Quantum repeaters, entanglement swapping, and quantum key distribution (QKD) hardware form the backbone of long-distance quantum communication networks. These technologies rely on precise material engineering and photonic device design to enable secure data transmission and entanglement distribution across vast distances. The following sections detail the underlying principles, hardware components, and material systems critical to their operation.

Quantum repeaters are essential for overcoming the exponential loss of photons in optical fibers, which limits the range of quantum communication. A quantum repeater node typically consists of three key elements: quantum memories, entanglement sources, and Bell-state measurement (BSM) units. Quantum memories store photonic qubits until entanglement swapping can be performed, with rare-earth-doped crystals like europium-doped yttrium orthosilicate (Eu:Y2SiO5) or praseodymium-doped yttrium orthovanadate (Pr:YVO4) being common choices due to their long coherence times. These materials exhibit narrow inhomogeneous linewidths and can be controlled via spectral hole burning, enabling efficient storage and retrieval of quantum states. Entanglement sources often employ spontaneous parametric down-conversion (SPDC) in nonlinear crystals such as beta barium borate (BBO) or periodically poled lithium niobate (PPLN). These crystals generate entangled photon pairs when pumped by a laser, with PPLN offering higher efficiency due to quasi-phase-matching. The BSM unit performs entanglement swapping by interfering photons from separate nodes and projecting them into a Bell state, requiring low-loss beam splitters and superconducting nanowire single-photon detectors (SNSPDs) made from materials like niobium nitride (NbN) or tungsten silicide (WSi).

Entanglement swapping is a protocol that extends entanglement across distant nodes without direct interaction. The process involves two independent entangled photon pairs, where one photon from each pair is sent to a central node for BSM. Successful measurement projects the remaining photons into an entangled state, even if they never interacted. This requires high-fidelity entangled photon sources, typically realized using type-II SPDC in nonlinear crystals. For telecom-band compatibility, PPLN waveguides are engineered to emit photon pairs at 1550 nm, matching the low-loss window of optical fibers. Indium gallium arsenide (InGaAs) avalanche photodiodes (APDs) or SNSPDs are used for detection, with the latter offering higher detection efficiency and lower dark counts. The success probability of entanglement swapping depends on the Hong-Ou-Mandel (HOM) visibility, which quantifies the indistinguishability of the interfering photons. Achieving high HOM visibility demands precise spectral and temporal matching, often accomplished using narrowband filters and dispersion-compensated fibers.

Quantum key distribution (QKD) hardware enables secure communication by distributing cryptographic keys encoded in quantum states. Two prominent protocols are BB84 and E91, which rely on single-photon transmission and entanglement, respectively. For BB84, weak coherent pulses from laser diodes are attenuated to the single-photon level, with indium phosphide (InP) lasers being the standard due to their reliability at 1550 nm. The photons are modulated using electro-optic modulators (EOMs) made from lithium niobate (LiNbO3), which impose phase or polarization shifts to encode the key. Polarization-encoding systems require high-extinction-ratio polarizers and calcite beam displacers, while phase-encoding systems use fiber-based Mach-Zehnder interferometers with active phase stabilization. For E91, entangled photon pairs generated via SPDC are distributed to two users, who perform measurements in randomly chosen bases. The hardware resembles that of entanglement swapping, with additional components for time-tagging and coincidence counting.

Nonlinear crystals play a central role in generating entangled photons for these applications. Beta barium borate (BBO) is widely used for type-I and type-II SPDC due to its high nonlinear coefficient and broad phase-matching range. However, its low damage threshold limits pump power, reducing pair generation rates. Periodically poled lithium niobate (PPLN) addresses this by enabling quasi-phase-matching, which allows longer interaction lengths and higher efficiencies. PPLN waveguides further enhance the efficiency by confining the pump light, with typical pair generation rates exceeding 10^6 pairs per second per mW of pump power. For telecom-band operation, PPLN is doped with magnesium oxide (MgO:PPLN) to reduce photorefractive damage, ensuring stable performance over time. Other materials like potassium titanyl phosphate (KTP) and lithium tantalate (LiTaO3) are also explored for their tailored dispersion properties and resistance to optical damage.

Telecom-band emitters are critical for minimizing fiber losses in quantum networks. InGaAs quantum dots grown on gallium arsenide (GaAs) substrates can emit single photons at 1300 nm or 1550 nm through careful strain engineering and bandgap tuning. These emitters exhibit high purity and indistinguishability, with g(2)(0) values below 0.01 and Hong-Ou-Mandel visibilities exceeding 90%. However, their inhomogeneous broadening necessitates spectral filtering, which reduces the usable photon rate. Alternatively, erbium-doped materials like erbium-doped yttrium orthosilicate (Er:YSO) can emit single photons at 1536 nm via optically pumped spin transitions. These systems benefit from long coherence times but suffer from low emission rates due to their weak oscillator strength. Hybrid approaches combine quantum dots with photonic crystal cavities to enhance emission rates via the Purcell effect, achieving photon extraction efficiencies above 80%.

The performance of quantum communication hardware is quantified by several metrics. For QKD systems, the secure key rate depends on the channel loss, detector efficiency, and quantum bit error rate (QBER). State-of-the-art systems achieve key rates of 1 Mbps over 10 km and 1 kbps over 100 km, with QBER below 2%. Entanglement distribution rates are limited by the pair generation efficiency and memory storage time, with current demonstrations achieving 1 Hz over 1200 km using multiplexed quantum memories. The fidelity of entanglement swapping typically exceeds 90%, with the primary losses arising from imperfect photon collection and detector dark counts.

Material advancements continue to push the boundaries of quantum communication. Nanophotonic engineering of nonlinear crystals enhances SPDC efficiency by exploiting resonant structures like microring resonators or photonic crystal waveguides. Superconducting materials for SNSPDs are being optimized to achieve higher detection efficiencies (>95%) and lower timing jitter (<20 ps). Quantum memories based on atomic vapors or silicon vacancies in diamond offer alternative platforms with potential for room-temperature operation. These developments promise to scale quantum networks to global distances while maintaining the security and reliability required for practical applications.