This article explores the SQUID, or Superconducting Quantum Interference Device. A SQUID is a sensitive magnetometer designed to measure magnetic fields. It can detect fluctuations thousands of times weaker than those in everyday life, such as the faint magnetic signatures produced by biological processes.
The Physics of Superconductivity
To understand how a SQUID operates, one must first grasp superconductivity. This phenomenon occurs in certain materials when cooled to extremely low, cryogenic temperatures. At a specific temperature, the material undergoes a phase transition where its electrical resistance completely disappears.
When a material becomes a superconductor, electrons flow through it without any energy loss. This is because the electrons form Cooper pairs, which move through the material’s atomic lattice without the collisions that cause resistance in normal conductors.
Another characteristic of superconductors is their ability to expel magnetic fields, a phenomenon known as the Meissner effect. As a material enters its superconducting state, it pushes magnetic field lines out from its interior. This process ensures the magnetic field inside the bulk of a superconductor is always zero.
The temperatures required for superconductivity are close to absolute zero, often requiring costly liquid helium. More recently, “high-temperature” superconductors have been discovered that can operate at the warmer temperature of liquid nitrogen. These materials have broadened the applications for superconducting technologies.
Core Principles of a SQUID
A SQUID harnesses the properties of superconductivity to function as a measurement tool. The most common type, the DC SQUID, is a small superconducting loop interrupted by two weak links called Josephson junctions. These junctions are thin insulating layers that separate the superconducting sections. The paired electrons of a superconductor can “tunnel” through these layers due to quantum mechanical effects.
This tunneling creates a supercurrent that circulates around the loop, governed by quantum interference. The waves associated with the electron pairs split, travel through each arm of the loop, and then recombine. How they recombine depends on the phase difference between the two paths.
This interference pattern is extremely sensitive to any external magnetic field passing through the loop. A minuscule change in the magnetic flux, which is the measure of the total magnetic field passing through an area, alters the phase of the electron pairs. This alteration changes the total current that can pass through the device in a predictable, periodic way.
This relationship allows the SQUID to act as a flux-to-voltage converter. As the external magnetic flux changes, the maximum supercurrent the loop can sustain oscillates. By applying a bias current and measuring the resulting voltage, operators can detect these oscillations. Each oscillation corresponds to the magnetic flux changing by a single magnetic flux quantum, allowing for fine measurements.
Detecting the Imperceptible
One prominent use for SQUIDs is in medicine, specifically in a technique called magnetoencephalography (MEG). The human brain’s neural activity generates very faint magnetic fields. SQUIDs are sensitive enough to detect these signals, allowing researchers to map brain function in real-time without physical contact.
SQUIDs are also tools in geology and materials science. Geologists use them for geophysical surveys to detect variations in the Earth’s magnetic field caused by underground mineral deposits or aquifers. In non-destructive testing, SQUIDs can identify tiny flaws or stress points in materials by detecting their weak magnetic signatures.
These devices also form the core of laboratory equipment. Magnetic Property Measurement Systems (MPMS) use SQUIDs to characterize the magnetic properties of novel materials. This is important for developing new technologies in data storage and electronics. The ability to measure the magnetic moment of a sample makes SQUIDs a useful tool for scientific research.
SQUIDs in Quantum Computing
In quantum computing, SQUIDs perform functions for building and operating quantum processors. Their ability to respond to single quanta of magnetic flux makes them well-suited for the quantum realm. One primary role is acting as a building block for a qubit. A specific type of superconducting qubit, the flux qubit, is a tiny SQUID loop.
In a flux qubit, the two states representing a quantum ‘0’ and ‘1’ are defined by the direction of the supercurrent flowing in the loop—clockwise or counter-clockwise. The quantum state can be controlled by applying precise magnetic flux pulses to the loop, which allows for the manipulation needed to perform quantum computations.
SQUIDs are also used as readout devices for other types of superconducting qubits. After a computation is complete, the final state of the qubits must be measured. A SQUID can be coupled to a qubit so that the qubit’s final state influences the magnetic flux through the SQUID loop. By measuring this flux, the SQUID reads the ‘0’ or ‘1’ state of the qubit without prematurely destroying its quantum information.