Although a SQUID primarily measures magnetic flux, it can measure all physical quantities which can be transformed into magnetic flux, such as electrical currents or voltages.  For example, by preparing a ( superconducting ) coil on top of the SQUID, which can be used to convert electrical current to magnetic flux, the SQUID can also be used as a current detector, a voltmeter or a low or high-frequency amplifier.  One of the advantages of a SQUID amplifier over a conventional ( cold ) semiconductor amplifier is its low noise and very low power dissipation ( less than a nanowatt, compared to tens of mW for a semiconductor amplifier ).  A conventional washer type dc SQUID with integrated input coil shows a reasonable gain only up to frequencies of about 100 MHz [1].  At higher frequencies, parasitic capacitance between the input coil and the SQUID causes the gain to drop substantially. 

We developed a new SQUID amplifier design in which the input coil integrated on top of the SQUID is operated as a half-wavelength microstrip resonator, in order to circumvent the problem of parasitic capacitance [2] ( see Fig. 1 ).  In contrast to the conventional input scheme in which the signal is connected to the two ends of the coil, the signal is instead coupled between one end of the coil and the SQUID washer, which provides the groundplane for the microstrip; the other end of the coil is left open.  At the fundamental resonance frequency of the microstrip so formed, the applied current is amplified by the quality factor Q and couples magnetic flux into the SQUID. 

Figure 2 shows the gain of such an amplifier vs. frequency for different devices having different resonator lengths ( 2.2 mm to 16 mm ).  For frequencies below one GHz a gain of 20 dB or more can be achieved and noise temperatures of about one quarter of the operating temperature of the device ( e.g., a noise temperature of 1 K for a bath temperature of 4.2 K ).  At very low bath temperatures ( < 100 mK ), the amplifier can be near quantum limited [3].
 
 

Figure 2. Gain vs. frequency for five microstrip SQUID amplifiers with different resonator length.

[1] C.Hilbert and J.Clarke, J. Low Temp. Phys. 61, 263 (1985). 
[2] M.Mück, M.O.André, John Clarke, J.Gail and C.Heiden, 
      Appl.Phys.Lett. 72, 2885 (1998). 
[3] M.Mück, J.B.Kycia and J.Clarke, Appl. Phys. Lett. 78, 967 (2001).