The Cleland group pursues research in two distinct areas:

1.     Quantum-limited behavior of electronic and mechanical systems, and

2.     Developing tools for biophysical and biomedical applications.

Specific research topics include:

⇒     Quantum integrated circuits, based on the Josephson phase qubit.

⇒     Nanomechanical and optomechanical resonators, quantum control of individual phonons and photons.

⇒     Microfluidic-based electronic biosensing of cells and molecules.

⇒     Nanoscale bolometers and nanocalorimeters.

Quantum integrated circuits

Josephson phase qubit:
Working in collaboration with the Martinis group, we are developing superconducting quantum integrated circuits for applications in quantum computation and quantum simulation. This research is based on the Josephson phase qubit as well as the transmon, superconducting devices whose operation relies on the physics of the Josephson junction. These devices are very appealing for engineering quantum integrated circuits because their relatively simple fabrication and operation. We can achieve energy relaxation times of order 10 μs and phase coherence times of order 1 μs; we are continuing to develop better materials, processing, and circuit engineering to improve these basic figures of merit. We have built circuits comprising of up to four qubits coupled to five electromagnetic resonators, which has allowed us to demonstrate factoring the number 15 using Shor's algorithm. Present research is focused on developing these devices for a surface-code based architecture, which promises highly fault-tolerant error management with topologically-protected logical operations.

Superconducting electromagnetic resonators:
We have developed very high quality factor, superconducting electromagnetic resonators, for use as computational and memory elements for quantum computation. With resonance frequencies in the 5-10 GHz band, we can achieve quality factors as high as 10,000,000 at low temperatures and low excitation powers. The resonators have energy relaxation times as long as 30 μs and phase coherence times just under twice this value. We have coupled an individual Josephson qubit to a resonator, and using a sequence of qubit preparation and tune-to-resonance protocols, we have demonstrated that we can pump photons one at a time, on demand, into the resonator. By subsequently measuring the resonator with the qubit, we have shown that the resulting Fock (photon number) states in the resonator have very high purity, with up to 20 photons stored in the resonator at one time.

By employing a protocol developed for ions interacting in a harmonic ion trap, we have extended our ability to generate Fock states in the resonator to being able to create arbitrary quantum superpositions of Fock states, demonstrating a fundamental tenet of quantum mechanics: That a quantum system can be "in two places at the same time", creating states with e.g. zero, three and six photons simultaneously, with full quantum control over the relative amplitude and phase of the quantum superpositions.

The resonators provide a robust quantum memory for quantum information processing, as well as allowing large on-off coupling when used as an intermediary between qubits.

Mechanical quantum resonators:
We have developed mechanical resonators that allow us to cool a mechanical system to its quantum ground state. These resonators are based on the film bulk acoustic resonator (FBAR), a piezoelectric dilatational resonator based on aluminum nitride. We can achieve dilatational resonance frequencies as high as 10 GHz, using the piezoelectric response to provide actuation and detection of the mechanical resonance. We have coupled these very high frequency mechanical resonators to a Josephson phase qubit and demonstrated quantum ground state operation and control of individual phonons with these devices. We are now developing optomechanical devices that up-convert microwave electrical signals to optical frequencies. These are being implemented using aluminum nitride as a high-performance optomechanical material, with good optical properties in the 1550 nm telecommunications band and strong piezoelectric response to microwave excitations. This project is aimed at developing an interface between a superconducting qubit and optical photons, with the goal of generating entangled photons that carry quantum information entangled with the superconducting qubit state.

Microfluidic biosensors

High throughput analysis of nanoparticles, microparticles, and biological cells:
We have developed an electronic sensing technology that allows the purely electronic detection of nanoparticles, microparticles and biological cells suspended in biologically-appropriate saline solutions. This relies on the use of nanolithographically patterned fluidic systems coupled with high-speed electronics, able to detect electrical impedance changes caused by the passage of small particles through the sensor volume. Using this intrinsically very high bandwidth technique, we can detect and count individual virus particles, cell fragments, cells (e.g. HeLa tumor cells) passing through a single sensor at rates up to one hundred thousand events per second. The sensors and associated microfluidic systems are simple and can easily be parallelized into arrays with tens to hundreds of parallel sensors.

Cell labeling technology:
We have also developed a cell labeling system that relies on lithographically-patterned, micrometer-scale "bar codes", made of photodefinable epoxy. The labels can be individually read out using our radiofrequency reflectometer, when they are passed in single file through the sensor volume, which is embedded in a microfluidic channel. The bar codes include a digital encoding that permits up to 1,024 individual codes. By functionalizing the bar codes with antibodies specific to epitopes that distinguish cell types, we can label and then quantify and sort extremely complex populations of cells. We can read out the bar codes at rates of over 100,000 per second, and with the parallelization possible with the electronic and microfluidic systems we are using, can achieve sorting rates higher than fluorescence-activated cell sorters (FACS), with sort complexities at least two orders of magnitude greater than can be achieved with optical fluorophores typically used in FACS systems.

Surface sensors:
Surface-based molecular sensors provide an excellent modality for detecting very small concentrations of target molecules in solution: By specifically functionalizing a surface, target molecules will bind to the surface, and for sufficient binding affinity, can achieve a much greater surface density than indicated by the volumetric concentration. We are trying to employ this "density amplification" by developing surface-sensitive electronic sensors, where an electronic signal is generated from changes in the electrical impedance at the surface of the sensor.

The electrical response of metal surfaces exposed to saline solutions is dominated by the Debye-Huckel double-layer formed at the interface, which has a thickness of order 1 nm. Electrically controlling and measuring the properties of the Debye layer allows a type of surface sensitivity appropriate for molecular binding, with typical oligonucleotide and protein diameters of order the Debye layer thickness. To this end, we have recently developed and demonstrated operation of a Debye layer transistor, a radiofrequency device whose input-output response is controlled by the Debye double layer, which in turn can be controlled and modulated using a third, gate electrode, which adjusts the potential of the saline solution relative to the metal electrodes. This transistor can operate to modulation frequencies as high as 5 MHz, and is dominated by the interfacial behavior 1 nm in thickness. We are now attempting to develop molecular sensors using this unique transistor.

Nanothermometry and nanocalorimetry

NIS tunnel junction thermometry:
We have developed nanoscale thermometers and bolometers, based primarily on the extremely strong temperature responsivity of normal metal-insulator-superconductor (NIS) tunnel junctions. At zero voltage bias and low temperatures, the resistance is exponentially dependent on the temperature of the electrons in the normal metal. Very small normal metal volumes can be fabricated, allowing calorimetry with heat capacities approaching a single kB. We have demonstrated radiofrequency readout of the temperature of one of these very small metal volumes, and measured the electron-phonon and phonon thermal conductivities of very small, mechanically suspended nanoscale calorimeters. Using a unique three-junction single electron transistor integrated with an NIS tunnel junction, we have demonstrated readout of the temperature of a nanoscale metal volume with almost zero back-action.

Single photon bolometry:
We are now attempting to couple a nanoscale normal metal volume to an impedance-matching, superconducting slotline antenna, in order to capture and detect individual mm-wave (THz frequency) photons, achieved by resolving the temperature change in the normal metal induced by Joule heating of a single photon. This would thus demonstrate single-photon bolometry.