Scheme of the generalized multipath delayed-choice experiment

Figure 1a shows a diagram of general d-path Mach–Zehnder interferometer (d-MZI) consisting of d arms (paths) and two d-mode beamsplitters (d-BSs). An array of individually reconfigurable phases of $$ {\left\{{\theta }_k\right\}}_{k=0}^{d-1}$$ are applied on the d-path between the two beamsplitters of d-BS1 and d-BS2. To implement the d-BSs, a general scheme is to nest (d² − d)/2 standard 2-BSs (see a bulk-optic example in Fig. 1b). For simplicity, we consider the case that d-BSs are well balanced, thus the state emerging from the d-BS1 is a maximally coherent state as $$ {\rho }_0={∣\mathrm{\Psi }⟩}_{00}⟨\mathrm{\Psi }∣$$ and $$ {∣\mathrm{\Psi }⟩}_0=\frac{1}{\sqrt{d}}{\sum }_{k=0}^{d-1}∣k⟩$$, where $$ {\left\{∣k⟩\right\}}_{k=0}^{d-1}$$ is the logical basis that defines the reference frame. The state of d-BS2 represents the measurement apparatus photons will confront. When the d-BS2 is inserted (removed), the d-MZI is closed (open), the detected probabilities at {D₀, … D_d−1} are dependent (independent) on the $$ {\left\{{\theta }_k\right\}}_{k=0}^{d-1}$$ configuration, thus revealing the d-path wave-like interference (d-mode particle-like quantization).

In order to probe the genuine wave-particle duality, the state of d-BS2 has to be determined after the photon has entered the d-MZI. In Fig. 1c, we adopt a modified version of the quantum-controlled delayed-choice experiment, recently proposed by Ionicioiu and Terno⁴² and implemented in several double-path experiments^12–15. By introducing a quantum-controlled BS that is in a coherent superposition of presence and absence, it represents a controllable experiment platform that can reveal wave or particle character, and their intermediate character⁴². In our multipath quantum delayed-choice scheme, the state of a general d-dimensional Hadamard operator $$ {\widehat{H}}_d$$ is coherently entangled with the control qubit of $$ {∣\psi ⟩}_C$$. The Hadamard $$ {\widehat{H}}_d$$ is implemented by a balanced d-BS, with elements $$ h_{i,j}^{\left(d\right)}=\frac{1}{\sqrt{d}}{\left(-1\right)}^{i\odot j}$$, where i ⊙ j denotes the bitwise dot product of the binary representations of i and j. Note that in general the implementation of d-dimensional controllable Hadamard operation however is highly challenging in the delayed-choice experiment^10,11. This is because of the difficulty of actively and rapidly operating the d-BS2—simultaneously operating the entire (d² − d)/2 array of 2-BSs, and thus quickly reconfiguring the whole d-MZI to an either open or closed state. We adopted a similar scheme as the double-path quantum delayed-choice experiments^12,13. The two multimode complementary measurements performed on a target photon are determined by the state of another control photon, in which all operations only require passive optical components without any active operation of the whole d-BS2 and d-MZI. The key idea is to create a coherent entanglement between a control qubit and the state of d-BS2.

We devise a large-scale silicon-integrated quantum nanophotonic device for the implementation of the delayed-choice d-path interferometric experiment, that features high phase stability and scalability⁴¹. Figure 1e illustrates a simplified diagram of the device to implement the circuit in Fig. 1c. Our task is to test the wave-particle dual nature of the target photon ρ₀ by choosing measurement apparatus $$ {\widehat{\mathrm{M}}}_m$$ (Fig. 1d). The device includes four parts: an entangled photon-pair source, a d-path MZI, a quantum-controlled d-BS, and a d-mode eraser. As shown by Fig. 1e, all these parts are monolithically integrated on a single silicon chip. We prepare a maximally entangled Bell state $$ \left({∣0⟩}_C{∣0⟩}_T+{∣1⟩}_C{∣1⟩}_T\right)/\sqrt{2}$$ in two integrated spontaneous four-wave mixing (SFWM) sources³⁵, where $$ {∣0⟩}_{C,T}$$ and $$ {∣1⟩}_{C,T}$$ are path-encoded logical states of the control and target photons. The target photon undergoes a $$ {\widehat{H}}_d$$ transformation by the d-BS1, and a phase operator $$ {\widehat{\theta }}_d$$ by the phase array. Depending on the control state of $$ {∣0⟩}_C$$ or $$ {∣1⟩}_C$$, the target photon coherently evolves either as a wave or particle, resulting in a state-process entanglement:

We operate the control photon state as $$ \sin \frac{\alpha }{2}{∣0⟩}_C+e^{i\delta }\cos \frac{\alpha }{2}{∣1⟩}_C$$, where {α, δ} represents the angles of {σ_y, σ_z} Pauli rotations, and selected the events when the detector $$ {\mathrm{D}}_8^{\prime }$$ is clicked (Fig. 1e). Owing to the presence of entanglement between the control photon and the quantum state of d-BS2, the d-BS2 is thus in a superposition of presence and absence as ($$ \cos \frac{\alpha }{2}\widehat{I}-i{e}^{i\delta }\sin \frac{\alpha }{2}{\widehat{H}}_d$$). Note the first term represents the measurement of particle character, while the second term represents the measurement of wave character. Figure 1d represents the framework of delayed choice of measurement $$ {\widehat{\mathrm{M}}}_m=∣m⟩⟨m∣$$ performing on the target photon ρ₀, where $$ ∣m⟩$$ forms the wave-particle measurement basis:

The choice of measurement apparatus $$ {\widehat{\mathrm{M}}}_m$$—determined by the {α, δ} state of the control photon in a delayed manner, allows us to observe the d-path wave-particle transition and to test the d-path duality relation of the target photon. In Eq. (3), α refers to the amplitude of wave and particle properties, and the inherent δ phase identifies the genuine quantum particle-wave superposition. If α = 0, d-BS2 is in the off-state and the $$ {\widehat{\mathrm{M}}}_m$$ discloses the particle-nature. Hence, the photon registers each of the detectors (Fig. 1e) with a probability of 1/d, leading to the observation of d-mode quantized distributions. If α = π, d-BS2 is in the on-state and $$ {\widehat{\mathrm{M}}}_m$$ reveals the wave nature. In this regard, the probability of detecting the photon relies on {θ_d}, building up d-path wave interference patterns. If 0 < α < π, d-BS2 is in a superposition of the on- and off-state and it thus allows the observation of wave-particle nature simultaneously. We remark that the choice of $$ {\widehat{\mathrm{M}}}_m$$ and the dual property of the target photon remain undetermined, until the control photon has been detected. This is because of the essence of entanglement that information is non-locally shared between the two photons.

Our quantum chip is designed for d-path (d ≤ 8) experiments, in which the number of paths and mode number of d-BSs can be reconfigured. The d-BSs are formed by a squared mesh of 2-BSs (each is a 2-path MZI for full reconfigurability). The chip integrates 95 phase-shifters that are individually addressed and electronically driven. A telecom-band laser was used to pump two integrated SFWM sources and generate a pair of entangled photons. The signal photon is regarded as the target photon, while the idler photon is regarded as the control. The two photons were ultimately routed off the chip for detection by superconducting nanowire single-photon detectors {$$ {\mathrm{D}}_i,{\mathrm{D}}_i^{\prime }$$}, i = 0, ... 8 (Fig. 1e). The fabricated devices and waveguides are shown in Fig. 1f–h. See Methods and Supplementary Note 1 for more details of device fabrication and setup.

Ruling out high-order interference

Born’s rule implies that multipath interference consists of all possible combinations of mutual interference. The possible presence of high-order interference could mask the test of wave-particle duality in the d-path interferometric experiment. Prior to testing the multipath wave-particle duality, we first rule out the presence of high-order interference. As an example, we implemented a four-path interference experiment. We measured the normalized Sorkin parameter κ, a ratio of high-order interference to second-order interference^23–25, and obtained the tight bound of −0.0031 ± 0.0047 for the fourth-order interference (Fig. 2). Our experimental results confirm the absence of high-order interference, within an accuracy of 10⁻³ of the bound, which is comparable to the most precise result obtained so far in ref. ⁴⁴. Main experimental errors come from the non-perfect opening/closure of paths and thermal crosstalk between paths, as well as background photon noises. See Supplementary Note 2 for more measurement details.

Fig. 2

Measurement of high-order interference in the d-path interferometer.

a Measured κ values for the fourth-order interference that is normalized to the second-order interference. In total, 60 measurements (red points) are performed independently. Error bars (±σ) are estimated from photon Poissonian statistics. The yellow line denotes the mean value of κ, and the shaded regime shows one standard deviation for all κ. b Histogram of all measured κ values. The shaded regime shows a fitted Gaussian profile distribution. All data are measured at the prime maxima of the complete wave interference fringes. The measured tight bound of κ = −0.0031 ± 0.0047 allows us to rule out the presence of high-order interference.

Multipath wave-particle transitions

Figure 3 reports experimental results for d-path wave-particle transitions. Here, probability distributions for the $$ {\widehat{\mathrm{M}}}_0$$-measurement are plotted, corresponding to the detection of the target photon at the ports of $$ \left\{{\mathrm{D}}_0,{\mathrm{D}}_0^{\prime }\right\}$$. Measurement results for other ports $$ {\left\{{\mathrm{D}}_i\right\}}_{i=1}^{d-1}$$ are provided in Supplementary Fig. 4. In our measurement, the phase θ_k was chosen as k(θ − π), where θ ∈ [0, 2π], but an arbitrary θ_k phase can be set. We obtained the continuous transition of d-path duality, from the full-particle nature at α = 0 to the full-wave nature at α = π, in two different scenarios: classical mixture (Fig. 3a–e) and quantum superposition (Fig. 3f–j). Classical wave-particle mixture represents the state $$ \left({\cos }^2\frac{\alpha }{2}∣\mathrm{P}⟩⟨\mathrm{P}∣+{\sin }^2\frac{\alpha }{2}∣\mathrm{W}⟩⟨\mathrm{W}∣\right)$$, while quantum wave-particle superposition represents the state $$ \left(\cos \frac{\alpha }{2}∣\mathrm{P}⟩-i{e}^{i\delta }\sin \frac{\alpha }{2}∣\mathrm{W}⟩\right)/\sqrt{N}$$, where N is a normalization coefficient, denoting the ultimate states after the quantum erasure. Details are given in Supplementary Note 3.

Fig. 3

Experimental observations of multipath wave-particle transition in the delayed-choice experiment.

Measured transitions between particle and wave properties in several different scenarios: a–c classical mixture; f–h, quantum superposition; and k–m, intrinsic coherent quantum superposition. They are quantified by the probability distributions (normalized coincidences) for different {α, δ} of the control and {θ_d} of the target (θ_k = k(θ − π)) was chosen, in the 2-, 4-, and 8-path experiments. Density distributions (colored) represent experimental data, while contour lines (dashed) represent theoretical results. The F denotes the classical fidelity $$ {\sum }_i\sqrt{p_i{q}_i}$$ summing over the whole space of (θ, α) or (θ, δ), where p_i and q_i are the measured and theoretical probabilities, respectively. High fidelities are obtained for all measurements. Results in a–c are consistent with classical optical multi-slit interference. The asymmetry of transition patterns in the quantum case f–h stem from quantum interference between wave and particle properties. Intrinsic coherent quantum superposition represents the intermediate particle-wave character, corresponding to the maximal wave-particle superposition, when α = 3π/2 or π/2. The δ-dependence of interference patterns in k–m, as an example measuring at α = 3π/2, confirms the existence of genuine wave-particle superposition. d, e Classical fringes, and i, j quantum fringes for the full-wave case at α = π, and full-particle case at α = 0, for d = 2, 4, and 8. The interference fringe becomes sharper for d-path interference; the multimode quantization results in a 1/d probability at the outport. Only when α = {0, π}, the classical and quantum fringes agree with each other. Examples of δ-dependence interference fringes for δ = {0, π/2} and α = 3π/2 in the n 2-path, and o 4-path experiments. The construction or destruction of interference appears by controlling the internal δ phase. Points represent experimental data, while lines represent theoretical values. All error bars (±3σ) are estimated from photon Poissonian statistics.

In the case of 2-path classical mixture, Fig. 3a shows a sinusoidal interference fringe at α = π representing the full-wave nature, and the detection probability approaches nearly 1/2 at α = 0 representing the full-particle nature. The observation of 2-path wave-particle transition is consistent with the results in refs. ^13–15. In contrast, in the d-path experiments (Fig. 3b, c), at α = π we observed interference patterns that feature sharper distributions with an increment of d, confirming the d-path wave nature; at α = 0 we observed a 1/4 (1/8) probability for d = 4 (8), confirming the d-mode particle nature; for 0 < α < π, intermediate particle-wave behaviors were revealed. The results for α = {0, π} are replotted in Fig. 3d, e, which are expected in classical optical multi-slit interference.

We now report the unique feature of quantum wave-particle superposition in d-path experiments. In Fig. 3f–h, the probability distributions represent asymmetry with respect to θ = π, while the distributions for classical mixture in Fig. 3a–c remain symmetric. The extraordinary asymmetry comes from quantum interference between the wave and particle properties (Supplementary Eq. 14). Only when choosing the full-particle (α = 0) or full-wave (α = π) point, the distributions for classical (Fig. 3d, e) and quantum cases (Fig. 3i, j) are in agreement. When α ≠ {0, π}, the quantum distributions are remarkably distinct from the classical ones. Quantum distributions however tend to be less asymmetric for high d-path interference, becoming more classical (see analysis in Supplementary Note 5). Figure 3k–m shows quantum interference of multipath wave and multimode particle properties regarding the inherent phase δ. We set α = 3π/2 as an example (it works as well for π/2) that corresponds to the maximal wave-particle superposition. The δ-dependence of distributions confirms the genuine quantum wave-particle superposition, while δ-variation is absent in the case of classical mixture (see the explicit forms in Supplementary Eqs. 14 and 17). It is notable that by controlling the δ phase, the quantum interference of the wave and particle properties can be steered. For example, in the 2-path case, Fig. 3n shows constructive interference for δ = 0 and destructive interference for $$ \delta =\frac{\pi }{2}$$. In the case of 4-path interference (Fig. 3o), more wave-like characters appear when δ is set as $$ \frac{\pi }{2}$$.

All measurements in Fig. 3 were performed in the computational basis {$$ ∣0⟩$$, $$ ∣1⟩$$}, which are well in agreement with theoretical predictions. We also estimated high-level classical fidelities (see definition in Fig. 3’s caption) of 0.998 ± 0.001 for 2d-path, 0.991 ± 0.003 for 4d-path, and 0.980 ± 0.007 for 8d-path experiments, respectively. Since entanglement is playing an enabling role in the delayed-choice measurement of d-path wave-particle duality, we repeated the experiments in the complementary basis {$$ ∣+⟩$$, $$ ∣-⟩$$} to verify the presence of coherent entanglement, where $$ ∣\pm ⟩$$ denotes $$ \left(∣0⟩\pm ∣1⟩\right)/\sqrt{2}$$. Our circuit allows any local qubit rotation by the MZIs together with posterior phase-shifters (Fig. 1e). In particular, local Pauli σ_x ⊗ σ_x operations on the Bell state were implemented to perform measurements in the {$$ ∣+⟩$$, $$ ∣-⟩$$} basis. We again obtained coherent wave-particle transitions with high fidelities in the complementary basis, as shown in Supplementary Fig. 5. Moreover, to exclude the presence of local hidden variables that may contribute the delayed-choice measurement, we verified entanglement by both performing quantum state tomography of the entangled state, and demonstrating the violation of the Bell-CHSH (Clauser–Horne–Shimony–Holt) type inequality⁴⁵. The quantum state fidelity of 0.962 ± 0.002 was obtained (Supplementary Fig. 3). And the Bell value of 2.75 ± 0.04 was measured, which violates the classical bound by 18.8σ, confirming the existence of strong entanglement through the device.

Generalized multipath wave-particle duality relation

We next report experimental results of a generalized multipath wave-particle duality relation in the delayed-choice interferometer. It is of fundamental interest to develop a general framework to describe the multipath duality and to quantify wave and particle properties^17,27–30. The conventional visibility defined as the contrast of 2-path interference fringe, fails to be a good wave measure of d-path (d > 2) interference²⁶; however, quantum coherence is believed to be a good quantifier^27–30. We adopt the l₁-norm coherence ($$ {\tilde{\mathcal{C}}}_{l1}={\sum }_{i\ne j}\mid {\rho }_{ij}\mid $$) proposed by Streltsov et al. as a wave measure³⁴. Moreover, the capability of distinguishing which path the photon taken represents the d-path distinguishability. The formation of path-distinguishability for the 2-path case can be generalized to the d-path case^27,28,46. The normalized coherence $$ {\mathcal{C}}_d$$ and path-distinguishability $$ {\mathcal{D}}_d$$ that we have used are given as:

The $$ {\mathcal{C}}_d$$ and $$ {\mathcal{D}}_d$$ were measured in the context of delayed choice of measurement $$ {\widehat{\mathrm{M}}}_0$$ under the control of {α, δ}. It is notable that the measurement of which-path information $$ {\mathcal{D}}_d$$ in our experiment is a posteriori one^11,13. This is because of the fact that $$ {\widehat{\mathrm{M}}}_0$$ enables the projection into the particle-wave superposition basis, that is delayed controlled from the full-wave to full-particle measurements. In order to measure the $$ {\mathcal{C}}_d$$, we here adopt a newly derived general visibility $$ {\mathcal{V}}_d$$ proposed by Qureshi et al.^32,46,

Fig. 4

Experimental results of generalized multipath wave-particle duality relation in the delayed-choice experiment.

Measurement of generalized visibility ($$ {\mathcal{V}}_d$$) that is equivalent to the normalized l₁-norm coherence ($$ {\mathcal{C}}_d$$) for a 2-path and b 4-path full-wave interference (α = π). The $$ {\mathcal{V}}_d$$ is determined by the disparity of the primary maxima $$ I_{\max }$$ and incoherent term I_inc, both directly read out from the d-path interference fringes, with no need to probe the full density matrix. I_inc was obtained by recording the probability of i-outcomes only having the i-path open, and the mean of measured I_inc was used to estimate the $$ {\mathcal{V}}_d$$. Measurement of generalized duality relation in the classical mixture scenario: c 2-path and d 4-path experiments; in the genuine quantum-superposition scenario: e 2-path, δ = 0; f 4-path, δ = 0; g 2-path, δ = π/2; and h 4-path, δ = π/2 experiments. The path-information $$ {\mathcal{D}}_d$$ is measured in a posteriori way, by the delayed choice of measurement apparatus $$ {\widehat{\mathrm{M}}}_m$$. The bound of $$ {\mathcal{C}}_d^2+{\mathcal{D}}_d^2$$ values are indicated within the orange-colored regimes. The generalization of duality relation $$ {\mathcal{V}}_d^2+{\mathcal{D}}_d^2\le 1$$ is thus confirmed in the d-path quantum-superposition experiment for different {d, α, δ} configurations (e–h). In the classical-mixture case (c, d), the duality relation only holds at the full-wave and full-particle points at α = {0, π}, independent on the setting of δ. ln all plots (a–h), points represent experimental data, while lines represent theoretical values. Error bars (±σ) are estimated from photon Poissonian statistics.

Figure 4c–h reports experimental results of the generalized multipath duality relation for both wave-particle quantum-superposition and classical-mixture cases. In the genuine wave-particle quantum-superposition case (Fig. 4e–h), we demonstrate that the generalized duality relation holds the tight bound of unity as $$ {\mathcal{C}}_d^2+{\mathcal{D}}_d^2=1$$ in the different setting of {d, α, δ}. This is however not true for the classical-mixture case, where one cannot reach the upper bound and the equality does not saturate at α ≠ {0, π}, resulting in $$ {\mathcal{C}}_d^2+{\mathcal{D}}_d^2<1$$. The bound can only be approximated at α = {0, π} for full-particle or full-wave states, as shown in Fig. 4c, d. The quantitative results in Fig. 4 are consistent with the qualitative observations in Fig. 3a–j. The quantity of $$ {\mathcal{L}}_d=1-{\mathcal{C}}_d^2-{\mathcal{D}}_d^2$$ represents the missing information, which is due to a classical lack of knowledge about the quantum system. The $$ {\mathcal{L}}_d$$ is directly related to the Tsallis 1/2-entropy as $$ {\mathcal{L}}_d=\frac{1}{d-1}\left(\frac{1}{4}{\left(S_{1/2}\left(\rho \right)+2\right)}^2-1\right)$$, where S_1/2(ρ) = 2(Tr(ρ^1/2) − 1) represents the Tsallis entropy. Note $$ {\mathcal{L}}_d$$ is exactly the linear entropy if the l₂-norm coherence is used²⁸. In Supplementary Note 5, we show how $$ {\mathcal{L}}_d$$ scales for a large value of d.

Device fabrication

The large-scale integrated quantum photonic device for the implementation of the multipath delayed-choice experiment was designed and fabricated on the silicon nanophotonics platform, a versatile system for photonic quantum information technologies. The quantum device was fabricated by the standard CMOS (complementary metal-oxide-semiconductor) processes. A layer of photoresist was the first spin on an 8-inch SOI (silicon-on-insulator) wafer with 220-nm-thick top silicon and 3 μm-thick buried oxide. The 248 nm DUV (deep ultraviolet) lithography was adopted to define the circuit patterns on the photoresist. Double inductively coupled plasma (ICP) etching processes were applied to transfer the patterns from the photoresist layer to the silicon layer, forming waveguides and circuits. Deep etching waveguides (see SEM image in Fig. 1g) with an etched depth of 220 nm were used for the SFWM photon sources, beamsplitters, and phase-shifters. Shallow etching waveguides (see SEM image in Fig. 1h) with an etched depth of 70 nm were used for the waveguide crossers and grating couplers. A SiO₂ layer of 1μm thickness was deposited on top of the SOI wafer by plasma-enhanced chemical vapor deposition (PECVD), working as an isolation layer between the waveguides and metal heaters to avoid potential optical losses. Then, a 10-nm-thick Ti glue layer, a 20-nm-thick TiN barrier layer, an 800-nm-thick AlCu layer, and a 20-nm-thick TiN anti-reflective layer, were consequently deposited by physical vapor deposition (PVD) and patterned by DUV lithography and etching process to form the electrode. A 50-nm-thick TiN layer for thermal-optical phase-shifters was deposited and also patterned by DUV lithography and etching process. Finally, another 1-μm-thick SiO₂ was deposited as the top cladding layer, and followed by the bonding pad opening process. More SEM images of the fabricated optical components and their characterizations are provided in Supplementary Note 1.

Experimental setup

Our experimental setup bases on off-the-shelf telecommunication instruments. A tunable continuous-wave laser (EXFO) central at a wavelength of 1550.11 nm was amplified to 40 mW by an erbium-doped fiber amplifier (EDFA, Pritel), and then fed in to the silicon-photonics quantum chip as a pump source for the SFWM nonlinear process. Before the chip, the polarization state of the pump light was optimized by a fiber polatisation controller in order to excite the transverse electric (TE) mode of silicon waveguides. A pair of path-coded entangled photons was on-chip generated by the two SFMW sources (Fig. 1), and further adopted for the implementation of multipath quantum delayed-choice experiment. We selected the signal photon at 1545.31 nm and idler photon at 1554.91 nm. After the chip, wavelength-division multiplexing (WDM) filters with a bandwidth of 1.1 nm and 200 GHz channel spacing were used to remove the residual pump light. Single photons were detected by an array of superconducting nanowire single-photon detectors (SNSPDs, Photonspot). A multichannel time interval analyzer (Swabian) was used to record two-fold photon coincidences. All of the 95 thermal-optical phase-shifters were accessed and controlled individually by multichannel electronics (Qontrol) with a 16-bits precision and KHz speed. The quantum chip was glued on a printed circuit board (PCB), and packaged by wire bonding (Fig. 1f). To achieve the full 2π operation of each phase-shifter, it requires about 40 mW power consumption. When all phase-shifters were on, total power consumption was about several watts. To ensure the stability of quantum operations and suppress thermal noises and thermal crosstalk, the chip was mounted on a Peltier-cell and a thermosink with liquid cooling in order to dissipate power rapidly and efficiently. A thermistor mounted on the chip together with a proportional integrative derivative controller was used to monitor and stabilize the temperature of the photonic device. See Supplementary Fig. 2 for more experimental details.