conversion efficiency of 27 % which exceeds the 23 % mentioned above. Can they comment on this? My main concern is if the increased conversion efficiency could be due to noise generation.\n7. In the conclusion, the authors claim that pump integration is easier in their platform than with silicon nitride and lithium niobate; I think some nuance in the description is needed. There are already heterogeneously and hybrid integrated lasers with nonlinear photonic devices in both of those platforms, and even work on heteroepitaxial growth on silicon substrates, etc. Of course, III-v platforms offer more straightforward integration (in principle), but in practice it seems a bit misleading to claim that as a major limitation of those platforms.\n8. Similarly, the authors comment on various deficiencies associated with resonant QFC systems, but of course resonant geometries are used in many contexts in integrated photonics and with simple thermal heaters that are widely available, they can be used for a range of wavelengths (and have been used in QFC experiments already). Anyway, I think the authors might consider a somewhat more nuanced comparison here.", "reviewer_2": "Thanks for the authors’ response. They have addressed all my concerns and added some discussions. I believe the outcome of this work indeed demonstrates the potential of a newly developed integrated photonic material platform in QFC. I have no other comments. I recommend the paper for publication.", "reviewer_3": "I have read the authors' rebuttal and the revised manuscript and find that most reviewer remarks and comments were taken into accouunt and that the clarity of the presentation improved considerably. However, there is one point left which two reviewers raised and which seems still confusing, i.e. \"the single photon flux spectral density\". This is introduced as \"a metric for ideal single photon source: a single photon pulse of duration τ and thus bandwidth B=1/ τ corresponds to 1/ τ/B=1 cps/Hz.\" I don't think this makes sense as is, not without giving a repetetion rate or a signal-to-background ratio. Let's assume a photon duration of tau = 1 ns and a corresponding bandwidth of 1 GHz. If one photon is emitted in a 1s time interval, then at a noise flux of 2x10^-5 cps/Hz this results in 2*10^4 noise photons in the spectral bandwidth of 1 GHz or a SBR 1:2*10^4. If, pushing to the limits, 10^9 photons are emitted per second, the SBR would be 10^5:1. In real experiments, photon count rates are typically way smaller than the theoretical limit (finite losses and efficiencies). Thus, alongside the noise flux, the single photon rate in the experiment, or equivalently, the SBR should be quoted."}, "author_response": "Response to the Reviewers\nWe thank the referees for their helpful comments regarding the science and presentation of\nthe material in our manuscript. In what follows the authors’ responses are in red and the\nrevised manuscript paragraphs are in blue.\nReviewer #1 (Remarks to the Author):\n[...]\nThank you for the support of our work and your helpful comments. Below we provide a\npoint-to-point response.\n1. I think the authors should include their internal efficiency numbers in the abstract, as\nquoting pump powers and noise fluxes without efficiency really lacks context. They could\neven consider quoting efficiency, pump power, and noise for both their 2.5 mm and 6 mm\nlong waveguides, since they present an interesting trade-off.\nModification: We have added internal conversion efficiency in the abstract.\n2. I think the authors should define cps in the abstract (or just write it out), since it isn’t an\nespecially widely used unit\nModification: We have added the definition of cps in the abstract.\n3. What is the gray band in Figure 2e? Is this uncertainty associated with the theoretical\npredictions, and if so, what factors into this uncertainty?\nResponse: The gray band indicates the uncertainty in estimation of pump power\ncorresponding to the peak efficiency. Specifically, it plots the theoretical pump power\nrequired for peak efficiency as a function of waveguide length, bounded by varying degrees\nof phase mismatch. The primary factor contributing to this variance is the different phase-\nmatching quality in waveguides of different lengths. The 2.5-mm waveguide suffers from\nless phase mismatch. Therefore, the data point for the 2.5-mm waveguide falls on the\nlower edge of the theoretical band (i.e., requiring less power). The 6-mm waveguide, which\naccumulates more phase mismatch, lies on the upper edge.\nModification: We have added a brief explanation of gray band in section ‘Optimized SFG\nand DFG’.\n4. Equation 2 sets a limit on conversion efficiency based on the average loss in the two\nbands. Since the authors quote 20 dB/cm loss in the 780 nm band and 1 dB/cm loss in the\n1550 nm band, I would assume that for a 6 mm long waveguide, the maximum internal\nefficiency would be (-10.5 dB/cm)*(0.6 cm) = -6.3 dB = 23 %. However, the authors quote\n30 %, which is substantially different. Can they comment on this discrepancy? In addition,\nthe external efficiency should be reduced by the insertion losses of 80 % and 50 %, which\nwould suggest an external efficiency of 9.4 % (which differs from the 12 % they quote).\nResponse: The reviewer’s calculation is correct based on the nominal losses provided.\nHowever, we would like to clarify that the 20 dB/cm loss for the 780-nm TM mode is an\n00\nestimation based on the typical quality factor of ring resonators. Furthermore, the 80% and\n50% coupling efficiencies represent average values across multiple measurements.\nBecause the actual loss of the specific waveguides might be lower than 20 dB/cm and the\ncoupling efficiency during the measurement might be different from the average, the\nexperimentally measured maximum efficiency can exceed the theoretical limit derived\nfrom these averaged parameters.\nModification: We have quoted more precise efficiency of 27% for the 6-mm-long waveguide\nand specified the 780 loss is approximately 20 dB/cm and the average coupling efficiencies\nare 80% and 50%.\n5. I may have missed it, but I don’t see a place where the authors have explained how they\nmeasured their conversion efficiencies experimentally, and what kind of uncertainties there are on them.\nResponse: We appreciate the reviewer's attention to this detail. We have revised the\nmanuscript to explicitly describe how the conversion efficiencies were experimentally\ndetermined. Furthermore, we clarify that the main uncertainty in these measurements\narise from variations in the input/output coupling efficiency.\nModification: We added description of efficiency measurement and uncertainty of\nmeasurements in section ‘Optimized SFG and DFG’.\n6. Figure 2b suggest a peak internal conversion efficiency of 27 % which exceeds the 23 % mentioned above. Can they comment on this? My main concern is if the increased conversion efficiency could be due to noise generation.\nResponse: As we explained in our responses to Comments 4 and 5, the 23% figure is a\ntheoretical estimation based on nominal parameters. The measured 27% efficiency results\nfrom the deviation of coupling efficiency and propagation loss from nominal values. The\ndifference is not due to noise generation, because for the efficiency measurement we used\na signal power (a few μW) much higher than those used for the quantum measurements in\nFigs. 3 and 4 and as a result the noise is negligible comparing to the signal.\nModification: We provided the signal power for SFG and DFG efficiency measurements in\nsection ‘Optimized SFG and DFG’.\n7. In the conclusion, the authors claim that pump integration is easier in their platform than with silicon nitride and lithium niobate; I think some nuance in the description is needed. There are already heterogeneously and hybrid integrated lasers with nonlinear photonic devices in both of those platforms, and even work on heteroepitaxial growth on silicon substrates, etc. Of course, III-v platforms offer more straightforward integration (in principle), but in practice it seems a bit misleading to claim that as a major limitation of those platforms.\nResponse: Thanks for your suggestion. We added the information of integrated laser on\nsilicon and lithium niobate platform and modify our statement in the conclusion.\nModification: We added this information in the conclusion.\n8. Similarly, the authors comment on various deficiencies associated with resonant QFC systems, but of course resonant geometries are used in many contexts in integrated photonics and with simple thermal heaters that are widely available, they can be used for a range of wavelengths (and have been used in QFC experiments already). Anyway, I think the authors might consider a somewhat more nuanced comparison here.\nResponse: We thank the reviewer for pointing this out. We added this information in the\nconclusion.\nModification: We have added ‘thermal tuning of cavity’ in the comparison of the\nconclusion.\nReviewer #2 (Remarks to the Author):\nThanks for the authors’ response. They have addressed all my concerns and added some\ndiscussions. I believe the outcome of this work indeed demonstrates the potential of a\nnewly developed integrated photonic material platform in QFC. I have no other comments.\nI recommend the paper for publication.\nThank you for the support of our work.\nReviewer #3 (Remarks to the Author):\nI have read the authors' rebuttal and the revised manuscript and find that most reviewer\nremarks and comments were taken into account and that the clarity of the presentation\nimproved considerably. However, there is one point left which two reviewers raised and\nwhich seems still confusing, i.e. \"the single photon flux spectral density\". This is introduced\nas \"a metric for ideal single photon source: a single photon pulse of duration τ and thus\nbandwidth B=1/ τ corresponds to 1/ τ/B=1 cps/Hz.\" I don't think this makes sense as is, not\nwithout giving a repetition rate or a signal-to-background ratio. Let's assume a photon\nduration of tau = 1 ns and a corresponding bandwidth of 1 GHz. If one photon is emitted in\na 1s time interval, then at a noise flux of 2x10^-5 cps/Hz this results in 2*10^4 noise\nphotons in the spectral bandwidth of 1 GHz or a SBR 1:2*10^4. If, pushing to the limits,\n10^9 photons are emitted per second, the SBR would be 10^5:1. In real experiments,\nphoton count rates are typically way smaller than the theoretical limit (finite losses and\nefficiencies). Thus, alongside the noise flux, the single photon rate in the experiment, or\nequivalently, the SBR should be quoted.\nThank you for the support of our work and your helpful comments.\nResponse: We sincerely thank the reviewer for this rigorous physical insight. We fully agree\nthat quoting the \"1 cps/Hz\" theoretical limit is practically meaningless without accounting\nfor the repetition rate, and we acknowledge that the Signal-to-Background Ratio (SBR) is\nindeed the essential metric for evaluating device performance. While our absolute noise\nfloor may not be the lowest reported, it is sufficiently low to enable high-fidelity single-\nphoton conversion, as demonstrated by our experimental results. To eliminate any\nremaining confusion and align with your recommendation, we have removed the\ntheoretical \"1 cps/Hz\" reference from the manuscript. Instead, we now explicitly compare\nthe noise flux to the real single-photon rate used in our experiment to provide a practical\nbenchmark.\nModification: We have deleted the statement regarding the theoretical 1 cps/Hz limit.\nInstead, we have added a direct comparison between the measured noise flux and the\nactual single-photon rate used in our experiment in page 4, which is 2x10-3 cps/Hz."}]

Decision letter