Quantum entanglement is a phenomenon where the quantum states of particles are intrinsically linked, such that the state of one cannot be described independently of the others, even when separated by vast distances. As of 2025, the field has transitioned from foundational physics to large-scale systems engineering, focusing on two distinct scaling dimensions: quantity (multi-partite entanglement) and range (distance scaling).
Scaling Qubit Count (Multi-Partite) Expanding the number of simultaneously entangled qubits is essential for powerful quantum computing.
• Physical Qubits: Researchers using the Zuchongzhi superconducting quantum computer set a record by verifying the entanglement of 51 qubits, creating a complex system with no classical counterpart. Additionally, Caltech researchers successfully trapped 6,100 neutral atoms in an array, maintaining superposition for 13 seconds.
• Logical Qubits: A significant milestone in fault tolerance was achieved by Microsoft and Atom Computing, who entangled 24 logical qubits (groups of physical qubits corrected for errors) using neutral atoms. This indicates progress toward reliable, large-scale computation.
Scaling Distance (Quantum Networking) Distributing entanglement over long distances is the prerequisite for a global Quantum Internet.
• Satellite Links: A record-breaking ultra-secure link was established over 12,900 km between China and South Africa using the Jinan-1 microsatellite, marking the first quantum satellite link in the Southern Hemisphere.
• Fiber and Memory: Researchers demonstrated secure quantum key distribution over 62 miles of fiber without relying on trusted devices. Furthermore, breakthroughs in erbium-doped crystal memories extended coherence times to 24 milliseconds, theoretically enabling fiber connections up to 4,000 km.
• Repeaters: To overcome signal loss in fiber, "entanglement swapping" is used to chain short entangled links into longer ones, a function performed by quantum repeaters. New architectures suggest using "Quantum Data Centres" where modular processors are interconnected via entanglement to act as a single machine.
Applications in Metrology Entanglement allows sensors to surpass the Standard Quantum Limit (SQL). By utilizing entangled states, measurement precision can approach the Heisenberg limit (Δθ∝1/N) rather than the classical shot-noise limit (Δθ∝1/N). This "entanglement-enhanced metrology" is being applied to improve the stability of atomic clocks and magnetic field sensors
