Vijay Patel

Rapid single flux quantum T-flip flop operating up to 770 GHz

Rapid Single Flux Quantum (RSFQ) T-flip flops (TFFs) operating up to 770 GHz have been demonstrated at 4.2 K. The devices, consisting of either resistively shunted or unshunted Josephson junctions, are fabricated using a planarized Nb/AlO/sub x//Nb trilayer process. Electron beam lithography is used to pattern all levels with a minimum junction area less than 0.1 /spl mu/m/sup 2/. Critical current densities of 0.5 mA//spl mu/m/sup 2/ and 2.5 mA//spl mu/m/sup 2/ are used for the shunted (tested at 1.8 K) and unshunted devices (tested at 4.2 K) respectively. The input and output frequencies of the TFFs are obtained from the input and output voltages by the Josephson relation. The output voltage is exactly half of the input voltage when the divide-by-two operation is correct.

Superconductor digital frequency divider operating up to 750 GHz

A superconductor frequency divider based on rapid single flux quantum (RSFQ) logic has been demonstrated to operate from dc to 750 GHz with an error rate less than the measurement limit (25 MHz). This high operating frequency is made possible by the use of small (0.25 μm2), high critical current density Josephson junctions along with an optimized design having parameter margins of ±30%. Simulations based on the Werthamer model are in reasonable agreement with the data and give a SFQ pulse width of 1 ps and a maximum power dissipation of 1.5 μW.

Self-shunted Nb/AlO/sub x//Nb Josephson junctions

We describe the fabrication and properties of high critical current density (J/sub c/) Nb/AlO/sub x//Nb Josephson junctions with deep-submicron dimensions. The junctions are fabricated using a planarized process in which all levels are patterned using a combination of optical and electron-beam lithography. The base and counter electrodes are defined by reactive ion etching using quartz etch masks to give a minimum feature size of 0.2 microns. For J/sub c/=2.1 mA//spl mu/m/sup 2/ and junction area less than 0.1 /spl mu/m/sup 2/ the devices are self-shunted and exhibit nonhysteretic I-V characteristics. A small hysteresis in the larger junctions is caused by heating in the electrodes.

Substrate and process dependent losses in superconducting thin film resonators

We measured the quality factor (Q) and hence the losses of thin film superconducting Nb coplanar waveguide resonators fabricated with processes and materials similar to those used for Josephson effect qubits, where such losses can cause significant decoherence. Intrinsic Q-values range from several thousand to almost 106 depending on the process details. Reactive ion etching appears to reduce the resonator Q-values and the lift-off process can also degrade the Q-value for some resists. The resistivity of the Si substrates affects the intrinsic Q at 1 K, where the resonators were measured. The Q-values obtained for optimized processing are sufficiently high as to suggest that qubits fabricated by a similar technique would not be limited by losses associated with the film or substrate.

rf-SQUID qubit readout using a fast flux pulse

We report on development of a set-up for measuring intrawell dynamics in a Nb-based rf-SQUID qubit described by a double well potential, by rapidly tilting the potential, allowing escape to the adjacent well with high probability for an excited state but low probability for the ground state. The rapid tilt of the double well potential is accomplished via a readout flux pulse inductively coupled to the qubit from a microstrip transmission line on a separate chip suspended above the qubit chip. The readout pulse is analogous to the current bias pulse used to readout phase qubits and hysteretic dc-SQUID magnetometers. The coupling between the transmission line and the qubit is carefully controlled via a window in the ground plane between the signal conductor of the microstrip and the qubit loop. Since the high frequency transmission lines are on a separate chip, they can be independently characterized and reused for different qubit samples. Clean flux pulses as short as 5 ns with rise times of 0.5 ns have been coupled to the qubit to measure escape rates higher than 108 s−1, the lifetime of the excited state, and coherent oscillations between the ground and excited states within the same well.

Development toward high-speed integrated circuits and SQUID qubits with Nb/AlO/sub x//Nb Josephson junctions

Our Nb/AlO/sub x//Nb planarized process has been upgraded by adding extra dielectric and Nb wiring layers and the installation of an Inductively Coupled Plasma (ICP) etcher. Much higher quartz etch rates as well as reduced residue are achieved with ICP etch. Etch uniformities of both Nb and quartz are also improved significantly. Damage to Nb during the fabrication process has been investigated. We have found that dry etching in SF/sub 6/ plasma has a significant effect on the quality of Nb films under certain conditions with damage coinciding with the presence of in situ deposited Al.

Decoherence in rf SQUID qubits

We report measurements of coherence times of an rf SQUID qubit using pulsed microwaves and rapid flux pulses. The modified rf SQUID, described by an double-well potential, has independent, in situ, controls for the tilt and barrier height of the potential. The decay of coherent oscillations is dominated by the lifetime of the excited state and low frequency flux noise and is consistent with independent measurement of these quantities obtained by microwave spectroscopy, resonant tunneling between fluxoid wells and decay of the excited state. The oscillation’s waveform is compared to analytical results obtained for finite decay rates and detuning and averaged over low frequency flux noise.

Temperature dependence of critical current fluctuations in Nb/AlOx/Nb Josephson junctions

We measured the low frequency critical current noise in Nb/AlOx/Nb Josephson junctions. Unshunted junctions biased above the gap voltage and resistively shunted junctions biased near the critical current Ic have been measured. For both, the spectral density of δIc/Ic, Sic(f), is proportional to 1/f, scales inversely as the area A and is independent of Jc≡Ic/A over a factor of nearly 20 in Jc. For all devices measured at 4.2 K, Sic(1 Hz)=2.0±0.4×10−12/Hz when scaled to A=1 μm2. We find that, from 4.2 to 0.46 K, Sic(f) decreases linearly with temperature.

A fast turn-around time process for fabrication of qubit circuits

The authors describe a process for fabrication of Nb/AlO/sub x//Nb Josephson junction circuits for quantum computation. The process involves only one etch step and incorporates electron beam lithography and a self-aligned lift-off of the dielectric resulting in a short turn-around time of 4-5 days. Deep submicron junctions of size down to 0.15 /spl times/ 0.15 /spl mu/m/sup 2/ have been fabricated and tested. Results of junction quality measurements will be presented. In particular, a subgap resistance of /spl sim/1 G/spl Omega/ measured at /spl sim/0.4 K indicates that the junctions subgap leakage should not be a limitation for quantum computation.

Microstrip filters for measurement and control of superconducting qubits

Careful filtering is necessary for observations of quantum phenomena in superconducting circuits at low temperatures. Measurements of coherence between quantum states require extensive filtering to protect against noise coupled from room temperature electronics. We demonstrate distributed transmission line filters which cut off exponentially at GHz frequencies and can be anchored at the base temperature of a dilution refrigerator. The compact design makes them suitable to filter many different bias lines in the same setup, necessary for the control and measurement of superconducting qubits.