Quantum coherence can exist in an object of any size, be it one atom or 1024 atoms, as long as the conditions are right. For example, macroscopic superconducting circuits can be made to behave as simple spin ½ systems. These circuits, called quantum bits or qubits for short, are excellent test beds for studying the fundamental properties of quantum measurement. Moreover, they are also potential building blocks for quantum machines—such as the quantum computer—whose operation is based on quantum electrodynamics and not classical electromagnetism.
Nonlinear Dispersive Readout
One of the major challenges in qubit research is measuring the state of a quantum coherent system and transmitting this quantum information to the classical world in which we live. A canonical method to read out a quantum spin system is to couple it to an oscillator. This technique, first developed in atomic physics, is known as a dispersive measurement. By making our oscillator out of a non-linear, non-dissipative circuit element, a Josephson junction, we can perform "quantum limited" measurements with high gain.
We are also investigating a new type of highly symmetric superconducting qubit with tetrahedral symmetry. This "tetrahedral qubit" has been predicted to be highly noise resistant, by virtue of being intrinsically protected from external sources of decoherence originating, for example, from materials defects or from the measurement circuitry. Given the massive overhead involved in fault-tolerant quantum computation, such intrinsically noise resistant qubits may play an essential role in scalable quantum computing.
Carbon Nanotube Qubits
More information coming soon!
Single Molecule Magnets
We are trying to demonstrate superconducting magnetometers with sufficient sensitivity to observe time domain behavior of individual nanomagnets. In analogy with single charge detectors, we have dubbed this measurement device the single Bohr magneton detector (SBD). The SBD consists of three superconducting Josephson junctions in a loop. The electronic transport properties of the circuit are affected by the magnetic flux in the loop. The three junction SBD is optimized for measurement of nanometer sized particles. In the SBD, one of the three junctions is a micrometer sized conventional aluminum/aluminum oxide/aluminum tunnel junction. The other two junctions are weak-link type junctions formed by single-walled carbon nanotubes bridging superconducting electrodes.
Novel Josephson Devices and Ultra Low Noise Amplifiers
The Josephson junction is a unique electrical circuit element which, when operated in the superconducting state, is both non-linear and non-dissipative. In the superconducting state, the junction behaves like a non-linear inductor whose inductance is inversely proportional to the critical current. We plan on using a new, non-dissipative technique to readout the junction inductance/critical current by pumping it to a dynamical bifurcation point with a sufficiently strong, pure ac microwave drive current and probing the transmission and reflection of the scattered microwave signal. The result is an ultra-low noise amplifier--with zero intrinsic dissipation--for any input signal which can be coupled to the junction critical current. The main goal of the proposed research is to realize a bifurcation amplifier using planar, high temperature superconducting (HTS) tunnel junctions.