Superconducting materials allow electric current without resistance below a critical temperature. How they work and what they mean for quantum technology.
⚡ What Is Superconductivity
Imagine an electrical circuit where current flows with no energy loss — no heat, no resistance, no waste. This is not science fiction, but the reality of superconductivity: a phenomenon where certain materials, when cooled below a critical temperature Tc, completely lose their electrical resistance.
The discovery was made in 1911 by Dutch physicist Heike Kamerlingh Onnes at the University of Leiden. Onnes, a pioneer of low-temperature physics, was studying the electrical resistance of metals at extremely low temperatures using liquid helium as a coolant. When mercury was cooled to 4.2 Kelvin (−268.95 °C), its resistance dropped abruptly to zero.
This drop was not gradual — it was a sudden phase transition, analogous to water freezing. Above 4.2 K, mercury behaved as a normal conductor. Below that threshold, it became something entirely different — a superconductor. Onnes was awarded the Nobel Prize in Physics in 1913 for this discovery and his low-temperature research.
A remarkable experiment proved that in a superconducting ring, an electric current can circulate for years without any measurable decrease. In practice, the resistance is exactly zero — not merely very small, but literally <10⁻²⁵ Ω, at least 10¹⁵ times smaller than that of copper.
🧲 The Meissner Effect
Superconductivity is not only about zero resistance. In 1933, German physicists Walther Meissner and Robert Ochsenfeld discovered an equally striking phenomenon: when a material transitions to its superconducting state, it completely expels magnetic fields from its interior. This is known as the Meissner effect.
A superconductor is not simply a perfect conductor — it is a perfect diamagnet. Even if you place a superconductor inside an external magnetic field and cool it below Tc, the magnetic fields are actively expelled. Currents arise on the surface of the material that create an opposing magnetic field, completely canceling the external field inside.
The most spectacular demonstration of the Meissner effect is magnetic levitation: a small magnet placed above a superconductor hovers stably in the air, with no external energy. The repulsive force between the magnet and the surface currents of the superconductor balances gravity. This image — a magnet floating silently above a frozen material — is one of the most iconic experiments in modern physics.
🔬 BCS Theory: Cooper Pairs
For nearly half a century after Onnes's discovery, superconductivity remained a theoretical mystery. Why do electrons — which normally repel each other due to their negative charge — behave so differently below Tc?
The answer came in 1957 from John Bardeen, Leon Cooper, and John Robert Schrieffer, who developed BCS theory (named after their initials). Their key insight was that at low temperatures, electrons can form Cooper pairs — two electrons bound together through interactions with phonons (vibrations) of the crystal lattice.
When an electron moves through a crystal lattice, it attracts nearby positive ions, creating a local region of positive charge. This region then attracts a second electron. The two electrons form a “Cooper pair” — a particle with zero total spin (a boson). Millions of such pairs move in concert as a macroscopic quantum state, passing through the lattice without scattering — that is, without resistance.
BCS theory also explains why there is an energy gap in superconductors. To break a Cooper pair, a minimum amount of energy is required. At very low temperatures, thermal energy is insufficient to break the pairs, so all electrons remain in a coherent quantum state. Bardeen, Cooper, and Schrieffer were awarded the Nobel Prize in Physics in 1972.
🌡️ High-Temperature Superconductors
For decades after Onnes, known superconductors required cooling near absolute zero — temperatures achievable only with liquid helium, a rare and expensive coolant. This severely limited practical applications. Everything changed in 1986, when Georg Bednorz and Karl Alex Müller at IBM's laboratories in Zurich discovered superconductivity in ceramic materials (cuprates) at temperatures far higher than expected.
The discovery triggered a “golden age” in superconductor research. Within months, researchers discovered YBCO (Yttrium Barium Copper Oxide, YBa₂Cu₃O₇), which becomes superconducting at 93 Kelvin — above the boiling point of liquid nitrogen (77 K). This meant superconductivity could now be achieved with a cheap and abundant coolant, dramatically reducing costs. Bednorz and Müller received the Nobel Prize in Physics in 1987 — just one year after their publication.
The quest for room-temperature superconductors remains one of physics' great challenges. In 2023, the LK-99 claim — a material said to be a room-temperature, ambient-pressure superconductor — generated enormous global excitement. However, multiple laboratories failed to reproduce the results, and the scientific community concluded that LK-99 is not a superconductor. This failure reminded everyone how difficult the goal remains, but did not diminish its significance.
🏥 Applications in Modern Technology
Despite technical challenges, superconductors are already at the heart of many critical technologies. The most well-known application is Magnetic Resonance Imaging (MRI): every MRI machine contains superconducting coils that generate extremely powerful magnetic fields (1.5-3 Tesla), necessary for detailed imaging of the human body. Over 40,000 MRI machines operate worldwide, saving millions of lives through non-invasive diagnosis.
In fundamental research, the Large Hadron Collider (LHC) at CERN uses thousands of superconducting magnets made of niobium-titanium alloy, cooled to 1.9 K, to steer protons at speeds close to light. Without superconductors, the LHC — and the discovery of the Higgs boson — would not have been possible.
SQUID sensors (Superconducting Quantum Interference Device) exploit the quantum nature of superconductors to measure extremely weak magnetic fields — even those produced by brain activity (magnetoencephalography). Maglev trains (magnetic levitation) use superconducting magnets for frictionless levitation, reaching speeds >600 km/h.
In quantum computing, transmon qubits — the most widespread type of quantum bits — are based on superconducting circuits. Companies like IBM, Google, and Rigetti build quantum computers with tens to hundreds of superconducting qubits, cooled near absolute zero inside dilution refrigerators.
🚀 The Future of Superconductivity
The search for room-temperature superconductors continues with unrelenting intensity. Under extremely high pressures, hydrides such as LaH₁₀ have shown superconductivity at 250 K (−23 °C) — close to room temperature, but at pressures of hundreds of GPa, impossible for practical applications. The goal remains clear: a superconductor at room temperature and atmospheric pressure.
If achieved, the implications would be revolutionary. Electrical grids could transmit energy with no losses — today, roughly 5-10% of generated electricity is lost during transmission due to resistance. Quantum computers could operate without expensive cooling systems — something that would democratize access to quantum computing.
Meanwhile, new materials are continuously discovered. Iron-based superconductors (iron pnictides), discovered in 2008, offered an entirely new family of superconductors. Topological superconductors — combining superconductivity with topological properties — are considered candidates for error-resistant quantum bits. Artificial intelligence is now used to predict new superconducting materials, accelerating discovery.
Superconductivity — a purely quantum phenomenon — already lies at the heart of critical technologies, from hospital imaging to fundamental particle physics research. Its full understanding and exploitation may represent the greatest technological promise of the 21st century.