We demonstrate a novel, passive method for processing frequency-bin encoded quantum information using linear interferometry and time-resolved detection, eliminating the need for electro-optic modulation or nonlinear optics. This approach is compatible with multimode propagation, a crucial step for free-space quantum communication, and favorable for size, weight, and power (SWaP) requirements. Using a high-brightness multi-resonator source, we generate frequency-bin entangled photons and show the ability to perform arbitrary projective measurements in both single- and multi-mode propagation scenarios. For the first time, we measure the joint temporal amplitude of the generated state, directly linking its entangled nature to the temporal oscillation of non-local correlations. We leverage this to certify entanglement with a Bell curve visibility exceeding 95%. Our technique enables full quantum state tomography, yielding a state fidelity of up to 91% for a maximally entangled state. This work paves the way for robust entanglement deployment over free-space and satellite-based links.
We demonstrate a novel, passive method for processing frequency-bin encoded quantum information using linear interferometry and time-resolved detection, eliminating the need for electro-optic modulation or nonlinear optics. This approach is compatible with multimode propagation, a crucial step for free-space quantum communication, and favorable for size, weight, and power (SWaP) requirements. Using a high-brightness multi-resonator source, we generate frequency-bin entangled photons and show the ability to perform arbitrary projective measurements in both single- and multi-mode propagation scenarios. For the first time, we measure the joint temporal amplitude of the generated state, directly linking its entangled nature to the temporal oscillation of non-local correlations. We leverage this to certify entanglement with a Bell curve visibility exceeding 95%. Our technique enables full quantum state tomography, yielding a state fidelity of up to 91% for a maximally entangled state. This work paves the way for robust entanglement deployment over free-space and satellite-based links.
Abstract Frequency-bin entangled photons can be efficiently produced on-chip which offers a scalable, robust and low-footprint platform for quantum communication, particularly well-suited for resource-constrained settings such as mobile or satellite-based systems. However, analyzing such entangled states typically requires active and lossy components, limiting scalability and multi-mode compatibility. We demonstrate a novel technique for processing frequency-encoded photons using linear interferometry and time-resolved detection. Our approach is fully passive and compatible with spatially multi-mode light, making it suitable for free-space and satellite-to-ground applications. As a proof-of-concept, we utilize frequency-bin entangled photons generated from a high-brightness multi-resonator source integrated on-chip to show the ability to perform arbitrary projective measurements over both single- and multi-mode channels. We report the first measurement of the joint temporal intensity between frequency-bin entangled photons, which allows us to certify entanglement by violating the Clauser-Horne-Shimony-Holt (CHSH) inequality, with a measured value of ∣ S ∣ = 2.32 ± 0.05 over multi-mode fiber. By combining time-resolved detection with energy-correlation measurements, we perform full quantum state tomography, yielding a state fidelity of up to 91%. We further assess our ability to produce non-classical states via a violation of time-energy entropic uncertainty relations and investigate the feasibility of a quantum key distribution protocol. Our work establishes a resource-efficient and scalable approach toward the deployment of robust frequency-bin entanglement over free-space and satellite-based links.
Frequency-bin quantum encoding shows great promise for quantum communication given its high-dimensional scaling, compatibility with photonic integrated circuits and synergy with classical optical communication technology. However, to date all demonstrations have been performed over single-mode and static channels, while the transmission over fluctuating and turbulent channels has not been addressed. We propose and demonstrate a novel approach that leverages field-widened interferometers to decode frequency-bins transmitted over free-space channels without any adaptive optics or modal filtering. Moreover, we investigate the phase stability requirements so that frequency-bin encoding could be feasible for satellite to ground quantum links. Our passive approach expands the versatility of frequency-bin encoding, paving the way towards long-range and fluctuating channels.
Frequency-bin quantum encoding shows great promise for quantum communication, given its high-dimensional scaling, compatibility with photonic integrated circuits, and synergy with classical optical communication technology. However, to date, all demonstrations have been performed over single-spatial mode and static channels, while the transmission over fluctuating and turbulent channels has not been addressed. We propose and demonstrate a novel approach that leverages field-widened interferometers to decode frequency-bins transmitted over free-space channels without any adaptive optics or modal filtering. Moreover, we investigate the phase stability requirements so that frequency-bin encoding could be feasible for satellite-to-ground quantum links. Our passive approach expands the versatility of frequency-bin encoding, paving the way towards long-range and fluctuating channels.
Time-bin encoded photonic qubits are robust against decoherence in birefringent media such as optical fibers, which has led to many successful quantum communication demonstrations, including a Mach-Zehnder interferometer-based system, plug-and-play quantum key distribution (QKD), differential phase shift (DPS) and coherent one-way (COW) protocols, quantum teleportation, and elements of quantum repeaters. Despite these successes, time-bin encoding is considered impractical for turbulent free-space or multimode fiber channels, because spatial and temporal mode distortions hinder the interference required for analysis of time-bin superposition states. Here we present an efficient time-bin qubit analyzer, assisted by passive relay optics, that matches the spatial mode of the interfering paths. Our system demonstrates interference visibility of 89 %, even for strongly distorted optical input modes emerging from multimode fiber under variable angles of incidence. We measure a high level of entanglement between a time-bin qubit and a separate polarization qubit, thereby demonstrating the feasibility of time-bin based quantum communication over optical multimodal links.
Quantum networking holds tremendous promise in transforming computation and communication. Entangled-photon sources are critical for quantum repeaters and networking, while photonic integrated circuits are vital for miniaturization and scalability. In this talk, we focus on generating and manipulating frequency-bin entangled states within integrated platforms. We encode quantum information as a coherent superposition of multiple optical frequencies; this approach is favorable due to its amenability to high-dimensional entanglement and compatibility with fiber transmission. We successfully generate and measure the density matrix of biphoton frequency combs from integrated silicon nitride microrings, fully reconstructing the state in an 8 × 8 two-qudit Hilbert space, the highest so far for frequency bins. Moreover, we employ Vernier electro-optic phase modulation methods to perform time-resolved measurements of biphoton correlation functions. Currently, we are exploring bidirectional pumping of microrings to generate indistinguishable entangled pairs in both directions, aiming to demonstrate key networking operations such as entanglement swapping and Greenberger–Horne–Zeilinger state generation in the frequency domain.
We demonstrate the first-time frequency-bin-encoded entanglement-based quantum key distribution (EBQKD) between two users, and a flexible entanglement distribution over long optical fiber links. Dynamic and resource-efficient solutions are crucial to realizing large-scale photonic quantum networks capable of accommodating a growing number of users over large distances and at the same time facilitating a reliable performance at a maintained security level. Here, we adopt the frequency-bin encoding approach for defining three frequency channels, namely, CH1, CH2, and CH3, from highly correlated signal and idler photon pairs from pulsed-excited spontaneous parametric down-conversion (SPDC) process. The adaptive frequency de/multiplexing capability of our approach facilitates dynamic allocation of multiple frequency channels to additional users without increasing the hardware overhead. We developed a frequency-bin-basis analyzer module (see Figure. 1 a & b) composed of a programmable filter (PF), a frequency mixer (FM) based on electro-optic phase modulation (EOPM), a fiber Bragg grating (FBG) for the frequency-to-time mapping (FTM), and a superconducting nanowire single photon detector (SNSPD) of high timing resolution. Through a fine-tuned frequency mixing, passive random projection measurements are realized in two mutually unbiased basis (MUB; hereZ={∣0⟩,∣1⟩}andX={∣+⟩,∣−⟩}). The FTM technique applied on the frequency-mixed spectrum of each frequency channel allows for a time-resolved access to the projection measurement results in four basis states using only a single SNSPD. Remarkably, we showcase the capability of our approach for time-resolved detection of the four projection states corresponding to CH1, CH2, and CH3 using a single SNSPD per user. The performance metrics of the BBM92 protocol are shown in Figure. 1 c-e under different optical attenuations, at maximum 15.6 dB (equivalent to a 73 km fiber link). The qubit error ratio (QBER) was obtained for all channels well below the 11% upper threshold [1]. The maximum asymptotic secret key lengthlAof 310-bits, 283-bits, and 149-bits were obtained for channels CH1, CH2, and CH3, respectively. From the finite key analysis, a positive non-zero secret key rate was obtained for less than 15.6 dB attenuation. The capability of the system was also assessed in preserving high quality frequency-insenstive entanglement for distribution across remote fiber links (see Figure.1 f-h). Overall, the presented approach enables passive random projection measurements in two MUB by each user and benefits from frequency-multiplexing capability for adaptive multi-user operations. A single SNSPD per user renders this approach resource-efficient compared to conventional implementation schemes requiring 4×N detectors corresponding to N channels [2]. A single SNSPD means a reduced vulnerability to detector side channel attacks, a minimized dark count contribution, and a diminished imbalance arising from detection probability mismatch. Our approach thus enables a resource-efficient and a scalable implementation of EBQKD required for the future large-scale quantum networks.