Frequency correlated photon generation at telecom band using silicon nitride ring cavities
Zhenghao Yin, Kenta Sugiura, Hideaki Takashima, Ryo Okamoto, Feng Qiu, Shiyoshi Yokoyama, Shigeki Takeuchi
FFrequency correlated photon generation attelecom band using silicon nitride ring cavities Z HENGHAO Y IN , K ENTA S UGIURA , H IDEAKI T AKASHIMA , R YO O KAMOTO , F ENG Q IU , S HIYOSHI Y OKOYAMA , AND S HIGEKI T AKEUCHI Department of Electronic Science and Engineering, Kyoto University, Kyotodaigakukatsura, Nishikyo-ku,Kyoto 615-8510, Japan Institute for Materials Chemistry and Engineering, Kyushu University, 6-1 Kasuga-koen, Kasuga-city,Fukuoka 816-8580, Japan These authors contributed equally to this work. * [email protected] Abstract:
Frequency entangled photon sources are in high demand in a variety of optical quantumtechnologies, including quantum key distribution, cluster state quantum computation and quantummetrology. In the recent decade, chip-scale entangled photon sources have been developed usingsilicon platforms, offering robustness, large scalability and CMOS technology compatibility.Here, we report the generation of frequency correlated photon pairs using a 150-GHz siliconnitride ring cavity. First, the device is characterized for studying the phase matching conditionduring spontaneous four-wave mixing. Next, we evaluate the joint spectrum intensity of thegenerated photons and confirm the photon pair generation in a total of 42 correlated frequencymode pairs, corresponding to a bandwidth of 51.25 nm. Finally, the experimental results areanalyzed and the joint spectral intensity is quantified in terms of the phase matching condition. © 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The field of quantum information processing technology using photons has been growing in lasttwo decades, including quantum cryptography [1, 2], quantum sensing [3, 4] and linear opticalquantum computation [5–7]. For these applications, entangled photon sources [8] are useful forthe generation of heralded single photons [9] and also as a resource for beating the quantum limitfor phase measurement using an optical interferometer [10, 11]. Especially, frequency entangledphotons are attracting attention for realizing high-dimensional quantum entangled states [12, 13].They can also be used for quantum optical coherence tomography (QOCT) [14, 15] and densequantum key distribution [16].The broad bandwidth of the frequency entangled photons is quite important for these ap-plications. The depth resolution of QOCT image is given by the inverse of the bandwidth.Thus the broader the bandwidth of frequency entangled photons, the better the depth resolutionof QOCT. Another example is the two photon absorption using entangled photons [17]. Thelarger bandwidth means the shorter correlation time of the bi-photons, resulting in the largerenhancement of TPA using frequency entangled photons. In addition, the number of frequencycorrelated modes can be increased using a broadband photon pair source for a given modespacing.Ultra-broad frequency entangled photons have been successfully generated using chirped quasi-phase matched devices [18, 19]. As an alternative approach, on-chip frequency-entangled photonsources realized by CMOS compatible platforms are attractive in terms of system scalabilityand initialization. Frequency entangled photon generation using silicon waveguides has beenreported. However, for two-photon absorption in silicon, the flux of generated photon pairs is a r X i v : . [ qu a n t - ph ] F e b .4 μ m μ m 1.7 μ m μ m . μ m Si N SiO (b)(a) Fig. 1. (a) Top view of the silicon nitride ring resonator used in this study. The diametercorresponds to a near-150-GHz mode spacing. (b) 1.7 µ m × µ m cross section ofring cavity, enabling anomalous dispersion behavior. limited. To overcome this limitation, on-chip ring resonators using high-index contrast dopedglass (HICDG) [12, 20–25] and silicon nitride (SiN) [26, 27] have been studied. In Ref. [22],the bandwidth of 140 nm has been observed in the single-photon spectrum and the frequencycorrelation has been measured for the bandwidth of 10 nm. However, the confirmed bandwidthof the frequency-correlated photon pair generation has, to date, been limited to 16 nm usinga SiN ring device [27] and to 23.6 nm using a HICDG ring device [25], which is the largestbandwidth to the best of our knowledge.In this paper, we report the broadband generation of photon pairs using a SiN ring resonator. Byadopting a device structure in which the material dispersion is well compensated by the structuraldispersion, we obtain very small dispersion for a broad wavelength range. From the obtained jointspectrum using a frequency-resolved coincidence measurement using superconducting nanowiresingle photon detectors, we confirm photon pair generation correlated in frequency over a 51-nmrange, which is more than two times broader than the previous record of the confirmed bandwidthof the frequency correlated photon pairs [25]. Furthermore, the observed joint-spectral intensitiesare well reproduced by the theoretical calculation.In the following, we describe the structure of the device in section 2. In section 3, acharacterization of the device using a transmission spectrum is provided. In sections 4 and 5, theexperimental setup and the results of broad-band frequency-correlated photon pair generation areexplained and discussed.
2. On-chip silicon nitride ring resonator
In a silicon nitride ring cavity pumped with a continuous wave laser, signal and idler photonpairs arise simultaneously due to the perturbation of Kerr nonlinearity. To increase the frequencycorrelation range of generated photon pairs, the phase matching condition is satisfied in abroadband form, in both the frequency domain and momentum domain [28]. This is a challengefor device fabrication since the cavity dimension affects the frequency dispersion significantly.Over the last decade, the fabrication process based on silicon nitride material with a Kerrfrequency comb and optical soliton generation has improved, especially for high quality factorring cavities [26, 29]. In our study, we customized silicon nitride ring cavities via the photonicdamascene process [30, 31] to obtain a low anomalous group velocity dispersion. To cope withboth small dispersion and the high 𝑄 factor, we made ring resonators with 5 different ring sizeswith 4 different gap distances between the ring and the bath line, and selected the ring diameter of314.90 µ m and the gap distance of 0.4 µ m . The device is illustrated in Fig. 1. The ring diametercorresponds to the 150-GHz free spectra range (FSR), which corresponds to the spacing of theneighboring frequency-correlated photon pair mode. The widths of the bus waveguide and ringwaveguide are both 1.7 µ m . The thickness of the silicon nitride layer is 800 nm, and the layer issurrounded with silicon dioxide as a low-index cladding layer. . Device characterization To study the device performance and phase matching condition, we first measure the devicetransmission using the setup shown in Fig. 2(a). A tunable semiconductor laser (TSL, santecTSL-710) is used to sweep the wavelength from 1480 nm to 1640 nm. After aligning the inputmode to launch only the fundamental TE mode using a fiber polarization controller (PC), thedevice is coupled with two lensed fibers, whose typical facet-to-facet loss is around − Q -factor of1 . × . The Q -factors of all the resonant dips within the wavelength range between 1480 nmand 1640 nm are counted and presented in Fig. 2(d). The maximum quality factor is 1 . × and the mean quality factor is 1 . × . Next, we analyzed the device dispersion of the ring resonator. Following the integrated dispersionapproach [32], we evaluated the second-order mode dispersion of the silicon nitride ring cavity.After the cavity resonant frequencies 𝜔 𝜇 are extracted from the device transmission, a Taylorseries can be used to expand the resonant frequencies with respect to the relative mode index 𝜇 ,where 𝜇 = 𝐷 int ( 𝜇 ) = 𝜔 𝜇 − ( 𝜔 + 𝜇𝐷 ) = 𝐷 𝜇 + 𝐷 𝜇 + · · · (1)where 𝐷 int is the integrated dispersion and 𝐷 is the FSR of the angular frequency. As shown inFig. 2(f), a polynomial fitting of the integrated dispersion of resonant frequency gives 𝐷 = . 𝐷 = .
37 kHz, showing that the frequency mismatch between tens of modes is lessthan the width of the cavity resonant mode (132 MHz).
4. Broadband photon pair generation
Finally, mode-resolvable photon pair generation is demonstrated using the setup shown in Fig. 3.To increase the pump power in the ring cavity, an erbium doped fiber amplifier (EDFA, AlnairLabs, HPA-200C) is employed as well as the TSL. The input polarization is aligned using apolarization controller. A bandpass filter is used to reject the sideband noise of the EDFA.The pump wavelength 𝜆 p is set to 1550.63 nm to match the cavity resonant mode. The powermeasured before the device coupling is 24.5 mW. Considering the thermal instability duringthe pump wavelength location, a simple feedback loop is added in the scheme. At the deviceoutput port, a 99:1 beam splitter is connected for fine control of the pump wavelength to maintainresonance alignment. 1% of the device output is detected with a photodiode to monitor the ratioof the transmitted power to the input power, which is around −
10 dB during our experiments. Theother 99% of the output power is filtered to reject the pump light with a fiber Bragg grating (FBG,OE Land) and then split into signal and idler channels using another 50:50 beam splitter. Tunablebandpass filters (TBPFs, WL Photonics) pass the needed signal and idler modes, with a bandwindow of 0.12 nm, much narrower than the device mode spacing. Both channels are filtered o un t s Wavelength (nm)
145 GHz 1.06pm(1.16 nm) (a)(b) (c) (d)(e)(f)
Wavelength (nm)Wavelength (nm)Frequency (THz)Frequency (THz)Trigger DAQ DUTTSL Power Ref. Trans. PCGPIB USB PM
Fig. 2. Silicon nitride cavity for generating frequency-correlated photon pairs. (a)Schematic diagram of device transmission measurement. TSL: tunable semiconductorlaser, PC: polarization controller, PM: power meter, DAQ: data acquisition device. Theorange lines are single mode single mode optical fibers and the black lines are electriclines. (b) Free spectral range of device near 1550 nm. (c) Resonance dip at 1550.64 nm,indicating a Q -factor of 1 . × . (d) Q -factors counts, up to 10 order for almostall the resonances. (e) Device transmission from 1480 nm to 1640 nm. (f) Integrateddispersion extracted from the device transmission and calculated based on the resonantwavelengths. o identify the signal or idler modes, and are finally detected with superconducting nanowiresingle photon detectors (SNSPDs, SCONTEL). Finally, a time-to-digital converter (ID Quantique,id900) records all the signal and idler counting events to allow evaluation of the photon pairgeneration rate for specific mode pairs. In order to confirm the frequency correlation for largebandwidth, we realized a stable measurement system where the wavelength of the narrow-bandCW pump laser was actively controlled by proportional-integral-differential (PID) controllerwith monitoring the output pump laser power from the device.To evaluate the photon pair generation rate, the losses of both the signal and idler channels aremeasured. The transmission of the lensed-fiber coupling is − . − . − .
5. Results and discussion
Fig. 4 shows the photon flux at cavity resonant modes, with red and blue corresponding to thesignal and idler modes respectively. Here, mode 0 is set as the pump frequency. The photon fluxof the − − . × cps. BPF
TimeController
Fig. 3. Schematic diagram of mode-resolvable photon pair generation. TSL: tunablesemiconductor laser, EDFA: erbium doped fiber amplifier, PC: polarization controller,BPF: bandpass filter, PD: photodetector, FBG: fiber Bragg grating filter, TBPF: tunablebandpass filter, SNSPD: superconducting nanowire single photon detector. The orangelines are optical fibers and the black lines are electric lines.Fig. 4. Photon flux of all the resonant modes filtered into signal and idler channels withan accumulation time of 10 s. Several modes around 1550 nm are affected by the FBGused to remove the pump light in our setup. a)(b)(c)
Fig. 5. Result of mode resolvable photon pair generation. a Measured joint spectralintensity, including 46 ×
46 mode pairs. The diagonal terms are replotted in b and arecompared with a numerical analysis result in c . The insert in b shows the coincidencecounts at the 40th and 41st mode pairs, which are lower than the maximal counting ofoff-diagonal terms. Next, the joint spectral intensity (JSI) of generated photon pairs is evaluated by coincidencecounting (CC) with a time window 𝑡 𝑐 of 1 ns and an accumulation time of 10 s for eachmode-by-mode photon count. As shown in Fig. 5(a), for the ranges from mode 1499.09 nm( 𝜇 = −
47) to 1548.32 nm ( 𝜇 = −
2) in the idler band, and from 1552.95 nm ( 𝜇 =
2) to 1605.79nm ( 𝜇 =
47) in the signal band, the JSI map covers all the 46 × = ,
116 mode pairs. To reducethe effect of accidental counting events during the measurement, the accidental coincidencecount (ACC), defined as 𝑁 s 𝑁 i 𝑡 𝑐 , is subtracted from the result. The mean ACC for all the modesis 10.5 cps, and the coincidence to accidental ratios are of diagonal terms from 2th mode to 45thmode are larger than 1. Along the diagonal direction, the maximal CC is 2 . × cps andecreases significantly as the relative mode index increases. This can be explained in terms of thedispersion of the device. As illustrated in Fig. 5(b), the highest value of the off-diagonal terms,i.e. non-phase matched modes, is 5.61 ± − 𝐶 JSI ( 𝜇 ) = 𝜂 𝜇 𝜂 − 𝜇 ∫ ∞−∞ d 𝜔 p 𝑔 ( 𝜔 p ) ∫ ∞−∞ d Ω 𝐴 𝜇 ( 𝜔 p + Ω ) 𝐴 − 𝜇 ( 𝜔 p − Ω ) sinc ( Δ 𝜙 ) (2)where 𝜂 𝜇, − 𝜇 is a constant of the cavity mode, including the detection channel loss coefficient; 𝑔 ( 𝜔 p ) is the pump light lineshape function; Ω is the frequency deviation from pump wavelength; 𝐴 𝜇 ( 𝜔 ) is the cavity transmission given by a Lorentzian function: 𝐴 𝜇 ( 𝜔 ) = √ 𝛾 𝜇 𝛾 𝜇 / − i ( 𝜔 − 𝜔 𝜇 ) (3)where 𝛾 𝜇 is the full width at half maximum (FWHM); and Δ 𝜙 = ( 𝜔 s + 𝜔 i − 𝜔 p ) 𝜏 + Γ 𝑃 p = 𝐷 int ( 𝜇 ) 𝜏 + 𝐷 int (− 𝜇 ) 𝜏 + Γ 𝑃 p (4)is the mismatch due to the frequency mismatch and power-dependent self-phase modulationduring four wave mixing phase, where 𝐷 int ( 𝜇 ) is the mode integrated dispersion, 𝜏 is the cavityround-trip time, Γ is the Kerr nonlinear coefficient and 𝑃 p is the intracavity power of the pumplight.Thus, the diagonal terms in the JSI map can be evaluated numerically by substituting theretrieved cavity resonant frequencies and linewidths into Eq. 1. The result is presented inFig. 5(c) and compared with the experimental CC result in Fig. 5(b). It can be seen that thenumerical analysis agrees well with the experimental result of CC and thus validates the phasematching theory of SFWM. As shown in Fig. 5 (b) and 5 (c), the CCs around the 20th modeare significantly low. This can be explained by the term ∫ ∞−∞ d Ω 𝐴 𝜇 ( 𝜔 p + Ω ) 𝐴 − 𝜇 ( 𝜔 p − Ω ) in Eq.(2), which relates the resonant frequencies and energy conservation of SFWM. The frequencymismatch between pump photons (2 𝜔 p ) and signal photon and idler photons ( 𝜔 𝜇 + 𝜔 − 𝜇 ) increasesaround the 20th mode.
6. Conclusion
In conclusion, we realized broadband frequency-correlated photon generation using a siliconnitride ring cavity. From the experimentally obtained JSI, we confirmed that the generated photonpairs are correlated in 37 continuous frequency mode pairs and a total of 42 mode pairs, whichcorresponds to a bandwidth of 51.25 nm in the signal band. Note that the full bandwidth of thephoton-pairs from the − unding JST-CREST (JPMJCR1674); MEXT Q-LEAP (JPMXS0118067634); JSPS-KAKENHI (26220712);Grant-in-Aid for JSPS Research Fellow (19J20968); MEXT WISE Program; Research Programfor Next Generation Young Scientists of “Five-star Alliance" in “NJRC mater. & Dev.”.
Acknowledgments
We wish to acknowledge the helpful comments provided by Bo Cao, Takayuki Kiyohara andXiaoyang Cheng.
Disclosures
The authors declare no conflicts of interest.
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