DC SQUIDs 1980: The state of the art

The four primary areas of concern in the development of high performance dc SQUIDs are white noise, 1/f noise, readout schemes, and coupling. For an optimized dc SQUID the intrinsic energy sensitivity in the white noise region is given by ε
w
=γ
1
hk
B
T(eI
0
R)
^-1
+γ
2
h, where I
o
is the critical current per junction and R is the shunt resistance per junction. γ
1
, which multiplies the thermal noise contribution, and γ
2
, which multiplies the shot noise/zero point fluctuation contribution, are numerical factors of order unity. Values of ε
w
approaching h have recently been measured for several members of a new generation of low noise dc SQUIDs. The intrinsic energy sensitivity in the 1/f noise region, ε
f
, is predicted to scale as (I
0
R)
²
for tunnel junctions. This may impose significant low frequency limitations on SQUIDs with very low values of ε
w
. Readout schemes for high sensitivity dc SQUIDs will require further development. At the moment primarily small signal amplifier readout schemes are being used to evaluate the new generation of low noise SQUIDs. Planar thin-film coupling schemes are about to have a big impact on dc SQUID design. Such schemes can achieve tight coupling between SQUID and input coil. and can be implemented using the same fabrication techniques that produce SQUIDs with low values of ε
w
.