Thomas Ohki

Imaging of laser--plasma x-ray emission with charge-injection devices

This work details the method of obtaining time-integrated images of laser–plasma x-ray emission using charge-injection devices (CIDs), as has been demonstrated on the University of Rochester’s 60-beam UV OMEGA laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. The CID has an architecture similar to a charge-coupled device. The differences make them more resistant to radiation damage and, therefore, more appropriate for some application in laser–plasma x-ray imaging. CID-recorded images have been obtained with x-ray pinhole cameras, x-ray microscopes, x-ray spectrometers, and monochromatic x-ray imaging systems. Simultaneous images obtained on these systems with calibrated x-ray film have enabled determination of the absolute detection efficiency of the CIDs in the energy range from 2 to 8 keV.

Dressed States of Josephson phase qubit coupled to an LC circuit

We study the dynamics of a current biased Josephson phase qubit capacitively coupled to an LC circuit. We find that the eigenstates of this system are dressed states that are entangled states between the phase qubit and the LC resonator. We demonstrate that these dressed states can be probed by measuring the avoided crossing in the spectrum of the system. We present our experimental setup to investigate them. This system is interesting not only in demonstrating entanglement, the essential element for quantum information processing (QIP), but also in serving as a first step toward a solid-state analog of cavity QED.

Niobium Tunable Microwave Filter

A superconductor bandpass filter with tunable central frequency in the range of 2-3.5 GHz has been implemented using superconducting quantum interference devices (SQUIDs). The filter is designed using two pi-network resonators connected by a transmission line. Both resonators have a SQUID inductor with a tuning range of 65-200 pH, controlled by current magnetically coupled to the SQUIDs. Over a frequency tunability of 1.5 GHz, the filter has a mid-band insertion loss of 0.5-3.0 dB and corresponding maximum unloaded quality factor of 40. Due to the presence of active elements, tunability of the filter depends on the power of the microwave signal. A maximum power of -52 dBm corresponds to a frequency tuning range of 15%. Spectral measurements by controlling the central frequency of the filter with variable pulsewidth shows that the filter can be tuned at a rate of 120 GHz/mus.

Miniaturized superconducting microwave filters

In this paper we present methods for miniaturization of superconducting filters. We consider two designs of 7th order bandpass Chebyshev filters based on lumped elements and a novel quasi-lumped element resonators. In both designs the area of the filters, with a central frequency of 2-5 GHz, is less than 1.2 mm2. Such small filters can be readily integrated on one board for multi-channel microwave control of superconducting qubits. The filters have been experimentally tested and the results are compared with simulations. The miniaturization resulted in parasitic coupling between resonators and within each resonator that affected primarily stopband and bandwidth increase. The severity of the error depends on the design in particular, and was less prawn when groundplane was used under the inductances of the resonators. The best performance was reached for the quasi-lumped filter with central frequency of 4.5 GHz, quality factor of 100 and 28 dB stopband.

Picosecond on-chip qubit control circuitry

Fast on-chip control of superconducting qubits has engaged complex and power consuming RSFQ circuits that currently pose more of an experimental burden than an asset. Measurements of quantum coherent oscillations of qubits require dilution refrigerator temperatures. The motivation of this design is to minimize the necessary bias leads and power dissipation for an SFQ based control circuit. Elimination of redundant circuit elements by innovative use of fundamental elements allows small-scale control circuitry.

Unshunted QOS Comparator for Qubit Readout

State measurements of flux-based superconducting quantum bits (qubits) require the sensitive discrimination of small differences of magnetic flux, with measurement times short compared to the timescales of coherent quantum mechanical behaviour. The single-flux-quantum (SFQ) superconducting comparator may be the best device for this purpose. An SFQ comparator design using a quasi-one-junction SQUID (QOS) acting as the comparator junction has two advantages over earlier designs—increased sensitivity and the possibility to eliminate shunt resistors. Here we report the first measurements of the sensitivity of an unshunted QOS comparator. The gray zone of this device at 4.2K is 6 mΦo.

Low-Jc Rapid Single Flux Quantum (RSFQ) Qubit Control Circuit

We designed a 30 A/cm2 RSFQ-qubit control circuit and tested operation at 4 K. The integration of RSFQ technology and qubits monolithically requires great attention to thermal budgeting and electromagnetic compatibility. These issues are of primary concern when developing an RSFQ-qubit experiment. The interface of an SFQ circuit to a qubit requires tunable coupling since RSFQ inherently limits logical elements to single-flux storage. By measuring the quantum-tunneling rate of a phase qubit, we can determine the effect of changing the coupling strength between the classical and quantum systems. By looking at the same system, we can diagnose the impact, whether thermal or electromagnetic, of an active digital circuit. These tests would set an upper bound on the deleterious effects of monolithic integration.

Thermometry using thermal activation of Josephson junctions at MilliKelvin temperatures

We use the thermal activation of Josephson junctions as a thermometer to investigate heat flow from a hot resistor at milliKelvin temperatures on a silicon chip used for superconducting qubit experiments. The experiments are compared to computer simulations and agree well. These results indicate that on-chip resistors can be used below a certain power level, but not above that level.

Thermal design of superconducting digital circuits for milliKelvin operation

Niobium based rapid-single-flux-quantum (RSFQ) digital circuits generally operate at temperature 4 K. It is desirable to develop RSFQ circuits for operation at much lower temperatures, in particular to use as control and interface circuitry for superconducting qubits, and eventually for a full scale quantum computer. The total heat load is moderate - current designs generate 0.5 /spl mu/W per bias resistor, so simple RSFQ integrated circuits are easily compatible with commercial helium dilution refrigerators, and this power can readily be reduced by several orders of magnitude for complex future designs - but thermal conductivity will be a bottleneck. We present a simple model of heat flow through standard RSFQ structures. We find that circuits designed for 4 K operation can be used with little or no modification below one Kelvin. At lower temperatures however the heat generated on chip cannot be removed, and the temperature of a working circuit will rise. We suggest fabrication design rule changes to address this problem.

A simple circuit to supply constant flux biases for superconducting quantum computing

We present and demonstrate experimentally a persistent-current bias loop that is intended to provide an on-chip stable dc magnetic field to flux-bias the flux qubits in a superconducting quantum computer. The bias loop is initialized to approximately the desired persistent current and then the external source is disengaged; this procedure prevents the fluctuations in the bias sources from causing decoherence in the qubit.