Our central theme is the development, noise limitations and applications of Superconducting QUantum Interference Devices (SQUIDs) made from low-transition temperature (Tc) superconductors. These include ultralow frequency nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI), the use of SQUIDs to measure the quantum state of superconducting flux qubits (quantum bits), and the development of the "microstrip SQUID amplifier" that is within 20% of the quantum limit of noise at frequencies in the 0.5-1 GHz frequency range.
We have developed a low-Tc SQUID-based spectrometer for measurements of ultralow-frequency NMR and MRI. In contrast to the conventional, tuned-circuit detection scheme, the SQUID retains its sensitivity at arbitrarily low frequencies. This fact, together with prepolarization of the nuclear spins, enables us to detect NMR and MRI signals at frequencies down to a few hertz with high signal-to-noise ratio. One application is a novel technique to perform MRI in zero magnetic field. Others include "magnetoelastography," in which we image variations in the elastic constants of liquid-like materials by applying a uniaxial stress, and remote detection of MRI in which the nuclear spins are polarized at one location and detected at another. We are particularly interested in imaging tumors as a potential clinical technique. At low magnetic fields, there is substantial differentiation of normal and cancerous tissue because their spin relaxation times (T1) differ much more than in the high fields of conventional MRI. We have imaged surgically removed tissue specimens using a T1-weighted contrast technique at a magnetic field of 0.132 mT, and find that the cancerous tissue is clearly defined.
We are investigating the properties of flux qubits. This qubit consists of a superconducting loop interrupted by three Josephson junctions. The two macroscopic quantum states are represented by a trapped magnetic flux in the "up" state or "down" state, produced by anticlockwise and clockwise supercurrents in the loop, respectively. This system is described by a two-well potential, and has a symmetric ground state and an antisymmetric first excited state--in exact analogy with the ammonia molecule. The quantum state of the qubit is measured by applying current pulses to a SQUID surrounding it. We have used microwave pulses to determine the spectroscopy of transitions between these two states, as well as Rabi oscillations, Ramsey fringes and flux echoes. We recently demonstrated that we can control the coupling between two such qubits, and even reverse its sign.
The microstrip SQUID amplifier (MSA) involves a resonant stripline to couple magnetic flux efficiently into the SQUID at frequencies around 1 GHz. At an operating temperature of 20 mK, the noise of this amplifier is within 20% of the quantum limit at a frequency of about 0.8 GHz. The corresponding "noise temperature" is 40 times lower than that of any other amplifier. One application of this amplifier is to determine the flux coupled by a qubit into its readout SQUID without dissipating any energy. This technique should enable us to perform experiments with near quantum-limited resolution. Another application of the MSA is to detect signals from an axion detector operating at Lawrence Livermore National Laboratory; the axion is a candidate for cold dark matter.