Superconducting Quantum Interference Devices (SQUIDs)
Superconducting quantum interference devices (SQUIDs) are the most sensitive
sensors for magnetic flux and magnetic field so far. They consist
of a small superconducting ring (less than 1 mm in diameter), which is
interrupted by one or two so-called Josephson junctions, which essentially
act as a "weak" superconductor. Only a small current (on the order
of microamps) can flow through such a weak link without dissipation.
Above a certain current, the so-called critical current Ic, a voltage drop
develops across the junction. A magnetic flux through the SQUID,
induced by, e.g., an external magnetic field, alters the value of the critical
current. Specifically, it changes the phase of the Cooper pair wave
function across the junction(s).
In the case of the so called direct current SQUID (dc SQUID, a superconducting
ring with two junctions), the variation in critical current with varying
magnetic flux can be detected by simply passing a dc current (hence the
name dc SQUID) through the SQUID. The value of this dc current is
chosen to be slightly larger than the sum of the critical currents of the
two Josephson junctions (the critical current of the SQUID). Then
a dc voltage drop develops across the SQUID, which varies periodically
with varying magnetic flux. The periodicity of this so-called flux-to-voltage
transfer function of the SQUID is the flux quantum (approx. 2 fVs [femto
volt-seconds]). If the SQUID ring contains only a single junction
(the so-called rf SQUID), it is usually coupled to a tuned circuit, consisting
of an inductor in parallel with a capacitor, which is driven by an rf current.
The amplitude of the resulting rf voltage across this circuit oscillates
with varying flux through the SQUID, similar to the oscillating dc voltage
in case of the dc SQUID.
A citerion for the sensitivity of a SQUID is its energy resolution.
This is defined as the smallest change in energy which the SQUID can detect
in a bandwidth of 1 Hz. Typical values are 10-32 J/Hz
for dc SQUIDs operated at 4 K. This is not far away from the limit
which is set by Heisenberg's uncertainty principle.
Although a SQUID primarily measures magnetic flux, it can measure all
physical quantities which can be transformed into magnetic flux, such as
electrical currents, voltages or displacements. SQUIDs therefore
can play an essential role in a large variety of applications, both in
fundamental science and in industrial measurements.
Our SQUIDs are prepared in thin film technology (see figure below).
The SQUID ring is a square loop made of niobium. The Josephson junctions
are fabricated as tunnel junctions in a vertical geometry: A top niobium
layer is separated from a bottom niobium layer (the SQUID ring) by a very
thin aluminum oxide layer. On top of the SQUID loop, we pattern a
spiral coil which can be used to convert small electrical currents to magnetic
flux, enabling the SQUID to also be used as a current detector or low or
Optical-microscope image of a thin-film niobium dc SQUID.