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Photonicchipbased dense entanglement distribution
PhotoniX volume 4, Article number: 12 (2023)
Abstract
The dense quantum entanglement distribution is the basis for practical quantum communication, quantum networks and distributed quantum computation. To make entanglement distribution processes stable enough for practical and largescale applications, it is necessary to perform them with the integrated pattern. Here, we first integrate a dense wavelengthdivision demultiplexing system and unbalanced MachZehnder interferometers on one largescale photonic chip and demonstrate the multichannel wavelength multiplexing entanglement distribution among distributed photonic chips. Specifically, we use one chip as a sender to produce highperformance and wideband quantum photon pairs, which are then sent to two receiver chips through 1km standard optical fibers. The receiver chip includes a dense wavelengthdivision demultiplexing system and unbalanced MachZehnder interferometers and realizes multiwavelengthchannel energytime entanglement generation and analysis. High quantum interference visibilities prove the effectiveness of the multichip system. Our work paves the way for practical entanglementbased quantum key distribution and quantum networks.
Introduction
Multiuser entanglement distribution and analysis at different locations are practical requirements for quantum networks and distributed quantum computation. The systems often involve numerous independent building blocks, such as beam splitters (BSs), modulators and delay lines [1, 2], which always need a large space and bear mechanical and thermal disturbances [3, 4]. Compared to freespace optics and fiber optics, photonic integrated circuits (PICs) are promising platforms to overcome these difficulties due to their small footprint, scalability, programmability, and stability [5,6,7,8,9]. PICs provide opportunities to demonstrate largescale quantum information processing that contains numerous optical components with millimeter scale, that is, quantum photonic integrated circuits (QPICs) [10, 11]. For example, multidimensional quantum entanglement [12], quantum teleportation [13] and various quantum algorithms [14,15,16,17] have been demonstrated on QPICs.
In quantum networks, to enhance the transmission capacity, make full use of wideband entangled photons [18,19,20,21] and increase the number of users, dense wavelengthdivision multiplexing (DWDM) techniques are often used for quantum communication and play a key role in many quantum network architectures [1, 22, 23]. Besides, compared to polarization and frequency entanglement, entanglement with time basis is insensitive to polarization and frequency dispersion in fiber transmission and can be maintained in fiber networks, and thus is widely used. For example, combining unbalanced MachZehnder interferometers (UMZIs) with DWDM components, energytime entanglement states are used for longdistance quantum entanglement distribution among multiple users [24,25,26]. To improve the compactness and practicability of the system, key components need to be integrated monolithically. In fact, we have seen many attempts in recent years. For example, UMZIs are integrated on the silicon nitride (SiN) platform to generate time bins and analyze entanglement [27, 28]. As a main choice for integrated DWDM systems [29], arrayed waveguide gratings (AWGs) have been integrated on silica [30, 31], silicononinsulator (SOI) [32,33,34], SiN [35], thinfilm lithium niobate on insulator (LNOI) [36] and polymer platforms [37]. However, these works are subject to limited scale, and in particular, the two most critical components—DWDMs and UMZIs have not been completely integrated on one single chip.
In this work, we realize the integration of a 32channel highperformance AWG and 32 UMZIs on a single chip based on a highindex doped silica (HIDS) platform for largescale quantum networks. Furthermore, we achieve the multiwavelength energytime entanglement distribution among three chips. As shown in Fig. 1, one fiberpackaged silicon nanowire works as the sender to generate wideband quantum photon pairs, and the other two receivers are integrated on the HIDS platform for the preparation and analysis of the energytime entanglement. On the receiver chips, multiple wavelength channels support simultaneous operations by different users. Even if the chips are separated with 1kilomiterlong fibers, the high quantum interference visibility is achieved and proves the effectiveness of the multichip system. The system will play an important role in dense quantum communication and building up practical quantum networks.
Results
Details of the photonic chips
The quantum photonpair source serves as the sender. It includes one SOI helical waveguide with a cross section of 220 nm × 500 nm. Wideband correlated photons are generated via the spontaneous fourwave mixing (SFWM) process in the waveguide [38]. The HIDS chips serve as the receivers. The 1 × 32 AWG, 32 balanced MZIs and 32 UMZIs are integrated on a single chip (shown in Fig. 2(a)). The waveguide has a crosssection of 2 × 2 μm to support single mode propagation. The waveguide transmission loss is very low, ~ 0.05 dB/cm, which is ideal for onchip longtime delay. Two arms of the UMZI have a relative time delay of 400 ps (unbalanced length is ~ 7.38 cm, which introduce an extra loss of ~ 0.37 dB), which is used to distinguish the photon coincidence counting peaks. A balanced MZI is employed in the short arm of the UMZI for energy compensation through modulation of the onchip platinum microheater (not shown in Fig. 1(a)). The phase differences of the two arms of UMZIs are tuned using microheaters covering the delay spiral waveguides. The size of the onchip integrated 1 × 32 AWG is 6.2 mm × 8.0 mm (Fig. 2(b)). The 3dB passband width of the AWG channel is approximately 0.3 nm, and the insert loss variation is less than 4 dB for all channels. The crosstalk among all channels is less than − 30 dB. The wavelength and channel spacing of the AWG meet the standard international telecommunications union (ITU) grids. All the input and output waveguides are arranged in a column with a pitch of 127 μm coupled by a 128channel fiber array. The coupling loss is less than 1.5 dB/facet. We select 17 channels, and their transmission spectra are shown in Fig. 2(d).
Entanglement distribution of one sender and one receiver
Firstly, we distribute the multiwavelength correlated photons to one receiver chip. As shown in Fig. 3(a), a continuouswave laser (TSL550, Santec) centered at 1537.40 nm (center of the C50 channel of ITU grids) is used as the pump (23 mW off chip) to generate photon pairs through the SFWM process in an SOI waveguide on Chip I. We use one PC before chip I to vary the polarization of the pump to guarantee maximum photonpair generation. A prefilter of 100 GHz is utilized to eliminate noise, mainly the broadband background fluorescence of the amplified spontaneous emission effect in the pump laser. A post filter of 200 GHz is used to reject the residual pump and protect the single photon detectors. Both filters exceed 100 dB isolation. After narrowing the bandwidth with one bandpass filter, the generated photon pairs are injected into Chip II, which is composed of an AWG and UMZIs. We use this chip for the energytime entanglement preparation and analysis. The polarization of photons injected into Chip II is also fixed by another PC (not shown) to ensure TE_{0} mode excitation.
The AWG serves as a critical DWDM component. It has 32 output ports, as shown in Fig. 2(b). We select 17 of them to demonstrate the entanglement distribution experiment (transmission spectra shown in Fig. 1(d)) and label them as CH1 to CH17 by wavelength from long to short. Their functionalities and the correspondence with standard DWDM channels are given in Table 1. The output photon pairs are detected by superconducting nanowire singlephoton detectors (SNSPDs, Scontel) with time jitter 45 ps. The generated electric signals are imported into the timecorrelated single photon counting (TCSPC) system for the correlation analysis.
In the experiment, the spiral on Chip II provides a time delay (ΔT) of 400 ps, which meets the condition for energytime entanglement generation. That is, τ_{2} ≪ ΔT ≪ τ_{1}, where τ_{2} is the coherence time of the single photons (~ 25 ps, which is estimated by the reciprocal of the photon bandwidth) and τ_{1} is the coherence time of the continuous pump laser [39]. The photonic state after the AWG and UMZIs can be described as \(\frac{\left({e}^{i{\varphi}_i}{\leftL\right\rangle}_i+{\leftS\right\rangle}_i\right)}{\sqrt{2}}\) and \(\frac{\left({e}^{i{\varphi}_s}{\leftL\right\rangle}_s+{\leftS\right\rangle}_s\right)}{\sqrt{2}}\), where φ_{i} and φ_{s} are the phase differences introduced by UMZIs for the idler and signal photons, and “L” and “S” represent the “Long” and “Short” time bins, respectively. The twophoton state is the direct product of singlephoton states, which has the form of
and will be recognized by the TCSPC system as three coincidence peaks separated by “ΔT” (see Fig. 3(bc)). The central peak (a time delay of 0) indicates the energytime entanglement state, which has the form of
The central peak coincidence C is proportional to (1 + V cos(φ_{i} + φ_{s})), where V is the visibility of the coincidence curve. By tuning the phases φ_{i} + φ_{s} with onchip heaters, we test every correlated channel pair and list their quantum interference visibilities in Table 1. All raw visibilities are over 90%, which proves that the integrated circuits have high quality and are ready for further quantum information applications. The typical coincidence curve of CH11&CH7 (C52&C48 of ITU) and their single channel counts as a function of phase on the long arm of UMZI are shown in Fig. 3(d). Coincidence curves for other channels can be found in the Supplementary Materials.
Entanglement distribution of one sender and two receivers
The results of the former experiment prove that our chips work well in generating and analyzing energytime entanglement. Furthermore, we verify the feasibility of the chips for entangled distribution among three chips. In this part, we add another receiver chip (see Fig. 4 (a)). The photons generated by the SOI waveguide are divided by one BS and then sent to two receiver chips, namely, Chip IIa and Chip IIb for energytime entanglement analysis. Two 1kmlong standard SMF28e fibers connect the BS with the following two chips. We selected three equally spaced channel pairs and measured their coincidence versus phase. The quantum interference visibilities are listed in Table 2. All the raw visibilities are over 85%.
Discussion
In the experiment, the arm length difference of the UMZIs is 400 ps, which is mainly limited by the detectors’ time jitter and coherence time of the single photons. Using a temporally narrowband light source and smaller time jitter detectors, we are able to decrease the arm length difference, which facilitates further construction of highdimensional energytime entanglement on the single chip. Due to the ultralow transmission loss of the HIDS waveguide, long time delay UMZIs can be easily achieved. On the basis of our structure, a multiwavelength highdimensional energytime entanglement distribution can be demonstrated by the cascade of UMZIs, thereby further increasing the system capacity. With another onchip UMZI to modulate the pulsed pump laser, our scheme can be easily extended to another commonly used scheme, the timebin entanglement distribution [40]. Because multiplewavelength channels of the receiver chips support simultaneous operations by different users and more nodes can be constructed with more similar integrated chips, our work advances the quantum technologies for largescale and complex quantum networks.
Conclusions
This work first integrates DWDM components with UMZIs monolithically and demonstrates multichip entanglement distribution with high interference visibilities. Benefiting from the low loss, low optical nonlinearity and high integration density of the HIDS platform, this work is promising for extending to highdimensional energytime and timebin entanglement distributions among multiple users. With many other integrated structures, more complex and larger quantum networks will be built.
Methods
Thermal control is critical to the experiment. The resistance of heaters on the short balance MZIs is ~ 170 Ω, which requires a current of ~ 50 mA to adjust a 2π phase. The heaters on the spiral waveguide of UMZIs have resistance of ~ 1.5 kΩ, and a current supply of ~ 14 mA is enough to finish a coincidence curve measurement. In our experiment, a custommade highpower current source is used. To stabilize the operation temperature during the experiment, the integrated chip is mounted on a thermoelectric cooler (TEC), which is driven using a TEC controller (TED200C, Thorlabs). Conductive silver glue is used to fill the contact surfaces (see Supplementary Materials). The temperature coefficient of the AWG central wavelength is ~ 18 pm/°C, which can be utilized to align the central wavelength of the AWGs with the standard of ITU grids and ensure that the system works properly.
Availability of data and materials
Supplementary information is available in the online version of the paper.
Abbreviations
 BS:

Beam splitter
 PIC:

Photonic integrated circuits
 QPICs:

Quantum photonic integrated circuits
 DWDM:

Dense wavelengthdivision multiplexing
 WDM:

Wavelengthdivision multiplexing
 UMZI:

Unbalanced MachZehnder interferometer
 QKD:

Quantum key distribution
 SiN:

Silicon nitride
 AWG:

Arrayed waveguide grating
 SOI:

Silicononinsulator
 LNOI:

Lithium niobate on insulator
 HIDS:

Highindex doped silica
 MZI:

MachZehnder interferometer
 CMOS:

Complementary metal oxide semiconductor
 SFWM:

Spontaneous fourwave mixing
 ITU:

International telecommunications union
 PC:

Polarization controller
 VOA:

Variable optical attenuator
 EDFA:

Erbiumdoped fiber amplifier
 FSR:

Free spectral range
 TE:

Transverse electric
 SNSPD:

Superconducting nanowire singlephoton detectors
 TCSPC:

Timecorrelated single photon counting
 TEC:

Thermoelectric cooler
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Acknowledgements
This work was partially carried out at the USTC Center for Micro and Nanoscale Research and Fabrication.
Funding
This work was supported by the National Natural Science Foundation of China (NSFC) (Nos. 62061160487, 12004373, 62075238), the Innovation Program for Quantum Science and Technology (No. 2021ZD0303200), the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB24030601), the CAS Project for Young Scientists in Basic Research (No. YSBR049), the Postdoctoral Science Foundation of China (No. 2020 M671860) and the Fundamental Research Funds for the Central Universities.
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All authors contributed extensively to the work presented in this paper. W.Q.W. and B.Z.D. prepared the samples. Y.J.C. prepared the quantum source. S.Y.R. and L.T.F. performed the measurements, data analysis and discussions. L.H., W.Z. and G.C.G. provided support in the experiment and conducted the theoretical analysis. S.Y.R., W.Q.W., L.T.F., W.F.Z. and X.F.R. wrote the manuscript, and L.T.F., W.F.Z. and X.F.R. supervised the project. The authors read and approved the final manuscript.
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Supplementary Information
Additional file 1: S0.
Characterization of the quantum source. Fig. S0. Coincidence peak for i_{1}&s_{1}. The resolution histogram is set to be 40 ps. S1. Coincidence curves for other channels in the experiment of dense energytime interference. Fig. S1. The raw coincidences of the central peak versus phase from i_{2}s_{2} to i_{7}s_{7}. S2. Coincidence curves for other channels in multichip entanglement distribution over 1km standard fibers. Fig. S2. The raw coincidences of the central peak versus phase for i_{4}s_{4} and i_{7}s_{7}. S3. The effect of the free spectral range (FSR) of arrayed waveguide gratings (AWGs) on coincidence. Fig. S3. Coincidence peaks with/without bandpass filters for i_{5}&s_{5}. S4. Packaged chip with electric supply. Fig. S4. (a) Packaged and mounted chip ready for testing. (b) Fiber distribution box for 128channel I/O. S5. The classical characterization of the onchip balanced MachZehnder interferometer (MZI) and unbalanced MachZehnder interferometer (UMZI). Fig. S5. (a) The classical interference curve of the onchip balanced MZI. Fig. S5. (b) The classical interference curve of the onchip UMZI.
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Ren, SY., Wang, WQ., Cheng, YJ. et al. Photonicchipbased dense entanglement distribution. PhotoniX 4, 12 (2023). https://doi.org/10.1186/s43074023000891
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DOI: https://doi.org/10.1186/s43074023000891