F. P. Milliken

50 Ω characteristic impedance low-pass metal powder filters

We have fabricated several 50 omega characteristic impedance low-pass metal powder filters. The filters are made with bronze or copper metal powder with varying amounts of metal powder in a metal powder/epoxy mixture. Our goal is to make a filter with a characteristic impedance Z = 50 omega at frequencies up to 10 GHz. Using a 78% bronze powder/epoxy mixture in a suitable geometry, we achieved an impedance Z = 54 omega at 4.2 K, with a cutoff frequency fc approximately/= 0.3 GHz and an attenuation A = Vout/Vin=0.0001 (-80 dB) at 10 GHz. We also made several non-50 omega low-pass bronze powder filters with fc = 1 MHz and A = 0.0001 at 10 MHz. Fabrication details and performance data will be presented for both types of filter.

Magnetic field-induced noise in directly coupled high Tc superconducting quantum interference device magnetometers

We have measured the noise of several directly coupled high Tc superconducting quantum interference device (SQUID) magnetometers as we change a static magnetic field B while the devices are superconducting. Devices without “flux dams” show an increase in noise at relatively low magnetic fields B∼1 μT. Devices with flux dams can show no deterioration of the noise characteristics for B as large as 34 μT. The flux dams are part of the pickup loop and limit the circulating current Icirc. If Icirc is kept sufficiently low no vortices are forced into the SQUID loop and the noise performance of the magnetometer remains good.

Quantum information storage using tunable flux qubits

We present details and results for a superconducting quantum bit (qubit) design in which a tunable flux qubit is coupled strongly to a transmission line. Quantum information storage in the transmission line is demonstrated with a dephasing time of T(2)∼ 2.5 µs. However, energy lifetimes of the qubit are found to be short (∼ 10 ns) and not consistent with predictions. Several design and material changes do not affect qubit coherence times. In order to determine the cause of these short coherence times, we fabricated standard flux qubits based on a design which was previously successfully used by others. Initial results show significantly improved coherence times, possibly implicating losses associated with the large size of our qubit.

Sensitive high-T/sub c/ SQUID magnetometers for unshielded operation

We have developed sensitive highly reliable high-T/sub c/ SQUID magnetometers a reproducible SNS junction fabrication process. In order to enable unshielded operation in the earths field, we have incorporated flux dams into direct-coupled SQUID magnetometers. By using up to four pickup coils in parallel, each with its own flux dam, an effective area of 0.57 mm/sup 2/ has been achieved on 24 mm/spl times/24 mm substrates using 100 /spl mu/m wide pickup coils. We have demonstrated a magnetic field noise sensitivity of 80 fT//spl radic/Hz 10 Hz and 77 K in an externally applied field 60 /spl mu/T. This magnetic field sensitivity unchanged from the zero-field measurement.

Materials issues related to the fabrication of HTS SQUIDs

A number of materials related problems have hindered the development of a reproducible process for the fabrication of high-quality SQUIDs and magnetometers. In this paper we discuss the use of GdBa/sub 2/Cu/sub 3/O/sub 7/ and YBa/sub 2/Cu/sub 3/O/sub 7/ thin films for the fabrication of step-edge and bicrystal magnetometers and how materials selection can influence issues such as reproducibility, yield and noise performance. Magnetometers with noise performance as low as 63 fT//spl radic/Hz have been made with variations in junction parameters (I/sub c/ and R/sub n/) that are as low as 25% on-chip and 57% from chip-to-chip.

The subgap current in Nb∕AlOx∕Nb tunnel junctions

We have measured the subgap current at 0.8 K in several hysteretic 1.5×1.6 and 2.6×2.6μm2 Nb∕AlOx∕Nb tunnel junctions. In zero magnetic field, the critical current Ic∼8μA and the subgap resistance R∼0.3MΩ. When we apply a magnetic field, Ic can be as low as 1 nA, and R is larger than 1 GΩ. This means that in a superconducting qubit made using Nb tunnel junctions, the decoherence rate from the quasiparticle conductance may be as low as 1∕(100μs).

The response of high-Tc SQUID magnetometers to small changes in temperature

We have investigated the response of the flux-locked output of several high-Tc SQUID magnetometers to small changes in temperature and for magnetic fields 0–30 μT. The temperature response DT≡dΦS/dT is observed to be linear in the applied magnetic field Ba and can be as large as 500 mΦ0/K where ΦS is the flux through the SQUID loop and Φ0 is the flux quantum. Our measurements can be explained using a simple model that takes into account the geometry of a given device and is based on the idea that DT is due to the temperature dependence of the superconducting penetration depth. Our results can be used to optimize device performance in applications where the noise of a device is dominated by ambient temperature fluctuations.