One of my main interests is in the development, noise limitations and applications of Superconducting Quantum Interference Devices (SQUIDs) made from both low-transition temperature (Tc) and high-Tc superconductors. I am particularly intrigued by quantum-limited detectors. The applications include reading out of potential superconducting “qubits”, axion detectors, sensing of magnetically-tagged biomolecules, nuclear magnetic resonance and nondestructive evaluation. I am also involved in research on arrays of nanometer-sized arrays of tunnel junctions which display charging effects arising from the Coulomb blockade of single electrons or electron pairs.
A possible qubit for quantum computation consists of a tiny superconducting loop interrupted by a nanometer-sized Josephson tunnel junction. The two macroscopic quantum states are represented by a trapped magnetic flux in the “up” state or “down” state, represented by anticlockwise and clockwise supercurrents in the loop, respectively. This system is described by a two-well potential; operation of the device as a qubit requires it to oscillate between the two quantum states. This oscillation can in principle be detected by an ultrasensitive SQUID, provided the backaction can be reduced sufficiently. A major effort is under way to observe these macroscopic quantum oscillations.
We have recently developed a “microstrip SQUID amplifier” which involves a resonant input circuit to couple magnetic flux efficiently into the SQUID. At frequencies around 0.5 GHz and at 20 mK, the noise is within a factor of two of the quantum limit. At this frequency, the energy resolution is 40 times better than that of any other amplifier. A major application is in a search for cold, dark matter -- the reading out of signals from the axion detector at Lawrence Livermore National Laboratory.
We have spent several years developing SQUID magnetometers based on high-Tc superconductors and operating in liquid nitrogen at 77 K. Our primary interest has been in understanding the noise limitations of these devices. We have received two patents for magnetometers involving laser-deposited multilayer structures. Although not quite as sensitive as their low-Tc counterparts, these high-Tc SQUIDs are now sufficiently sensitive for a wide variety of applications. This technology has enabled us to construct a “SQUID microscope,” in which a SQUID at 77 K, mounted below a thin window, is typically only 15 µm from room temperature specimens. Using this instrument, we have developed a novel immunoassay technique that involves tagging an antibody or antigen with a superparamagnetic nanoparticle. Another potential application includes measurements on molecular motors labeled with a ferromagnetic nanoparticle and of the unwinding of DNA or nanotubes similarly tagged.
We have developed a low-Tc SQUID-based spectrometer for measurements of ultralow-frequency nuclear magnetic resonance (NMR). Unlike the conventional tuned-circuit detection scheme, the SQUID retains its sensitivity at arbitrarily low frequencies. This fact, together with the use of a prepolarization technique and a novel pulse sequence, enables us to detect NMR at frequencies down to a few hertz with a very high signal-to-noise ratio. A particularly exciting potential application of this new technique is high-resolution magnetic resonance imaging (MRI) in magnetic fields even lower than that of the earth.
A rather different area of research is the investigation of one-dimensional arrays of nanometer-sized tunnel junctions deposited on a GaAs substrate containing a two-dimensional electron gas (2DEG). Of particular interest are the effect of changing the dissipation on this model quantum system in situ, and its implications for both phase transitions and decoherence of possible qubits.
In the News
Four University of California, Berkeley, faculty members – physicists John Clarke and Bernard Sadoulet, chemist John Hartwig and ecologist Mary Power – have been elected members or foreign associates of the National Academy of Sciences, bringing UC Berkeley’s total NAS membership to 141.