Large-scale quantum communication systems often require different channels to be connected to multiple networks via frequency conversion, which requires frequency converters to act as relay points for system connectivity. We propose a quantum network for single-photon frequency conversion based on time-bin entanglement, where legitimate users can realize the spanning of communication bands between users by independently choosing the target conversion frequency mode. The results show that the converter in the network can be used as a bridge to completely connect the two networks by cascading the quadratic nonlinear process. Meanwhile, the converted quantum entangled states are demonstrated in entanglement measurements and can be used for entanglement swapping. This scheme is expected to be widely used in scenarios where different networks share quantum resources, switching communication channels and effectively connecting different multiplexed spectrum. This concept also provides significant applications in large-scale metropolitan area networks.
Large-scale quantum communication systems often require different channels to be connected to multiple networks via frequency conversion, which requires frequency converters to act as relay points for system connectivity. We propose a quantum network for single-photon frequency conversion based on time-bin entanglement, where legitimate users can realize the spanning of communication bands between users by independently choosing the target conversion frequency mode. The results show that the converter in the network can be used as a bridge to completely connect the two networks by cascading the quadratic nonlinear process. Meanwhile, the converted quantum entangled states are demonstrated in entanglement measurements and can be used for entanglement swapping. This scheme is expected to be widely used in scenarios where different networks share quantum resources, switching communication channels and effectively connecting different multiplexed spectrum. This concept also provides significant applications in large-scale metropolitan area networks.
Perfect entanglement swapping, which can be realized without the postselection by using the nonlinear optical technology, provides an important way toward generating the large-scale quantum network. We explore an entanglement-swapping-based dense wavelength division multiplexed network in the experiment. Four users receive single quantum states at different wavelengths, and we perform a time-energy entanglement swapping operation based on the sum-frequency generation to make users fully connected in the network. The results show that the fidelity of the entangled state is larger than 90% and is independent of the number of users. Our Letter demonstrates the feasibility of a proposed multiuser network, and hence paves a route toward a variety of quantum applications, including entanglement-swapping-based quantum direct communication.
Single-photon frequency conversion for quantum interface plays an important role in quantum communications and networks, which is crucial for the realization of quantum memory, faithful entanglement swapping and quantum teleportation. In this chapter, we will present our recent experiments about single-photon frequency conversion based on quadratic nonlinear processes. Firstly, we demonstrated spectrum compression of broadband single photons at the telecom wavelength to the near-visible window, marking a critical step towards coherent photonic interface. Secondly, we demonstrated the nonlinear interaction between two chirped broadband single-photon-level coherent states, which may be utilized to achieve heralding entanglement at a distance. Finally, we theoretically introduced and experimentally demonstrated single-photon frequency conversion in the telecom band, enabling switching of single photons between dense wavelength-division multiplexing channels. Moreover, quantum entanglement between the photon pair is maintained after the frequency conversion. Our researches have realized three significant quantum interfaces via single-photon frequency conversion, which hold great promise for the development of quantum communications and networks.
Frequency conversion of single photons is an important technology for quantum interface and quantum communication networks. Here, single-photon frequency conversion in the telecommunication band is experimentally demonstrated via cascaded quadratic nonlinear processes. Using cascaded quasi-phase-matched sum and difference frequency generation in a periodically poled lithium niobate waveguide, the signal photon of a photon pair from spontaneous down-conversion is precisely shifted to identically match its counterpart, i.e., the idler photon, in frequency to manifest a clear nonclassical dip in the Hong-Ou-Mandel interference. Moreover, quantum entanglement between the photon pair is maintained after the frequency conversion, as is proved in time-energy entanglement measurement. The scheme is used to switch single photons between dense wavelength-division multiplexing channels, which holds great promise in applications in realistic quantum networks.
High-speed long-range quantum communication requires combining frequency multiplexed photonic channels with quantum memories. We experimentally demonstrate an integrated quantum frequency conversion protocol that can convert between wavelength division multiplexing channels in the telecom range with an efficiency of 55±8% and a noise subtracted Hong-Ou-Mandel (HOM) dip visibility of 84.5%. This protocol is based on a cascaded second order nonlinear interaction and can be used to interface a broad spectrum of frequencies with narrowband quantum memories, or alternatively as a quantum optical transponder, efficiently interfacing different regions of a frequency-multiplexed spectrum.
Single-photon frequency conversion for quantum interface plays an important role in quantum communications and networks, which is crucial for the realization of quantum memory, faithful entanglement swapping and quantum teleportation. In this talk, I will present our recent experiments about single-photon frequency conversion based on quadratic nonlinear processes. Firstly, we demonstrated spectrum compression of broadband single photons at the telecom wavelength to the near-visible window. A positively chirped single-photon-level laser pulse and a negatively chirped classical one are converted to a narrowband single-photon pulse, with a spectrum compression factor of 58, through sum-frequency generation (SFG), marking a critical step towards coherent photonic interface. Secondly, we demonstrated the nonlinear interaction between two chirped broadband single-photon-level coherent states. A high SFG efficiency of 1.06 × 10−7 is realized, which may be utilized to achieve heralding entanglement at a distance. Finally, we theoretically introduced and experimentally demonstrated single-photon frequency conversion in the telecom band, enabling switching of single photons between dense wavelength-division multiplexing channels. Using cascaded quasi-phase matched sum/difference frequency generation, the signal photon of a photon pair from spontaneous down-conversion is precisely shifted to identically match its counterpart, i.e. idler photon, in frequency to manifest a clear non-classical dip in the Hong-Ou-Mandel interference. Moreover, quantum entanglement between the photon pair is maintained after the frequency conversion. Our researches have realized three significant quantum interfaces via single-photon frequency conversion, which hold great promise for the development of quantum communications and networks.
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.