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Quantum integrated circuits
Josephson phase qubit:
Superconducting electromagnetic resonators:
Mechanical quantum resonators:
Microfluidic biosensors Cell cytometry and labeling:
Surface sensors:
Nanothermometry and nanocalorimetry NIS tunnel junction thermometry:
Single photon bolometry:
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, a superconducting device whose operation relies on the physics of the Josephson junction. These devices are very appealing for engineering quantum integrated circuits because their low electrical impedance (of order 50 Ω) and relatively large capacitance (of order 1 pF) make wiring these devices together a straightforward process. As of 2009, we can achieve energy relaxation times of order 0.5 μs and phase coherence times of order 150 ns; we are continuing to develop better materials, processing, and circuit engineering to improve these basic figures of merit. We have built circuits comprising up to two qubits coupled together, which which we have demonstrated entanglement, and shown good operation, through full process tomography, of a "square root of i-SWAP" gate.
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 500,000 at low temperatures and low excitation powers. The resonators have energy relaxation times as long as 10 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.
We are now working on coupling multiple qubits to a single superconducting resonator, which will allow us to create complex qubit-resonator entanglements, and possibly begin to explore demonstrations of quantum algorithms.
We are developing mechanical resonators that should allow us to cool a mechanical system to its quantum ground state, a long-awaited and anticipated goal. These resonators are based on the film bulk acoustic resonator (FBAR), a dilatational resonator developed by Agilent for use as a cell-phone primary filter. Our resonators are aluminum-aluminum nitride-aluminum trilayers, patterned by lithographic processing into discs a few μm in diameter, and mechanically suspended by small suspension legs. We can achieve dilatational resonance frequencies as high as 10 GHz, using the piezoelectric response of the oriented-growth aluminum nitride to provide actuation and detection of the mechanical resonance. We are working to couple these very high frequency mechanical resonators to a Josephson phase qubit; at an operating temperature of 20 mK, we hope to cool the mechanical degree of freedom to the quantum ground state, and verify this by using the qubit to measure the resonator. We then hope to use the qubit to create and detect a single phonon, the quantum of mechanical vibration, in the resonator.
We have developed an electronic sensing technology that allows the purely electronic detection of cells or other small (sub-micrometer or larger diameter) particles in a microfluidic system. This relies on the use of radiofrequency reflectometry, which by appropriate impedance-matching techniques can be made extremely sensitive to very small, transient changes in the electrical response of a saline solution, such as the 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 cells (e.g. HeLa tumor cells) passing through a single sensor at rates up to one million cells per second. The sensors and associated microfluidic systems are simple and can easily be parallelized into arrays with tens to hundreds of parallel sensors.
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-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.
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.
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.