Electronics Guide

BB84 Protocol

The BB84 protocol, developed by Charles Bennett and Gilles Brassard in 1984, was the first quantum key distribution (QKD) scheme. The sender (Alice) transmits single photons encoded in one of two randomly chosen bases: rectilinear (horizontal/vertical polarization) or diagonal (45/135 degree polarization). The receiver (Bob) measures each photon using a randomly chosen basis.

After transmission, Alice and Bob publicly compare their basis choices without revealing the actual bit values. They keep only the bits where they happened to choose the same basis, discarding the rest. By sacrificing a portion of these matching bits to check for errors, they can detect eavesdropping. High error rates indicate interception, while low rates confirm a secure key has been established.

E91 Protocol

Artur Ekert's E91 protocol uses entangled photon pairs to establish secure keys. A source generates pairs of photons in an entangled state and sends one photon to each party. Both parties measure their photons using randomly chosen measurement angles. The quantum correlations between entangled photons enable key generation while also providing security verification.

The security of E91 rests on Bell's theorem: any eavesdropper attempting to gain information about the key must disturb the quantum correlations in ways detectable through Bell inequality violations. This device-independent security means the protocol remains secure even if the measurement devices are partially compromised.

Continuous-Variable QKD

Continuous-variable QKD encodes information in the continuous properties of light, such as the amplitude and phase quadratures of coherent states. Rather than detecting single photons, CV-QKD uses homodyne or heterodyne detection with standard telecommunications photodetectors. This approach enables higher key rates over shorter distances and compatibility with existing fiber infrastructure.

The security of CV-QKD relies on the quantum uncertainty principle: precise measurement of one quadrature introduces uncertainty in the conjugate quadrature. Protocols such as GG02 (Gaussian-modulated coherent states) provide practical implementations with proven security against collective attacks.

Decoy-State Protocols

Practical QKD implementations often use attenuated laser pulses rather than true single photons. Multi-photon pulses create vulnerabilities to photon-number-splitting attacks. Decoy-state protocols address this by randomly varying the intensity of transmitted pulses. Analysis of detection rates for different intensities allows estimation of the secure key rate even with imperfect sources, enabling practical long-distance QKD.

The Distance Challenge

Optical fiber and atmospheric transmission channels absorb and scatter photons exponentially with distance. At typical telecom wavelengths, fiber losses of 0.2 dB/km limit direct QKD to roughly 100-200 kilometers before signal levels become impractical. Unlike classical signals, quantum signals cannot be amplified without destroying their quantum properties, as the no-cloning theorem forbids copying unknown quantum states.

Quantum Repeater Architecture

Quantum repeaters overcome distance limitations through entanglement swapping and quantum error correction. The communication channel is divided into segments, with quantum memories at each node storing entangled states. Through entanglement swapping operations, short-range entanglement is extended across multiple segments to create long-distance entangled pairs.

First-generation repeaters use probabilistic entanglement generation and purification, requiring quantum memories to hold states while multiple attempts succeed. Second-generation designs incorporate quantum error correction to improve efficiency. Third-generation repeaters aim for fault-tolerant operation with encoded logical qubits, enabling deterministic long-distance entanglement distribution.

Quantum Memory Technologies

Quantum memories store quantum states for later retrieval, essential for synchronizing probabilistic operations in quantum repeaters. Technologies include atomic ensembles using electromagnetically induced transparency, nitrogen-vacancy centers in diamond, rare-earth ion-doped crystals, and trapped ions. Key metrics include storage time, efficiency, fidelity, bandwidth, and wavelength compatibility with telecommunications infrastructure.

Entanglement Purification

Imperfect channels and operations degrade entanglement quality. Entanglement purification protocols combine multiple low-fidelity entangled pairs to produce fewer pairs with higher fidelity. Through local operations and classical communication, parties can distill high-quality entanglement from noisy resources, essential for reliable quantum communication over long distances.

Teleportation Principles

Quantum teleportation transfers an unknown quantum state from one location to another using entanglement and classical communication. The sender performs a joint measurement on the state to be teleported and their half of an entangled pair, then communicates the measurement result classically. The receiver applies a corresponding operation to their entangled particle, recreating the original state.

Importantly, teleportation does not violate the no-cloning theorem: the original state is destroyed during the sender's measurement. It also does not enable faster-than-light communication, as the classical message is essential for completing the protocol. Teleportation provides a primitive for quantum communication, enabling state transfer without direct quantum channel transmission.

Implementation Technologies

Photonic teleportation uses entangled photon pairs and Bell state measurements. Linear optical implementations are inherently probabilistic, succeeding at most 50% of the time without ancillary resources. Matter-based systems using trapped ions or superconducting qubits can achieve deterministic teleportation within local systems. Hybrid approaches teleport states between different physical systems.

Long-Distance Teleportation

Extending teleportation to long distances requires distributing entanglement across the channel. Ground-based demonstrations have achieved teleportation over 100 kilometers through optical fiber and free-space links. Satellite-based systems have demonstrated teleportation over 1,200 kilometers, with the Micius satellite serving as an entanglement source between ground stations in China and Austria.

Network Topology

The quantum internet will comprise quantum nodes connected by quantum channels, overlaid on classical network infrastructure. End nodes include quantum computers, sensors, and secure communication terminals. Intermediate nodes provide quantum repeater functionality for long-distance entanglement. Network architecture must support routing of entanglement, resource allocation, and integration with classical networking protocols.

Protocol Stack

Quantum network protocols span multiple layers analogous to classical networking. The physical layer handles photon transmission and detection. The link layer manages entanglement generation between adjacent nodes. The network layer routes entanglement across multiple hops. The transport layer provides end-to-end entanglement with quality guarantees. Application layers support QKD, distributed quantum computing, and quantum sensing applications.

Stages of Development

The quantum internet is evolving through stages of increasing capability. Current networks support prepare-and-measure QKD. Near-term developments will enable entanglement distribution between nodes. Quantum memory integration will allow entanglement storage. Full quantum repeaters will extend range. Eventually, a fault-tolerant quantum internet will support arbitrary quantum operations between any connected nodes, enabling distributed quantum computing.

Hybrid Classical-Quantum Networks

Practical quantum networks will integrate closely with classical infrastructure. Classical channels carry synchronization signals, measurement results, and protocol coordination. Wavelength-division multiplexing can share fiber between quantum and classical signals. Network management, routing decisions, and security protocols require classical computation, making hybrid architectures essential for practical deployment.

Entanglement Sources

Reliable entanglement sources are fundamental to quantum networks. Spontaneous parametric down-conversion in nonlinear crystals generates photon pairs entangled in polarization, time-energy, or spatial modes. Quantum dots and atomic systems can produce entangled photons deterministically. Source metrics include pair generation rate, entanglement fidelity, photon indistinguishability, and wavelength compatibility.

Distribution Channels

Optical fiber provides a natural channel for photon transmission but introduces loss and polarization mode dispersion. Free-space channels avoid fiber losses for satellite links but require precise pointing and are affected by atmospheric turbulence. Wavelength conversion interfaces connect different spectral regions, enabling optimal wavelengths for sources, channels, and detectors.

Network Entanglement Management

Managing entanglement as a network resource requires new protocols for generation, storage, routing, and consumption. Entanglement must be generated, stored, and delivered to applications on demand. Resource allocation algorithms must balance competing requests while accounting for decoherence. Entanglement fidelity tracking ensures quality of service for applications with different requirements.

Quantum Randomness Sources

Quantum random number generators (QRNGs) produce true randomness derived from quantum measurements rather than algorithmic pseudo-randomness. Sources include photon detection timing, vacuum fluctuations, radioactive decay, and spin measurements. Unlike classical random number generators, QRNGs produce output that is fundamentally unpredictable, not merely computationally unpredictable.

Device-Independent Randomness

Device-independent QRNGs certify randomness without trusting the internal operation of the devices. By testing Bell inequality violations, users can verify that outputs are genuinely random regardless of device construction. This provides the highest security guarantee, though at the cost of lower generation rates and more complex implementations.

Applications and Standards

QRNGs provide randomness for cryptographic key generation, scientific simulations, gaming, and statistical sampling. Commercial QRNGs are available as standalone devices, PCIe cards, and integrated modules. Standards for QRNG certification and testing are being developed by organizations including NIST and the European Telecommunications Standards Institute.

Signature Principles

Quantum digital signatures (QDS) provide message authentication with information-theoretic security, unlike classical digital signatures that rely on computational assumptions. QDS protocols distribute quantum states that enable recipients to verify message authenticity while preventing forgery or repudiation. Multiple recipients can independently verify signatures without the ability to forge them.

Protocol Implementations

Early QDS protocols required quantum memory, limiting practicality. Modern protocols use phase-encoded coherent states or similar prepare-and-measure schemes compatible with QKD technology. The sender distributes signature states to recipients, who later use these to verify signed messages. Security analysis must account for multiple potentially colluding recipients.

Integration with Classical Systems

QDS complements classical signature schemes in hybrid security architectures. For documents requiring long-term security guarantees, quantum signatures provide protection against future quantum computer attacks. Integration requires protocols for signature distribution, verification, and interoperability with existing document and transaction systems.

Secret Sharing Concepts

Quantum secret sharing distributes a secret among multiple parties such that only authorized subsets can reconstruct it. In a (k, n) threshold scheme, any k of n parties can recover the secret, but fewer than k parties gain no information. Quantum implementations use entangled states, with different parties holding components of a larger entangled system.

Applications in Distributed Systems

Quantum secret sharing enables secure distributed storage, where data remains protected even if some storage nodes are compromised. In distributed quantum computing, secret sharing protects sensitive inputs while allowing collaborative computation. Multi-party quantum protocols for voting, auction, and contract verification build on secret sharing primitives.

Implementation Considerations

Practical quantum secret sharing requires efficient entanglement distribution to all parties and high-fidelity measurements. Graph state approaches provide flexible access structures beyond simple thresholds. Verifiable secret sharing adds protocols for parties to confirm they received valid shares, protecting against malicious dealers.

Trust Assumptions

Standard QKD protocols assume trusted, correctly-functioning equipment. Device-independent protocols remove this assumption, certifying security based only on observed measurement statistics. Even if devices are manufactured by adversaries or contain hidden functionality, security can be guaranteed through Bell test violations. This provides the strongest possible security foundation.

Bell Tests and Security

Device-independent security relies on Bell inequality violations that are only possible with genuine quantum entanglement. No classical hidden-variable theory can reproduce quantum correlations, so observing these correlations certifies the quantum nature of the devices. Security proofs connect Bell violation magnitude to the amount of secure key that can be extracted.

Implementation Challenges

Device-independent protocols require closing experimental loopholes that might allow classical explanations of apparent Bell violations. The detection loophole requires high detection efficiency; the locality loophole requires space-like separation of measurements. Recent experiments have achieved loophole-free Bell tests, enabling practical device-independent protocols, though key rates remain lower than device-dependent alternatives.

Free-Space Quantum Channels

Satellite links overcome the distance limitations of fiber-based QKD by transmitting through the atmosphere and space. Above the dense lower atmosphere, free-space loss scales only as distance squared (beam diffraction) rather than exponentially. This enables quantum communication over thousands of kilometers, connecting continents and enabling global quantum networks.

Micius Satellite Achievements

China's Micius satellite, launched in 2016, demonstrated key satellite quantum communication capabilities. These include satellite-to-ground QKD over 1,200 kilometers, intercontinental quantum key distribution between China and Austria, satellite-based entanglement distribution to two ground stations separated by 1,200 kilometers, and ground-to-satellite quantum teleportation. These experiments proved the feasibility of global quantum networks.

Technical Challenges

Satellite quantum communication faces challenges including atmospheric turbulence, precise pointing requirements, background light rejection, satellite-ground synchronization, and limited observation windows. Adaptive optics compensate for atmospheric distortion. Single-photon detectors must discriminate faint quantum signals from background counts. Daylight operation requires narrow spectral filtering and spatial mode selection.

Future Satellite Networks

Multiple nations and organizations are developing quantum satellite capabilities. Constellations of quantum-enabled satellites could provide continuous global coverage. Integration with ground-based quantum networks will create hybrid systems with both terrestrial and space segments. Intersatellite quantum links may eventually enable direct space-based entanglement distribution without ground involvement.

Single-Photon Sources

Ideal QKD requires sources emitting exactly one photon per pulse. Practical sources include attenuated lasers with decoy-state protocols, spontaneous parametric down-conversion with heralding, quantum dots, and color centers. Source characteristics including wavelength, emission rate, photon purity, and indistinguishability affect system performance. Fiber-compatible telecom wavelengths (1310 nm, 1550 nm) minimize transmission loss.

Single-Photon Detectors

Detecting individual photons requires highly sensitive detectors. Avalanche photodiodes operate in Geiger mode, generating macroscopic signals from single photons. Superconducting nanowire detectors offer higher efficiency and lower timing jitter but require cryogenic cooling. Detector characteristics including efficiency, dark count rate, timing resolution, and dead time directly impact QKD key rates and maximum distances.

Quantum State Preparation and Measurement

Encoding quantum information requires precise control of photon properties. Polarization encoding uses wave plates and polarizers. Phase encoding modulates relative phases between time bins or path modes using interferometers. Measurement apparatus must distinguish between encoding states with high fidelity. Active stabilization systems maintain alignment against environmental drift.

Classical Control Electronics

QKD systems require sophisticated classical electronics for timing, synchronization, and data processing. Precise clock distribution coordinates transmitter and receiver operations. Fast random number generators select encoding bases. Time-tagging electronics record detection events. Classical post-processing computes error rates, performs error correction, and extracts secure keys.

Security Proofs

QKD security proofs establish conditions under which protocols are secure against arbitrary attacks. Proofs must account for finite-key effects in practical implementations, device imperfections, and various attack models. Security claims range from unconditional security against any quantum attack to practical security against known attack strategies.

Side-Channel Attacks

Real QKD devices may leak information through unintended side channels. Attacks have exploited detector efficiency mismatches, timing correlations, trojan horse vulnerabilities, and laser seeding attacks. Countermeasures include measurement-device-independent protocols, monitoring of device parameters, and careful engineering to eliminate information leakage.

Certification and Standards

Certification frameworks verify that QKD implementations meet security claims. Standards from ETSI, ITU, and ISO define QKD interfaces, performance metrics, and security evaluation criteria. Common criteria evaluation provides independent assessment of security properties. Ongoing standardization efforts address interoperability, testing procedures, and integration with classical security infrastructure.

Metropolitan QKD Networks

QKD networks operate in several cities worldwide, including Beijing, Shanghai, Tokyo, Vienna, and Geneva. These networks typically use trusted node architectures, where intermediate nodes are physically secured and temporarily decrypt traffic for relay. Key management systems distribute quantum-generated keys to encrypt traffic between network endpoints.

Financial and Government Applications

Banks, financial institutions, and government agencies have deployed QKD to protect high-value communications. Applications include securing interbank transactions, protecting classified government communications, and safeguarding critical infrastructure. The long-term security guarantee of QKD protects against future cryptanalytic advances and quantum computing threats.

Commercial QKD Systems

Commercial vendors offer QKD systems for point-to-point and network deployments. Products include compact modules for data center integration, rack-mounted systems for telecommunications, and turnkey network solutions. Key rates range from kilobits to megabits per second over metropolitan distances. Integration with classical encryptors enables transparent security for standard network traffic.

Integration with Quantum Computing

The quantum internet will connect quantum computers for distributed quantum computing. Blind quantum computation allows clients to perform computations on remote quantum servers without revealing algorithms or data. Quantum secure direct communication transmits messages rather than keys. These capabilities will emerge as quantum repeaters enable high-fidelity entanglement distribution.

Chip-Scale Integration

Integrated photonic circuits promise compact, low-cost quantum communication systems. Silicon photonics and other platforms integrate sources, modulators, filters, and detectors on single chips. Chip-scale devices will enable widespread deployment in data centers, mobile devices, and IoT applications, democratizing access to quantum security.

Standardization and Interoperability

Mature quantum communication infrastructure requires comprehensive standards for protocols, interfaces, and certification. Efforts by ETSI, ITU, IEEE, and national standardization bodies are defining requirements for commercial deployment. Interoperability between vendors will enable competitive markets and prevent technology lock-in.