The Promise of Quantum Communication
Quantum communication, which applies the principles of quantum mechanics for information transmission, enables fundamental improvements to security, computing, sensing, and metrology. This realm encapsulates a vast variety of technologies and applications ranging from state-of-the-art laboratory experiments to commercial reality.
The best-known example is quantum key distribution (QKD). The basic idea of QKD is to use the quantum states of photons to share secret keys between two distant parties. The quantum no-cloning theorem endows the two communicating users with the ability to detect any eavesdropper trying to gain knowledge of the key. Since security is based on the laws of quantum physics rather than computational complexity, QKD is recognized as a desired solution to address the ever-increasing threat raised by emergent quantum computing hardware and algorithms.
Beyond QKD, quantum teleportation has attracted extensive attention, which exploits quantum entanglement for transferring fragile quantum information in an effectively unhackable manner. Based on this, quantum networks can be conducted to connect various quantum devices, enabling unparalleled capabilities that are provably unattainable using only classical information techniques.
Quantum secure direct communication (QSDC) is another important branch of quantum communication, providing opportunities for secure data transferring without sharing encryption keys. This technique has been evolving quickly in recent years, enabling users to directly transmit confidential information over secure quantum channels.
Limitations of Discrete Optical Systems
Conventional quantum communication systems are typically built using discrete optical devices. Generally, these devices are separately assembled with optical glasses and crystals and connected via free space or optical fibers. Although it is convenient to optimize individual elements to fit with the strict requirements such as ultra-low loss, high efficiency, fast speed and high fidelity in quantum information applications, interconnects and packaging have always posed significant reliability and cost challenges for traditional discrete optical designs, especially when dealing with large-scale networks linking hundreds of thousands of users.
For instance, high mechanical and thermal stabilities are required to mitigate space and phase misalignment over time due to environmental stresses and temperature variations, which are yet difficult to achieve in a complex discrete optical system by global stabilization. Therefore, current bulky systems composed of discrete optical components may struggle to meet the growing demand for higher volume transmission capability, manifesting great merits of chip-scale quantum communication systems.
Quantum Photonic Chips: The Future of Quantum Communication
Quantum photonic chips are an ideal platform for new generation of quantum technology. In addition to miniaturization, two advantages over discrete optical systems, i.e., scalability and stability, are prominent. Scalability is enabled because the chips, with all their components, are printed as a unit by lithography rather than being constructed one component at a time. Stability is achieved as the circuits built on a robust and compact solid-state platform can minimize deviations due to vibrations or temperature variations.
These two advantages are critical for achieving the level of integration and performance required for quantum information processing and highly efficient quantum communication. Moreover, quantum photonic chips have a strong potential for low-cost production. While the initial cost of fabricating the required photomasks is high, the average cost per chip can be greatly reduced through mass production.
After decades of effort, photonic integration has been realized in all aspects of individual quantum communication systems, including photon sources, encoding and decoding photonic circuits, and detectors. In principle, integrated photonic chips can combine many desirable characteristics, such as efficiency, cost-effectiveness, scalability, flexibility and performance, that are required for quantum communication applications. Such characteristics, along with wafer-scale fabrication processes, make chip-based quantum communication systems a compelling platform for the future of quantum technologies.
Key Photonic Integration Platforms
Prevailing materials platforms for chip-based quantum communication implementations include silica waveguides (silica-on-silicon and laser-written silica waveguides), silicon-on-insulator (SOI), silicon nitride (Si3N4), lithium niobate (LN), gallium arsenide (GaAs), indium phosphide (InP) and silicon oxynitride (SiOxNy). Each platform has its own advantages and disadvantages in terms of waveguiding properties, available active components, and compatibility with related technologies.
For example, SOI provides a great refractive-index contrast for high-density integration, strong optical nonlinearity for nonclassical state generation, and excellent compatibility with advanced CMOS processes. However, the lack of lasing capability makes it challenging to fully integrate all the required components of a quantum communication system. III–V semiconductor platforms (GaAs, InP, etc.) allow for monolithic system integration, yet coming at the expense of higher cost and lower integration level.
The inevitable weaknesses of each material and its fabrication process indicate that no single platform can provide all the desired features for quantum communication applications. A viable solution is a hybrid integration that aims to combine the advantages of different platforms.
Integrated Quantum Light Sources
A photon source that generates designated quantum states of light is a key element of a quantum optical system. In general, single-photon states and entangled photon states are required in the architecture of quantum communication networks.
Quantum dots (QDs) are considered one of the most promising candidates for the on-demand generation of single photons or entangled photon pairs by virtue of the deterministic nature of their emission characteristics. QDs can be embedded in photonic crystal waveguides or heterogeneous waveguide structures for highly efficient coupling with waveguides.
In addition to QDs, several other solid-state quantum emitters, such as color centers in diamond, silicon carbide, carbon nanotubes, and defects in two-dimensional materials, have also been investigated and shown great potential for on-chip generation of single photons or entangled photon pairs.
Integrated probabilistic quantum light sources typically take advantage of spontaneous four-wave mixing (SFWM) or spontaneous parametric down-conversion (SPDC) in optical waveguides or other photonic structures. Due to the tight confinement of light, these nonlinear parametric processes are greatly enhanced on a chip, enabling efficient generation of high-quality photon states in miniaturized configurations.
Multiplexing techniques offer a promising way to solve the problems of non-deterministic photon generation and limited generation rates of these probabilistic sources.
Manipulation of Quantum States
Manipulation of quantum states of light is essential for the processing of quantum information in quantum communication, which can be readily implemented by using off-the-shelf passive and active components of integrated photonics.
In a typical quantum communication system, photons are generally handled in polarization, phase, spatial, spectral, and temporal domains. Thus, it requires building blocks that can influence these degrees of freedom of the photons, such as polarization splitters/rotators, phase shifters, intensity modulators, directional couplers, multi-mode interferometers, ring resonators, and delay lines.
For efficient optical connection between quantum photonic chips and optical fibers, one-dimensional grating couplers, off-plane coupling, and edge couplers can be used. Two-dimensional grating couplers supporting multi-polarization operation have also been demonstrated to convert path-encoded qubits to polarization-encoded qubits.
Integrated Single-Photon Detection
Efficient single-photon detection is of great importance to quantum communication applications. Fully integrated single-photon detectors (SPDs) are highly desirable because interfacing with off-chip detectors will lead to unavoidable coupling losses.
Recently, integrated waveguide-coupled Ge-on-Si lateral avalanche photodiodes and waveguide-integrated superconducting nanowire single-photon detectors (SNSPDs) have been reported, achieving high detection efficiency, low dark count rate, and photon-number resolving capability.
Balanced homodyne detectors, which have been widely exploited in continuous-variable (CV) quantum information applications, are another crucial detection element for quantum measurement. Integrated homodyne detectors with enhanced performance in terms of compact size, good stability, broad bandwidth, low noise, and high common-mode rejection have also been demonstrated.
Packaging for Practical Quantum Communication
While bare quantum photonic chips can be characterized using a probe station, they must be packaged into durable modules to develop working prototype devices. Photonic packaging involves a range of techniques and technical competencies needed to make the optical, electrical, mechanical, and thermal connections between a photonic chip and the off-chip components in a photonic module.
Fiber-to-chip coupling is one of the best-known aspects, where the large difference between the mode-field diameters of optical fibers and typical waveguides on the chip can be mitigated by using configurations that efficiently extract the mode from waveguide, such as inverted-taper edge couplers or grating couplers.
To access the electrical components on quantum photonic chips, electronic packaging is required to route signals from electronic drivers, amplifiers, and other control circuitry. This is often achieved by interfacing with dedicated printed circuit boards (PCBs) or 2.5-dimensional/3-dimensional integration with customized electronic integrated circuits (EICs).
Global thermal stabilization of quantum photonic devices is essential for prototypes that require high accuracy and repeatability or for field tests where seasonal temperature swings are common. This can be achieved using passive cooling techniques or a thermoelectric cooler.
Chip-based Quantum Key Distribution (QKD)
As the most developed quantum secure communication technology, QKD based on bulk or fiber optic components has already been used in banks and governments to provide high-level security for data transmission. Wider applications require QKD systems that are more robust, compact, and can be mass manufactured at a lower cost. Integrated quantum photonics provides a compelling platform to address these requirements.
A key component in QKD systems is the quantum random number generator (QRNG). Numerous integrated QRNG implementations have been demonstrated, leveraging various integration technologies with different levels of complexity. Through custom co-design of opto-electronic integrated circuits and side-information reduction, a record generation rate of 100 Gbps has been reported using an SOI photonic chip co-packaged with a GaAs transimpedance amplifier circuit.
In terms of QKD protocols, chip-based systems have been demonstrated for discrete-variable (DV) QKD and continuous-variable (CV) QKD. DV-QKD systems typically use integrated modulators, lasers, and detectors to implement protocols like BB84 and measurement-device-independent (MDI) QKD. CV-QKD systems, on the other hand, rely on integrated homodyne detectors instead of single-photon detectors, significantly simplifying the detection setup.
Recent efforts have also explored advanced QKD protocols that can greatly benefit from photonic integration, such as high-dimensional QKD and MDI-QKD. These protocols have been implemented using integrated photonic circuits, showcasing the versatility and potential of chip-based quantum communication systems.
Chip-based Quantum Teleportation and Beyond
Quantum teleportation has been demonstrated with many platforms, and photonic qubit is one of the most promising candidates to build the quantum channel in a quantum network. Photonic quantum teleportation has been implemented experimentally in both free space and fiber systems.
The first on-chip quantum teleportation was reported with off-chip photon source, achieving a fidelity of 0.89. Recent technological progress in integrated quantum photonics has enabled the implementation of entanglement-based quantum communication protocols beyond a single chip, such as chip-to-chip entanglement distribution and chip-to-chip quantum teleportation.
These chip-scale demonstrations of photonic qubit production, processing, and transmission show a promising way for the distributed quantum information processing internet. Moreover, entangled photon pairs across the visible-telecom range have been demonstrated on a Si3N4 chip, providing an entangling link between visible-band photons that can interface with quantum memories and telecom-band photons that feature low-loss transmission in optical fibers.
Challenges and Future Opportunities
While considerable progress has been achieved, the field of chip-based quantum communication is still in its early stages and naturally faces many challenges. On the component side, on-chip elements used in quantum communication require more stringent specifications than those used in classical optical communication to ensure high fidelity and prevent decoherence of quantum states.
On the system side, fully integrated quantum communication systems with photon sources, photonic circuits, and detectors have not yet been realized. The difficulties in achieving full integration are due to the fact that no monolithic platform can provide all the desired features, and different parts of an integrated quantum system may work in different conditions (e.g., cryogenic temperatures).
Furthermore, chip-based quantum communication faces potential loopholes due to the specific imperfections of integrated photonic devices, such as phase- and polarization-dependent losses. Comprehensive security analysis is needed to close the gap between theoretical models and practical integrated quantum communication systems.
Beyond prepare-and-measure QKD, entanglement-based QKD is another promising application for future chip-based QKD systems. Quantum memories and quantum relays are also crucial for realizing long-distance entanglement distribution and quantum teleportation, as well as large-scale implementations of quantum networks.
In conclusion, quantum photonic chips have rapidly matured to become a versatile platform that proves to be invaluable in the development of cutting-edge quantum communication technologies. It is anticipated that photonic integration will eventually assume a crucial role in building various quantum networks and potentially a global quantum internet, reshaping the landscape of future communication methodologies.
