SQUID Magnetometer

The superconducting quantum interference device (SQUID) consists of two superconductors separated by thin insulating layers to form two parallel Josephson junctions. The device may be configured as a magnetometer to detect incredibly small magnetic fields -- small enough to measure the magnetic fields in living organisms. Squids have been used to measure the magnetic fields in mouse brains to test whether there might be enough magnetism to attribute their navigational ability to an internal compass.

The great sensitivity of the SQUID devices is associated with measuring changes in magnetic field associated with one flux quantum. One of the discoveries associated with Josephson junctions was tha flux is quantized in units

If a constant biasing current is maintained in the SQUID device, the measured voltage oscillates with the changes in phase at the two junctions, which depends upon the change in the magnetic flux. Counting the oscillations allows you to evaluate the flux change which has occurred.

Josephson Devices

Devices based upon the characteristics of a Josephson junction are valuable in high speed circuits. Josephson junctions can be designed to switch in times of a few picoseconds. Their low power dissipation makes them useful in high-density computer circuits where resistive heating limits the applicability of conventional switches.

Parallel Josephson junctions are used in SQUID devices for the detection of minute magnetic fields.

Josephson Junction

Two superconductors separated by a thin insulating layer can experience tunneling of Cooper pairs of electrons through the junction. The Cooper pairs on each side of the junction can be represented by a wavefunction similar to a free particle wavefunction. In the DC Josephson effect, a current proportional to the phase difference of the wavefunctions can flow in the junction in the absence of a voltage. In the AC Josephson effect, a Josephson junction will oscillate with a characteristic frequency which is proportional to the voltage across the junction. Since frequencies can be measured with great accuracy, a Josephson junction device has become the standard measure of voltage.

The wavefunction which describes a Cooper pair of electrons in a superconductor is an exponential like the free particle wavefunction. In fact, all the Cooper pairs in a superconductor can be described by a single wavefunction in the absence of a current because all the pairs have the same phase - they are said to be "phase coherent" (Clarke). If two superconductors are separated by a thin insulating layer, then quantum mechanical tunneling can occur for the Cooper pairs without breaking up the pairs. Clarke envisions this condition as the wavefuntions for Cooper pairs on each side of the junction penetrating into the insulating region and "locking together" in phase. Under these conditions, a current will flow through the junction in the absence of an applied voltage (the DC Josephson effect).

Josephson Voltage Standard

When a DC voltage is applied to a Josephson junction, an oscillation of frequency

occurs at the junction. Since this relationship of voltage to frequency involves only fundamental constants and since frequency can be measured with extreme accuracy, the Josephson junction has become the standard voltage measurement.

Josephson junction standards can yield voltages with accuracies of one part in 10^10. NIST has produced a chip with 19000 series junctions to measure voltages on the order of 10 volts with this accuracy.

The Standard Volt

The standard volt is now defined in terms of a Josephson junction oscillator. The oscillation frequency of a Josephson junction is given by

so the relationship between frequency and voltage across the junction depends only upon the fundamental constants e and h. For one microvolt applied to the junction the frequency is

The standard volt is now defined as the voltage required to produce a frequency of 483,597.9 GHz.