Superconductivity is a remarkable quantum mechanical phenomenon where certain materials, when cooled below a characteristic critical temperature, exhibit zero electrical resistance and expel magnetic fields. Discovered in 1911 by Dutch physicist Heike Kamerlingh Onnes while studying mercury at cryogenic temperatures, this discovery opened a new frontier in physics. In ordinary conductors such as copper, electrons collide with lattice atoms, dissipating energy as heat—a limitation that plagues power grids and electronics. Superconductors, however, allow electrons to flow without any energy loss, enabling persistent currents that could theoretically last billions of years.
The critical temperature for mercury is about 4. 2 Kelvin (-269°C). For decades, scientists have sought materials that superconduct at higher temperatures, aiming for practical applications without expensive cooling. In 1957, John Bardeen, Leon Cooper, and Robert Schrieffer proposed the BCS theory, explaining superconductivity in conventional low-temperature superconductors. The key is Cooper pairs: two electrons with opposite spins and momenta that pair up via lattice vibrations (phonons). Normally, electrons repel each other due to Coulomb force, but at low temperatures, they can weakly attract through the lattice. The pairs behave as bosons and condense into a single quantum ground state, allowing them to flow without scattering.
This condensation creates an energy gap that prevents collisions with lattice imperfections. The theory predicted critical temperatures up to about 30 K, but later discoveries of high-temperature superconductors challenged this limit, suggesting additional mechanisms at play. Superconductors are categorised into two main types. Type I superconductors, such as lead and mercury, exhibit a sharp transition and completely expel all magnetic fields (Meissner effect) until a critical field strength, above which they become normal. Type II superconductors, like niobium-titanium and yttrium-barium-copper-oxide (YBCO), have two critical fields; they allow some magnetic flux to penetrate in the form of vortices, enabling higher critical fields and currents.
In 1957, John Bardeen, Leon Cooper, and Robert Schrieffer proposed the BCS theory, explaining superconductivity in conventional low-temperature superconductors.
This makes Type II materials more useful for practical applications like electromagnets. High-temperature superconductors, discovered in 1986 by Bednorz and Müller, operate above 77 K (boiling point of nitrogen), allowing cooling with liquid nitrogen, which is cheaper and more abundant than liquid helium. The mechanism for high-temperature superconductivity remains actively researched. One of the most widespread applications is in magnetic resonance imaging (MRI) machines, which use superconducting magnets to generate high, stable magnetic fields for medical diagnostics. Superconducting magnets also power particle accelerators like the Large Hadron Collider, where they steer proton beams at near-light speeds.
Moreover, superconducting quantum interference devices (SQUIDs) can measure extremely weak magnetic fields, finding use in geophysical surveys, materials testing, and brain mapping. In the energy sector, superconducting cables can transmit electricity with zero loss, potentially revolutionising power grids. Experimental maglev trains in Japan and China have achieved speeds over 600 km/h using superconducting magnets. These applications rely on the ability to carry large currents without resistance and have already transformed fields from medicine to transportation. The primary obstacle to widespread use of superconductivity is the need for extremely low temperatures.
High-temperature superconductors ease this requirement, but they are ceramic materials that are brittle and difficult to fabricate into wires. Moreover, their critical currents under magnetic fields are often limited. Current research focuses on understanding the mechanism of high-temperature superconductivity, searching for new materials, and improving engineering. Discoveries like iron-based superconductors and hydrogen sulfide under high pressure, which achieves near room-temperature superconductivity, have generated excitement. In 2023, researchers reported room-temperature superconductivity in a nitrogen-doped lutetium hydride under high pressure, but reproducibility remains controversial. The holy grail is a material that superconducts at ambient pressure and temperature.
Superconducting circuits are a leading platform for building quantum computers. Qubits based on Josephson junctions—sandwiches of two superconductors separated by a thin insulator—can exhibit quantum coherence and superposition. These qubits are manipulated using microwave pulses and read out via superconducting resonators. Companies like Google, IBM, and Rigetti have developed processors with dozens of qubits, achieving quantum supremacy demonstrations. However, decoherence and error rates remain significant hurdles. Superconducting qubits require dilution refrigerators to reach millikelvin temperatures, adding complexity. Despite challenges, the potential for solving problems intractable for classical computers, such as factoring large numbers or simulating molecular interactions, drives intense research.
The future of superconductivity is bright, with ongoing research aiming to discover room-temperature superconductors. If realised, it could transform energy transmission, transportation, medical imaging, and computing. Superconducting power lines could eliminate billions of dollars in energy losses; maglev transport could become ubiquitous; and quantum computers could revolutionise science and industry. Additionally, fusion reactors like ITER plan to use superconducting magnets to confine plasma, bringing clean energy closer. Governments and industries invest heavily in superconductivity research and development. While challenges remain, the fundamental science continues to deepen our understanding of quantum materials. Superconductivity exemplifies how quantum mechanics can lead to macroscopic phenomena with transformative potential.