An ultra-compact optical quantum router (QR) consisting of a Mach–Zehnder interferometer (MZI) and waveguide tapers is proposed and numerically simulated, using silicon-on-insulator (SOI). The interferometer is designed to work at the center wavelength of 1550 nm with visibilities of 99.65% and 98.80% for TE and TM polarizations, respectively. Using the principle of phase compensation and self-image, the length of the waveguide tapers is shortened by an order of magnitude with the transmission above 95% for both TE and TM polarizations. Furthermore, polarization beam splitters (PBS) with an ultra-compact footprint of 1.4 × 10.4 μm2 with transmissions of 98% for bi-polarizations are achieved by introducing anisotropic metamaterials. The simulated results indicate that the interferometer facilitates low loss, a broad operating spectral range, and a large tolerance to size variation in fabrications. The optical switch possesses the routing function while maintaining the polarization states, which promises to pave the point-to-point BB84 protocol into applications of network-based quantum communication.
An ultra-compact optical quantum router (QR) consisting of a Mach–Zehnder interferometer (MZI) and waveguide tapers is proposed and numerically simulated, using silicon-on-insulator (SOI). The interferometer is designed to work at the center wavelength of 1550 nm with visibilities of 99.65% and 98.80% for TE and TM polarizations, respectively. Using the principle of phase compensation and self-image, the length of the waveguide tapers is shortened by an order of magnitude with the transmission above 95% for both TE and TM polarizations. Furthermore, polarization beam splitters (PBS) with an ultra-compact footprint of 1.4 × 10.4 μm2 with transmissions of 98% for bi-polarizations are achieved by introducing anisotropic metamaterials. The simulated results indicate that the interferometer facilitates low loss, a broad operating spectral range, and a large tolerance to size variation in fabrications. The optical switch possesses the routing function while maintaining the polarization states, which promises to pave the point-to-point BB84 protocol into applications of network-based quantum communication.
We based on integrated silicon-on-insulator platforms design the key components of an on-chip interferometer, beam splitter and directional coupler included, valid in high-visibility interference at telecommunication wavelengths. Special attention is given to the equal-proportion beam splitting and directional coupling, which is achieved by carefully designing the geometric dimension of multi-mode interferometer structure. The proposed interferometer facilitates low loss, broad operating bandwidth, anticipated large tolerance on size variation induced in fabrication procedures, based on a particular wafer with silicon layer thickness of 320 nm. The most highlight property of polarization-insensitive, enables the path-selective qubits generation for bi-polarization that further makes possible quantum key distribution using high dimensional protocols. We numerically demonstrate interference at 1550 nm with visibilities of 99.50% and 93.99% for transverse-electric and transverse-magnetic polarization, respectively, revealing that the proposed interferometer structure is well capable of on-chip optical control especially in quantum optics regime.
Silicon-on-insulator (SOI) wafers have been developed for electronic integrated circuits, but are also one of the most promising platforms in photonic integrated circuits due to its low loss, high refractive index, ability to fast phase modulation, and well-developed fabrication technologies. With advantages to handle light with a small volume and high modulation speed, silicon photonics has been intensively investigated as the next generation platform of signal processing, interconnection, and communication devices. Recently, silicon photonics has been attracted great interests in quantum optics communities as optical quantum systems for linear optical quantum computing are achievable using silicon photonic components including beam splitters, phase modulators, and filters and the small footprint of a silicon photonic quantum system has unprecedented performances of high stability and scalability. Here we show our recent results on the fabrication of silicon photonic elements for quantum applications. We fabricated low-loss optical waveguides, efficient fiber-to-waveguide grating couplers, resonator-based optical filters, and tunable beam splitters and tested their performances. A micro-sized heater is attached on a Mach-Zehnder interferometer, showing stable operation as a slow phase modulator. In addition, phase modulators using plasma dispersion effects are under investigation. Finally, the quantum characteristics of photon pairs generated by spontaneous four-wave mixing in silicon waveguides are investigated. These results will be an essential cornerstone for the realization of quantum optical computing and quantum communication devices.
Abstract In photonic quantum applications, optical routers are required to handle single photons with low loss, high speed, and preservation of their quantum states. Single-photon routing with maintained polarization states is particularly important for utilizing them as qubits. Here, we demonstrate a polarization-maintaining electro-optic router compatible with single photons. Our custom electro-optic modulator is embedded in a configuration of a Mach–Zehnder interferometer, where each optical component achieves polarization-maintaining operation. We observe the performance of the router with 2%–4% loss, 20 dB switching extinction ratio, 2.9 ns rise time, and >99% polarization process fidelity to an ideal identity operation.
Efficient single-photon routing and switching are crucial for optical quantum computing and communication. For this purpose, all-optical switches are designed for gigahertz bandwidths. The switching mechanism is based on the optical Kerr effect via cross-phase modulation (CPM) of the single-photon signal by a strong 1550-nm pump pulse. For energy-efficient switching, this nonlinear effect is exploited in a microresonator that can either be used directly as an intensity switch in a typical add–drop configuration or as a phase shifter in a Mach–Zehnder interferometer (MZI) structure. To speed up resonance build-up and quenching, a pre-emphasis build and an off-resonance wipe pulse are used. The proposed designs are verified by traveling-wave simulations which demonstrate that 0.1 dB insertion loss and ~1 ns switching windows can be achieved. For a scalable out-of-the-lab transfer, we investigate the feasibility of the proposed switch designs for fabrication in a mature photonic integrated circuit (PIC) platform. In particular, silicon nitride PICs have demonstrated record-low losses which makes them suitable for single-photon applications. By parametric modelling of the microresonator’s directional couplers based on Lumerical EME and 2.5-dimensional varFDTD simulations, the required power transmission coefficients for both signal and pump wavelength can be achieved. This results in an all-optical switch design ready for fabrication in a commercial PIC foundry which can potentially enable scalable architectures for quantum photonic applications.
Abstract In this work, we experimentally studied silicon nitride Mach-Zehnder interferometer (MZI) with two directional couplers and 400 ps optical delay line for telecom wavelength 1550 nm. We achieved the extinction ratio in a range of 0.76-13.86 dB and system coupling losses of 28-44 dB, depending on the parameters of directional couplers. The developed interferometer is promising for the use in a compact random number generator for the needs of a fully integrated quantum cryptography system, where compact design, as well as high generation speed, are needed.
Integrated quantum key distribution (QKD) has achieved substantial advancements across different material platforms. Among these, silicon nitride has attracted widespread attention due to its low loss and compact size. However, silicon nitride exhibits pronounced polarization correlation, which makes it susceptible to unpredictable polarization state rotations induced by the fiber birefringence effect within the channel, thus affecting the normal operation of quantum processes. In this work, we design and fabricate the fundamental devices in QKD systems, namely 1×2 and 2×2 multimode interferometers (MMIs) with low polarization sensitivity and loss. We also improve the splitting balance via parameter optimization based on the particle swarm optimization (PSO) algorithm. Simulation results indicate that the 1×2 (2×2) MMI exhibits a loss of about 0.1 (0.2) dB, along with 0.001 (0.09) dB polarization-dependent loss (PDL) and a splitting ratio nearly 50:50 (±0.3%) at 1550nm, respectively. Test results for the fabricated devices demonstrate that the loss of both MMI types is confined to less than 0.25 dB, their PDL is kept within 0.2 dB, and splitting ratio imbalance is within 1% and exhibits temperature robustness. This research explores compact polarization-insensitive devices via a novel method, thereby laying the foundation for the development of integrated quantum information processing.