Optical Quantum Chips for Quantum Communications and the Internet Overview

In recent years, emerging optical quantum chips have made significant progress in the fields of quantum communication and quantum Internet. Characterized by scalability, stability and low cost, optical quantum chip chips open up new possibilities for miniaturized applications.

 

 

An article published on July 14 in LIGHT: Science & Applications outlines advances in optical quantum chips for quantum communications. The review summarizes the challenges of achieving high-performance chip-based quantum communications and looks at future opportunities for integrated quantum networks.

 

"Recent progress in quantum photonic chips for quantum communication and internet."

 

Quantum communication applies the principles of quantum mechanics to quantum information transfer, radically improving security, computing, sensing, and metrology. This field encompasses a wide range of technologies and applications, from state-of-the-art laboratory experiments to commercial realities.

 

The most famous example is quantum key distribution (QKD), the basic idea of which is to utilize the quantum state of a photon to share a key between two distant parties. The quantum unclonability theorem empowers the communicating parties to detect any eavesdroppers trying to obtain the key. Since security here is based on the laws of quantum physics rather than computational complexity, QKD is considered an ideal solution to the threats posed by emerging quantum computing hardware and algorithms.

 

Despite the controversy surrounding its practical security, QKD is leading the way in real-world applications. For example, fiber-optic and satellite-to-ground based QKD experiments have been demonstrated over 800 km of ultra-low-loss fiber optics and 2000 km in free space, respectively. The maximum secure key transfer rate for a single channel has exceeded 110 Mbit/s. Several field-test QKD networks have been established in Europe, Japan, China, and the UK. In addition, in order to overcome the current technical limitations, the security of practical QKD systems has been intensively studied. Post-quantum cryptography is combined with QKD to realize short-term security of authentication and long-term security of keys.

 

In addition to QKD, quantum invisible transmission has also attracted wide attention, which utilizes quantum entanglement to transmit fragile quantum information in an effectively unbreakable manner. On this basis, quantum networks can connect a variety of quantum devices to enable unparalleled functionality that is not possible using classical information technology alone. Quantum secure direct communication (QSDC) is another important branch of quantum communication that also offers opportunities for secure data transmission. This technology has evolved rapidly in recent years and enables users to directly transmit confidential information over secure quantum channels without sharing encryption keys. For example, a QSDC network with 15 clients has been demonstrated. Combined with post-quantum encryption, an end-to-end secure QSDC network can be built using existing technologies.

 

Conventional quantum communication systems are typically built using discrete optical devices. Typically, these devices are assembled from optical glass (e.g., fused silica and fused quartz) and optical crystals (e.g., calcite, barium beta borate, and lithium niobate), respectively, and are connected via free-space or optical fiber. While it is convenient to optimize individual components to meet the stringent requirements of ultra-low loss, high efficiency, high speed, and high fidelity for quantum information applications, interconnections and packaging have always been a major challenge for traditional discrete optical designs in terms of reliability and cost-especially when dealing with large-scale networks connecting hundreds of thousands of users.

 

For example, high mechanical and thermal stability is required to mitigate spatial and phase deviations due to ambient pressure and temperature variations, which are difficult to achieve through global stabilization in complex discrete optical systems. As a result, current bulky systems consisting of discrete optical components may have difficulty in meeting the growing demand for higher-capacity transmission capabilities, which demonstrates the great advantage of chip-scale quantum communication systems.

 

Optical quantum chips are ideal platforms for next-generation quantum technologies. Compared with discrete optical systems, optical quantum chips have two outstanding advantages in addition to miniaturization: namely, scalability and stability. Scalability is achieved because the chip and all its components are printed by photolithography, rather than being manufactured component by component. Stability is achieved because the circuits are built on a robust and compact solid-state platform that minimizes deviations caused by vibrations or temperature changes. These two advantages are critical to achieving the level of integration and performance required for quantum information processing and efficient quantum communications. In addition, optical quantum chips have great potential for low-cost production. While the initial cost of manufacturing the required photomasks is high, the average cost per chip can be significantly reduced through mass production.

 

After decades of effort, photonic integration has been realized in all aspects of a single quantum communication system, including photonic sources, encoding and decoding photonic circuits, and detectors. In principle, integrated photonic chips can combine many desirable characteristics required for quantum communication applications, such as efficiency, cost-effectiveness, scalability, flexibility, and performance. These characteristics, along with wafer-level fabrication processes, make chip-based quantum communication systems a compelling platform for future quantum technologies.

 

 

Three aspects of integrated quantum communications: photonic material platforms for large-scale integration; quantum photonic components, such as quantum light sources, high-speed modulators, and high-efficiency photodetectors; and typical applications in QKDs and quantum stealth transmissions.

 

 

Progress timeline for optical quantum chips for quantum communications. Key milestones include the first demonstration of an on-chip quantum interferometer for quantum encryption, quantum stealth transmissions on a photonic chip, chip-based DV-QKD, CV-QKD and MDI-QKD and chip-to-chip quantum stealth transmissions.

 

 

Latest technical specifications of monolithic integrated photonic platforms

 

1) Quantum Light Source

 

A photonic source capable of producing light in a specified quantum state is a key element of a quantum optical system. In general, quantum communication network architectures require single-photon states and entangled photon states , which can be obtained deterministically by single-photon emitters or probabilistically by parametric nonlinear processes.

 

Quantum dots, by virtue of the deterministic nature of their emission properties, are considered to be one of the most promising candidates for on-demand generation of single-photon or entangled-photon pairs. In particular, their small size and compatibility with semiconductor technology make them well suited for chip integration. In terms of single-photon generation, single InAs/GaAs self-assembled QDs and InGaAs QDs have achieved purity, extraction efficiency, and photon indistinguishability of 99.1%, 66%, 98.5% and 99.7%, 65%, 99.6%, respectively. However, these micropillar-based QD single photon sources have difficulties in waveguide integration due to their out-of-plane emission characteristics. Alternatively, QDs can be embedded in photonic crystal waveguide or heterogeneous waveguide structures to realize efficient coupling with waveguides. Entangled photon pairs can also be obtained by utilizing the double complex-exciton-exciton cascade radiation process in QDs. In addition to QDs, some other solid-state quantum emitters, such as color centers in diamond, silicon carbide, carbon nanotubes, and defects in two-dimensional materials, have also been studied and show great potential for generating single-photon or entangled-photon pairs on a chip.

 

 

On-chip quantum dot (QD) photon sources

 

Integrated probabilistic quantum light sources typically utilize spontaneous four-wave mixing (SFWM) or spontaneous parametric downconversion (SPDC) in optical waveguides or other photonic structures such as microdisks and ring resonators and photonic crystals. These nonlinear parametric processes are greatly enhanced on-chip due to the tight confinement of light, enabling efficient generation of high-quality photonic states in miniaturized configurations. In SFWM, the annihilation of two pump photons produces a pair of signal and inert photons, where the frequencies of the pump (ωp1, ωp2), signal (ωs), and inert photons (ωi) must obey ωp1 + ωp2 = ωs + ωi in order to maintain energy. In SPDC, a pump photon is split into a pair of signal and inert photons, where the frequencies of the pump (ωp), signal (ωs), and inert (ωi) must simultaneously satisfy ωp = ωs + ωi. Photon sources based on this three-photon process have been realized on platforms with second-order nonlinearities, and the main problem with these sources is that they produce photons in an indeterminate way and the production rate is limited by the brightness and the fundamental trade-off between multiphoton probability.

 

Multiplexing techniques offer a viable solution to these problems. For example, an integrated spatially multiplexed heralded single-photon source (HSPS) improves the single-photon generation probability by 62.4% for two individually pumped sources and 63.1% for two sources pumped through a common input. To further increase the efficiency, better ultra-low-loss and miniaturized delay lines are needed, as well as faster switches and faster electronics to synchronize the operation.

 

In practical quantum communication systems, single-photon and entangled-photon sources are not always needed. Weakly coherent pulses can be used as a reliable alternative to single-photon states for most preparation and measurement QKD applications, according to the decoy state protocol. Thus, an integrated photon source can be realized by simply attenuating the coherent pulses generated by on-chip lasers; such photon sources have already been demonstrated in several chip-based QKD systems.

 

 

Different types of chip-based parametric photon sources

 

2) Reconfigurable optical quantum elements

 

The manipulation of optical quantum states is crucial for quantum information processing in quantum communications, and this can be easily achieved using off-the-shelf integrated photonics passive and active elements.

 

In a typical quantum communication system, photons are usually processed in the polarization, phase, spatial, spectral and time domains. Therefore, it requires building blocks that can affect these degrees of freedom of photons, such as polarization splitters/rotators, phase shifters, intensity modulators, directional couplers, multimode interferometers (MMIs), ring resonators, and delay lines. Among them, phase shifters can be used for low-speed applications through the thermo-optic effect and high-speed applications through the bubbleglass effect.

 

Such devices have been demonstrated in a variety of integrated platforms, such as UV-written silicon-on-silicon photonic chips with thermo-optic phase shifters for quantum stealth state transfer, GaAs quantum photonic circuits with tunable Mach-Zehnder interferometers (MZIs) relying on the bubblegum effect, reprogrammable linear optical circuits consisting of arrays of 30 silicon-on-silicon waveguide directional couplers and 30 thermo-optic phase shifters, and large-size quantum photonic chips. size quantum photonic chips, and large silicon photonic quantum circuits integrating 16 SFWM photon pair sources, 93 thermo-optic phase shifters, and 122 MMI beam splitters. On-chip modulators based on the quantum confinement Stark effect (QCSE) can also be used for pulse generation and quantum bit coding at frequencies up to GHz. For polarization coding protocols, modulators based on polarization rotators and polarization beam splitters have been designed and demonstrated for generating BB84 polarized states.

 

 

Typical integrated components on an optical quantum chip

 

In addition to the components described above, additional integrated components are required for the optical connection between the quantum photonic chip and the optical fiber. When there is only one input or output polarization, one-dimensional grating couplers and out-of-plane couplers can be used. Otherwise, edge couplers, such as inverted-taper couplers for docking coupling, can be used when there are more polarizations and a wider spectral range. In addition, two-dimensional grating couplers supporting multi-polarization operation have been shown to convert path-encoded quantum bits into polarization-encoded quantum bits - the latter being more suitable for propagation in optical fibers.

 

3) Single Photon Detectors (SPDs) and Zero Difference Detectors

 

Efficient single-photon detection is important for quantum communication applications.

 

 

Detector overview

 

In particular, fully integrated single-photon diodes are highly desirable because of the unavoidable coupling losses that result from connecting to off-chip detectors. Recently, an integrated waveguide-coupled silicon-based Ge-on-Si transverse avalanche photodiode was demonstrated for single-photon detection with an efficiency of 5.27% at 1310 nm and a dark count rate of 534 kHz at 80 K. However, this single-photon avalanche photodiode tends to suffer from excessive dark counts at high efficiencies. As an alternative, single-photon avalanche photodiodes offer higher detection efficiency, lower time jitter, and photon number resolution (PNR) capability, which greatly reduces dark noise.

 

Waveguide-integrated SNSPDs have been reported on GaAs, Si, Si3N4, LN, and other platforms, with traveling-wave SNSPDs embedded in Si waveguides achieving detection efficiencies as high as 91% and dark count rates as low as 50 Hz, and demonstrating the on-chip compatibility of reconfigurable elements with SNSPDs at low temperatures. Waveguide PNR detectors can be realized by connecting multiple wires in series to form a pattern.

 

Equilibrium detectors have been widely utilized in continuously variable (CV) quantum information applications and are another key detection element for quantum measurements. Recent developments have greatly improved the performance of integrated synchrotron detectors, bringing them to a higher level in terms of small size, good stability, wide bandwidth, low noise and high common mode rejection. For example, a synchronous detector with a bandwidth of 150 MHz and a gap of 11 dB has been monolithically integrated on a silicon photonic chip. However, discrete amplification electronics greatly increase the device footprint. To reduce size and total capacitance, a silicon-germanium homodyne detector chip was integrated with the amplifier chip using wire bonding techniques, resulting in a 3-dB bandwidth of 1.7 GHz and shot-noise-limited bandwidths up to 9 GHz.

 

A similar approach has been used to build chip-scale InGaAs detectors consisting of low parasitic photodiodes and low-noise, high-speed transimpedance amplifiers. While it is convenient to employ commercial telecom transimpedance amplifiers, they typically introduce undesirable electrical noise. Co-designing and integrating a homodyne detector with a customized transimpedance amplifier effectively reduces noise and significantly improves performance, resulting in a shot noise-limited bandwidth of 20 GHz and a quantum shot noise removal rate of up to 28 dB.

 

4) Chip packaging and system integration

 

Although bare optical quantum chips can be characterized using a probe station, they must be packaged into rugged modules before a working prototype device can be developed. For this reason, a number of processes have been proposed to package photonic quantum chips into compact systems for practical applications.

 

In general, photonic packaging involves a range of technologies and capabilities required for the optical, electrical, mechanical, and thermal connection between the photonic chip and the off-chip components in the photonic module. Fiber-to-chip coupling is one of the best known aspects. The main challenge in coupling between an optical fiber and a typical waveguide on a chip is the large difference between their mode field diameters (MFDs). For example, a telecom single-mode fiber (SMF) has a mode-field diameter of about 10 μm at 1550 nm, whereas the corresponding ribbon silicon waveguide typically has a cross-section of only 220 × 450 nm. This mismatch can be mitigated by using configurations that can efficiently extract the modes from the waveguide, such as inverted-tapered fringing couplers coupled to lensed SMF fibers or ultrahigh-numerical-aperture fibers, and grating couplers coupled to SMF fibers, and grating couplers coupled to SMF fibers. For the grating coupler approach, coupling efficiencies of up to 81.3% (-0.9 dB) can be achieved on a 260 nm-thick SOI platform without the need for a back reflector or cover layer. Additionally, edge couplers fabricated on 200-mm SOI wafers have efficiencies in excess of 90 percent; coupling losses of about 1 dB at 1550 nm wavelengths have been reported.

 

Accessing the electronics on an optical quantum chip requires the use of electronic packages to transmit signals from electronic drivers, amplifiers and other control circuits. This is typically accomplished by connecting to a specialized printed circuit board (PCB). The connection between the PCB and the bonding pads on the chip is usually made using bonding wires. When a large number of electrical connections are required, or sub-nanosecond precision control of multiple channels is needed, 2.5- or 3-dimensional integration can be accomplished with custom electronic integrated circuits (EICs). This integration can be accomplished by interconnecting solder ball bumps or copper post bumps, providing a robust electrical, mechanical, and thermal interface to the photonic chip.

 

Global thermal stabilization of photonic devices is critical for prototypes requiring high accuracy and repeatability or for field testing where seasonal temperature fluctuations are common. This can be achieved through passive cooling techniques or thermoelectric coolers (TEC). The added overall stability of thermoelectric coolers allows for more efficient and repeatable localized temperature regulation of individual photonic components on the chip (e.g., micro-ring resonators, thermo-optic phase shifters, etc.). In addition, liquid cooling can be installed to further increase the cooling capacity of the system.

 

 

Examples of chip packaging and integration

 

As the most developed quantum secure communication technology, QKDs based on bulk or fiber optic assemblies have been used by banks and governments to provide a high level of security for data transmission. However, a wider range of applications require QKD systems to be more robust, compact, and able to be mass manufactured at a lower cost. In this section, we describe recent efforts to realize a fully chip-based QKD platform at the system level.

 

 

Typical integrated QKD implementations

 

1) Quantum Random Number Generator (QRNG)

 

The security of encryption depends on the quality or unpredictability of the key, which means that a true random number generator is an essential component of a quantum secure communication system. While pseudo-random numbers are easy to create, their inherent deterministic behavior prevents them from being considered truly unpredictable. QRNGs have therefore been developed to generate truly random numbers that are unpredictable, unrepeatable, and unbiased, all of which are guaranteed by the fundamental principles of quantum physics.

 

 

Integrated Quantum Random Number Generator (QRNG)

 

The most commonly used QRNG protocols include quantum phase fluctuation schemes and vacuum state schemes. These schemes easily achieve random bit rates up to Gbps by using photodetectors instead of single photon detectors. In addition to real-time output speed, module size is a key parameter for practical applications of QRNG.

 

The emerging technology of integrated optical quantum mechanics offers considerable advantages in terms of size reduction. Recently, a number of integrated QRNG implementations have been demonstrated utilizing various integration techniques of varying complexity. Using multiplexed detectors, a LiNbO3-based platform for QRNG achieved a real-time rate of 3.08 Gbps, while the InP platform constructed a quantum entropy source. Since the SOI platform has higher integration density and more mature technology compared with the III-V system, the realization of QRNG by measuring phase fluctuation and vacuum state on the SOI platform has also been reported.

 

However, it is worth noting that the germanium photodiode on SOI generates a large dark current, which degrades the performance of on-chip QRNG and requires careful optimization to mitigate. Alternatively, an integrated QRNG based on InGaAs photodiodes was constructed with a high-bandwidth transimpedance hybrid amplifier packaged with the SOI chip, with a real-time output rate of 18.8 Gbps. Another integrated QRNG is based on parallel arrays of independent single-photon avalanche diodes, uniformly illuminated by DC-biased light-emitting diodes, and co-integrated with logic circuits for post-processing. The CMOS-based QRNG achieves real-time bit rates of up to 400 Mbps. Recently, a record generation rate of 100 Gbps was achieved through custom co-design of opto-electronic integrated circuits and reduction of side information through digital filtering, using a SOI photonic chip co-packaged with GaAs transimpedance amplifier circuits.

 

2) DV-QKD System

 

In a typical QKD implementation, the key is encoded as a discrete variable (DV), such as the polarization or phase of a photon. A prominent example of such a DV-QKD protocol is the decoy state BB84, which has been widely used in state-of-the-art commercial applications. According to the protocol, a light source, modulator, single-photon detector, and basic passive optical components form the main framework of a DV-QKD system. The photonic integration of these elements begins with the asymmetric PLC MZI for differential phase-shifted QKD experiments. the on-chip interferometer operates more accurately and stably in phase decoding than its fiber-based counterpart.

 

Subsequently, a series of compact QKD devices were demonstrated. For example, miniature QKD transmitters were fabricated with dimensions similar to those of an opto-electronic modulator, containing a distributed feedback laser and a modulator. This small transmitter generates 1550 nm weakly coherent pulses encoded in BB84 polarized and decoy states. A client integrated in a handheld device receives the weak laser pulses from the QKD server and then attenuates and encodes each pulse before sending the information bits back to the server. In addition, the design and evaluation of a handheld QKD transmitter module is presented based on an integrated optical structure with an effective size of 25 mm × 2 mm × 1 mm. In this module, four vertical-cavity surface-emitting lasers are coupled to four miniature polarizers fabricated by focused ion-beam milling for generating polarized quantum bits. These quantum bits are combined with an array of waveguides fabricated from borosilicate glass to ensure spatial overlap.

 

The previously discussed devices demonstrate the feasibility of a partially integrated QKD system. However, a fully chip-based system is essential for improved performance, miniaturization, and increased functionality required for practical deployment. In fact, modulatorless QKD emitter chips can be realized based on the direct phase modulation method recently introduced in bulk optical emitters. Using a modulatorless chip, secure key rates of 270 kbps and 400 kbps at 20 dB attenuation have been achieved for the decoy state BB84 and distributed phase-shift protocols, respectively.Recently, a fully self-contained QKD system has been developed based on InP photonic integrated circuits assembled into compact modules. The system integrates a quantum transmitter, receiver, and QRNG chip and is capable of quantum random number generation and key distribution at gigahertz clock rates.

 

 

Chip-based QKD systems with hybrid material platforms

 

Silicon photonics is another attractive platform for all-chip QKD systems. Although the integration of light sources and SPDs remains challenging, several proof-of-principle demonstrations of silicon based QKD devices have been reported in recent years. Other demonstrations using silicon photonic technology have also been reported, including an integrated state encoder for free-space daylight QKD, a silicon photonic QKD transceiver based on a time-sharing protocol, a silicon photonic transmitter for high-speed distributed phase-reference QKD, and an integrated QKD receiver for multiusers.

 

 

Silicon Photonic Chips for Multiple QKD Protocols

 

Recently, the implementation of advanced QKD protocols using systems-on-chip has attracted more interest because these protocols would greatly benefit from photonic integration. High-dimensional QKD protocols based on multicore fiber optic space division multiplexing have been demonstrated using silicon photonic integrated circuits. These circuits provide a more efficient way to create high-dimensional quantum states with low and stable QBERs well below the coherent attack and individual attack limits. In addition, Measurement Device Independent (MDI) QKD eliminates all side-channel detection vulnerabilities and is well suited for chip-based client-server scenarios, where the client holds a low-cost photonic chip and the server acts as an untrusted node integrating the most costly components that can be shared among multiple users.

 

 

Different chip-based quantum communication systems for advanced QKD protocols

 

3) CV-QKD system

 

In addition to DV-QKD, several QKD protocols have been proposed to encode critical information as continuous variables, such as the values of the orthogonal components of a quantized electromagnetic field. A major technical difference is that the implementation of CV-QKD requires only homodyne detectors instead of the dedicated SPDs used in DV-QKD.This feature eliminates the need for additional cryogenic systems and greatly simplifies the detection setup. Thus, CV-QKD is naturally suited for photonic integration and is compatible with chip-based coherent detection schemes used in classical high-bandwidth communication systems. Indeed, a silicon photonic transceiver design has been proposed that includes all major CV-QKD components as well as complete subsystems; the feasibility of a homodyne detector integrated on a photonic chip to measure quantum states and generate random numbers has been demonstrated. Recently, a stable and miniaturized CV-QKD system compatible with existing fiber-optic communication infrastructures has been realized by integrating all optical components (except the laser source) on a silicon photonic chip.

 

The proof-of-principle demonstrated that the system is capable of generating a key rate of 0.14 kbps (under collective attack) over a simulated fiber distance of 100 km. The performance of the on-chip CV-QKD system can be improved by further optimizing the detection module. For example, a high-speed homodyne detector has been realized by connecting CMOS-compatible silicon and silicon-germanium nanophotonic elements with silicon-germanium integrated amplification electronics: the detector has a 3-dB bandwidth of 1.7 GHz, a shot-noise limit of 9 GHz, and requires only 0.84 square millimeters of miniature substrate surface.

 

 

Integrated Circuits for Continuously Variable (CV) QKD and High-Speed Homodyne Detection

 

Quantum invisible state transfer has been demonstrated for many platforms ranging from superconducting quantum bits, trapped atoms, nitrogen vacancy centers to continuously variable states. Among these implementations, the optical quantum bit is one of the most promising candidates for building quantum channels in quantum networks because of its robustness in noisy environments and ease of operation at room temperature. In addition, it can withstand longer propagation distances and minimize interference from the surrounding environment. To date, optical quantum invisible transmission has been experimentally realized in various ways, including through free-space and fiber-optic systems.

 

 

Chip-based quantum invisible state transfer and entanglement distribution system

 

Quantum stealth transmissions were first experimentally demonstrated when quantum bits were encoded in the polarization of photons generated by BBO crystals in a free-space system on an optical bench. Later, records of free-space transmission between the Mercury satellite and ground stations were pushed to over 1,400 kilometers - an achievement that paved the way for a globally interconnected quantum network. However, given the challenges of free-space transmission in terms of beam divergence, pointing, and collection, fiber-optic systems are more promising for cost-effective metropolitan quantum networks; currently, the farthest distance achieved based on optical fibers is 102 km.

 

One of the main challenges for invisible state transfer of optical quantum bits is that the theoretical efficiency of Bell state measurements is only 50% when using linear optics. In order to overcome this limitation, continuously variable optical modes can be used as an alternative method to achieve fully deterministic state teleportation. This approach has been demonstrated on a 6 km long fiber channel. However, since this scheme is sensitive to channel losses, its fidelity still needs to be improved. For other types of quantum bits, record distances of 21 meters have been achieved using captured atom systems.

 

As quantum stealthy state transfer continues to move towards real-world applications, integration is becoming increasingly important as a key technology. In future quantum networks, it will be possible to embed stealth transmitting chips in stationary hardware (e.g., repeaters in space stations) or mobile hardware (e.g., drones), transforming these devices into lightweight and compact quantum nodes. This would allow remote access to quantum devices, sharing quantum information or unleashing greater computing power. Such advances are possible because of the ability to generate and manipulate on-chip entangled photon pairs with different degrees of freedom, such as path-encoded entangled states in MZI, polarization-encoded entangled states engineered through birefringent structures, and time-banded entangled states in Fransson interferometers.

 

The first on-chip invisible transmitted state reportedly used an off-chip photon source with a fidelity of 0.89, although it was realized within a single chip. Recent technological advances in integrated optical quantum mechanics have enabled the realization of entanglement-based quantum communication protocols beyond a single chip. The first demonstration of chip-to-chip entanglement distribution was achieved with all key components integrated monolithically on a silicon photonic chip. On-chip entangled Bell states were generated and distributed from one quantum bit to another silicon chip by converting on-chip path-coded states and in-fiber polarization states via a two-dimensional grating coupler. In addition, more quantum circuits integrating on-chip sources realized inter-chip invisible state transfer with a fidelity of 0.88. This chip-scale demonstration of optical quantum bit production, processing, and transmission points to a promising path for a distributed quantum information processing Internet.

 

In addition, entangled photon pairs in the visible range were demonstrated on a Si3N4 chip with a well-designed micro-ring resonator, and further distributed over a range of 20 km. High photon number purity and brightness were achieved at a low pump consumption of a few hundred microwatts. Importantly, it provides an entangled link between visible-band photons, which can be connected to quantum memories, and telecom-band photons, which feature low-loss transmission in optical fibers.

 

This review discusses the rapid advances in chip-based quantum communications relying on the development of integrated photonic quantum mechanics. Photonic integration not only provides a solid strategy for miniaturization and scaling of quantum communication systems, but also facilitates the practical application of quantum communication and paves the way for future quantum communication networks and quantum Internet.

 

Despite the strides that have been made, the field of on-chip quantum communication is still in its early stages and naturally faces many challenges. In terms of components, on-chip components used in quantum communications require stricter specifications than those used in classical optical communications to ensure high fidelity and prevent decoherence of quantum states during preparation, manipulation, transmission and detection. Therefore, it is crucial to explore components with suitable properties. For example, high key rate QKDs require modulators that can operate at high clock rates while maintaining acceptable extinction ratios to reduce crosstalk between different quantum states. However, conventional silicon-based modulators do not always meet this requirement because carrier injection or carrier depletion techniques result in non-ideal loss characteristics. Fortunately, recent advances in ultra-high extinction (>65 dB) silicon modulators based on cascaded MZI structures, as well as LN180, Si-LN40, and barium titanate silicon modulators based on the electro-optic bubbleglass effect, offer possible solutions to this problem.

 

On the system side, fully integrated quantum communication systems containing photonic sources, photonic circuits and detectors have not yet been realized. The difficulty in realizing full integration lies in two challenges:

 

- The first challenge is that there is no single monolithic platform that can provide all the functionality required for quantum communication applications. Hybrid integration may be a viable solution to this problem; however, the technology is still under development and more effort is needed to achieve the ultimate goal.

 

- The second challenge is that different parts of an integrated quantum system may operate under different conditions. For example, QD single-photon sources and single-photon detectors typically operate at low temperatures. In contrast, conventional integrated modulators and thermo-optical phase shifters, which are designed for room temperature applications, do not work well under such extreme conditions. Therefore, manipulating photons at low temperatures has become a critical factor for fully integrated systems.

 

In terms of security, chip-based quantum communications face potential vulnerability threats due to the specific shortcomings of integrated photonic devices. For example, phase- and polarization-related losses are significant issues in quantum photonic chips, which, if left unchecked, may lead to overestimation of key rates, thus compromising the security of QKD systems. To address these issues, a post-selection scheme has recently been proposed that provides high key generation rates even in the presence of severe phase- and polarization-related losses. Experiments on BB84 QKD with a deceptive state considering polarization-related losses utilized this scheme and successfully distributed secure key bits over a 75 km long fiber optic link. In addition, the chip-based CV-QKD system reveals and analyzes the security vulnerabilities introduced by the plasma dispersion effect of free carriers and the integrated electronic control circuitry of the transmitter.

 

Since governmental organizations such as the U.S. National Security Agency (NSA) and the U.K.'s National Cyber Security Centre (NCSC) are still skeptical about the practical application of QKD, further comprehensive security analysis studies are needed to bridge the gap between theoretical models and actual integrated quantum communication systems.

 

In addition to preparing and measuring QKD, entanglement-based QKD is another promising application for future chip-based QKD systems. This application has become possible since the generation of time-band entangled states in GaAs, Si, and Si3N4 chips and the demonstration of chip-to-chip entanglement distribution and quantum invisible state transfer between two programmable Si chips.

 

Currently, on-chip quantum stealth state transfer is mostly based on backward and passive protocols. Future work may include feed-forward control by upgrading quantum communication systems from passive to active, so that receivers can apply conditional unit operations in real time to reconstruct quantum states. In addition, large-scale implementations of long-distance entanglement distribution and quantum invisible state transfer, as well as quantum networks, rely on quantum memories and quantum repeaters. For example, quantum memories in quantum nodes can create entanglement between long-distance parties, thereby extending the communication distance. However, experimental development of integrated quantum memories is still in its infancy. Much work remains to be done to achieve integrated quantum relays in the telecom band that are compatible with long-distance fiber-based quantum communication systems.

 

In conclusion, optical quantum chips have rapidly matured into a versatile platform that is invaluable in the development of cutting-edge quantum communication technologies. Considering these impressive results, it is expected that photonic integration will eventually play a key role in building various quantum networks and potentially a global quantum Internet, reshaping the landscape of future communication methods.