SQUID Magnetometer
A loop of superconductor with two atom-thin junctions — sensitive enough to hear the magnetic whisper of a single heartbeat from across the room.
Measures extremely weak magnetic fields by converting tiny changes in magnetic flux into a voltage. It is the core sensor in magnetoencephalography (MEG) systems that map brain activity without touching the skull, in magnetocardiography (MCG) that images the heart's electrical currents, and in geological survey instruments that detect buried ore deposits.
A superconducting loop — typically niobium, cooled to 4.2 K in liquid helium — contains two Josephson junctions: barriers only a few nanometres thick where electron pairs tunnel quantum-mechanically. When external magnetic flux threads the loop, it shifts the quantum phase of the tunnelling current. A feedback circuit called a flux-locked loop converts that phase shift into a linear output voltage. Because the loop's phase is periodic with one flux quantum (Φ₀ = 2.07 × 10⁻¹⁵ Wb), the device is intrinsically calibrated against a fundamental constant of nature.
In plain terms
Imagine the Earth's magnetic field is as loud as a jet engine. A SQUID can hear a sound one billion times quieter than that — like detecting a single person's heartbeat from a kilometre away, using only magnetism.
Also called: SQUID, superconducting quantum interference device, biomagnetic sensor.
Why it matters
- Achieves sub-femtotesla sensitivity — below 1 fT/√Hz for low-temperature niobium sensors — which no other room-temperature technology approaches.
- The only sensor capable of resolving the magnetic field of individual cortical neuron assemblies, enabling non-invasive functional brain mapping.
- Its output is anchored to Φ₀, a fundamental constant, making calibration traceable to quantum metrology.
- High-temperature versions (77 K, liquid nitrogen) extend use to field-portable geophysical surveys without liquid-helium logistics.
Where it lives
- MEGIN TRIUX neo MEG system — 306 SQUID channels (102 triple-sensor units) for whole-head brain imaging
- Tristan Technologies iMAG® HTS scanning SQUID microscope — 2 µm spatial resolution for PCB fault analysis
- Quantum Design MPMS3 — laboratory magnetometer for condensed-matter research
- Geophysical SQUID gradiometers for airborne electromagnetic mineral exploration
Key numbers
- LTS magnetic noise: < 1 fT/√Hz (Tristan LSQ/20 at 4.2 K; approaches quantum limit)
- HTS magnetic noise: < 100 fT/√Hz (Tristan HTM-10 at 77 K; no liquid helium required)
- Operating temp (LTS): 4.2 K (Liquid helium; niobium T_c = 9.2 K)
- Operating temp (HTS): 77 K (Liquid nitrogen; YBCO high-Tc superconductor)
- Flux quantum (Φ₀): 2.07 × 10⁻¹⁵ Wb (Fundamental calibration reference)
- 1/f corner frequency: < 0.5 Hz (LTS) / ~10 Hz (HTS) (Below brain-signal band (1–100 Hz))
Surprising facts
- The brain's magnetic field during a visual response is roughly 100 femtotesla — about one billion times weaker than the Earth's geomagnetic field of ~50 microtesla.
- A low-temperature SQUID's magnetic sensitivity can reach below 1 fT/√Hz, placing it within a factor of two of the quantum noise floor imposed by the uncertainty principle.
- The Josephson junction at the heart of every SQUID is an insulating barrier only 1–2 nm thick — roughly ten atoms — through which electron pairs tunnel without any energy loss.
- The first SQUID measurement of a human heart was made on New Year's Eve 1969 by James Zimmerman and David Cohen at MIT, launching the entire field of biomagnetism.
A short history
Brian Josephson predicted Cooper-pair tunnelling through a thin insulating barrier in 1962, earning the 1973 Nobel Prize in Physics. The first two-junction dc SQUID was demonstrated in 1964 by Robert Jaklevic, John Lambe, Arnold Silver, and James Mercereau at Ford Research Laboratories, shortly after the first single Josephson junction was fabricated by John Rowell and Philip Anderson at Bell Labs in 1963. The rf SQUID followed at Ford in 1965, with the term 'SQUID' coined by the team. The first biomedical application came on 31 December 1969, when James Zimmerman and David Cohen recorded a human magnetocardiogram at MIT using a SQUID inside a magnetically shielded room — the founding measurement of the field of biomagnetism.
Sources
- Tristan Technologies — SQUID sensor specifications (LSQ/20, HTM-10) (https://tristantech.com/sensors/)
- NIST IR 5095 — SQUIDs past, present, and future (Zimmerman symposium) (https://nvlpubs.nist.gov/nistpubs/Legacy/IR/nistir5095.pdf)
- Fagaly, Rev. Sci. Instrum. (2006) — SQUID instruments and applications (https://pubs.aip.org/aip/rsi/article/77/10/101101/349073/Superconducting-quantum-interference-device)
- Chwala et al. — SQUID-based receivers for EM exploration (https://static1.squarespace.com/static/5f10604a2eb3d3179de51001/t/66299d6bc06bd435d2ce973d/1714003308330/Chwala+SQUID+TEM+final.pdf)
- Russo et al., Sensors (2014) — 163-channel MEG system, 5 fT/√Hz noise floor (https://pmc.ncbi.nlm.nih.gov/articles/PMC4168467/)