A combination 95 GHz radar and 270/560 GHz spectrometer is being built as a space instrument prototype for probing plumes and jet phenomena in the solar system. Dubbed GAISR (Gas And Ice Spectrometer/Radar), the instrument's radar will make simultaneous range/Doppler measurements of 0.1-10 mm sized ice and dust particles out to a few km in range, while its tunable spectrometer will detect the abundance and velocities of gaseous water and other volatiles. Here we describe how the radar and spectrometer share a back-end architecture, and present some innovative elements of GAISR's frequency-modulated continuous-wave (FMCW) radar, including high-isolation and low-loss transmit/receive duplexing and a phase-noise-canceling RF architecture.

Clean two-dimensional electron systems in GaAs/AlGaAs heterostructures exhibit anisotropic collective phases, the quantum Hall nematics, at high Landau level occupancy and low temperatures. An as yet unknown native symmetry-breaking potential consistently orients these phases relative to the crystalline axes of the host material. Here we report an extensive set of measurements examining the role of the structural symmetries of the heterostructure in determining the orientation of the nematics. In single quantum well samples we find that neither the local symmetry of the confinement potential nor the distance between the electron system and the sample surface dictates the orientation of the nematic. In remarkable contrast, for two-dimensional electrons confined at a single heterointerface between GaAs and AlGaAs, the nematic orientation depends on the depth of the two-dimensional electron system beneath the sample surface.

Van Hove singularities (VHSs) are a hallmark of reduced dimensionality, leading to a divergent density of states in one and two dimensions and predictions of new electronic properties when the Fermi energy is close to these divergences. In carbon nanotubes, VHSs mark the onset of new subbands. They are elusive in standard electronic transport characterization measurements because they do not typically appear as notable features and therefore their effect on the nanotube conductance is largely unexplored. Here we report conductance measurements of carbon nanotubes where VHSs are clearly revealed by interference patterns of the electronic wave functions, showing both a sharp increase of quantum capacitance, and a sharp reduction of energy level spacing, consistent with an upsurge of density of states. At VHSs, we also measure an anomalous increase of conductance below a temperature of about 30 K. We argue that this transport feature is consistent with the formation of Cooper pairs in the nanotube.

We have fabricated and tested an Al/AlOx/Al dc SQUID phase qubit on a sapphire substrate with a qubit junction area 0.17 μm2 and critical current I0,J1 = 1.4 μA. The qubit is shunted by an interdigitated capacitor and isolated from the bias leads by an inductive isolation network using a larger Josephson junction with area 2 μm2 and critical current I0,J2 = 29 μA. Additional high-frequency filtering is provided by an on-chip LC filter, which consists of square spiral inductors and parallel plate SiNx capacitors. From detuned Rabi oscillation measurements, we estimate a relaxation time T1 ≈ 400 ns and dephasing time TΦ 1700 ns. Additionally, we observe that the qubit transition spectrum has multiple equally spaced subpeaks. These subpeaks are caused by coupling between the phase qubit and the LC filter, forming a Jaynes-Cummings system. Each individual subpeak corresponds to transitions with different photon numbers in the LC filter.

We report a one-dimensional nonlocal experiment where the conductance of a section of carbon nanotube shows regular oscillations due to phase-coherent and ballistic transport in an adjacent section. This occurs in spite of wide strongly coupled contact electrodes, which are expected to divide the nanotube into independent sections. Our simulations show that the electrodes can be modeled as shallow and wide barriers which maintain quantum coherence of electron transport between the adjacent sections for lengths of several micrometers. DOI: 10.1103/PhysRevB.87.045403

We examine a dc SQUID phase qubit with an on-chip low-pass resonant LC filter that transforms the line impedance, improving the qubit lifetime. Unusual features in the spectroscopy suggest dynamics more complicated than a simple two-level system. To model this behavior, we consider a lumped-element circuit model of the SQUID that includes the filter as part of the quantum system to be modeled. We show this model reduces to an effective Jaynes-Cummings Hamiltonian, in analogy with circuit QED.

A 675 GHz imaging radar has been developed for standoff personnel screening applications. A key performance metric is the radar's ability to penetrate thick clothing to reveal concealed threats. Here we investigate the limitations of terahertz (THz) radar imaging in the regime of weak penetration, and present evidence that noise carried by the radar signal itself, rather than the receiver's thermal noise floor, is the limiting factor for imaging through thick clothing. This means that higher power sources or lower noise figure receivers at 675 GHz will not improve radar penetration. Rather, efforts in this direction should focus on lowering the THz source's phase noise through better back-end electronics architecture, or operating at lower frequencies at the cost of poorer image resolution.

We describe a thin-film superconducting Nb microwave resonator, tunable to within 0.3 ppm of the hyperfine splitting of ^87Rb at f_Rb=6.834683 GHz. We coarsely tuned the resonator using electron-beam lithography, decreasing the resonance frequency from 6.8637 GHz to 6.8278 GHz. For in situ fine tuning at 15 mK, the resonator inductance was varied using a piezoelectric stage to move a superconducting pin above the resonator. We found a maximum frequency shift of about 8.7 kHz per 60-nm piezoelectric step and a tuning range of 18 MHz.

We fabricated a dc SQUID phase qubit with a sub- μm Al/AlOx/Al qubit junction and an interdigitated shunting capacitor on a sapphire substrate. The qubit junction had a critical current of 135 nA, and the isolation junction had a critical current of 8.3 μA. The shunting capacitance was about 1.5 pF. To reduce the unwanted effects of two-level systems and increase the relaxation time T1, we have removed unnecessary dielectrics, used a small qubit junction area (450 nm × 500 nm), isolated the qubit from the leads with an on-chip LC filter, and fabricated the device on a bare sapphire substrate. However, at a temperature of 20 mK, we found T1 ≈ 300 ns and the coherence time T2 ≈ 110 ns, which was much lower than one would expect from loss attributed to the leads and to dielectrics in the tunnel junction and substrate. Measurements of T1 versus applied flux (which tuned the qubit frequency) revealed a correlation between the strength of the coupling of the microwave excitation line to the qubit and the rate of energy dissipation in the qubit. This result suggests that the relaxation time was being limited by coupling to the microwave line.

Demand for new surveillance capabilities for usage in airport screenings and battlefield security check-points has led to the development of terahertz imagers and sensors. There are several advantages of imaging at terahertz frequencies compared to microwave or infrared: the wavelengths in this regime are short enough to provide high resolution with modest apertures, yet long enough to penetrate clothing. Moreover, unlike in infrared, the terahertz frequencies are not affected by dust, fog, and rain.

We have observed anomalous switching curves (s-curves) in a dc SQUID phase qubit. The SQUID has two Al/AlOx/Al Josephson junctions in a 1.5 nH loop. One junction as a phase qubit and the other acts as a detector junction. The qubit junction area is 2 μm2, the critical current is 0.08 μA and the junction has an added low-loss SiNx shunting capacitor. The 1.37 μA detector junction is isolated from the bias leads by an on-chip inductor-capacitor (LC) network with a 130 MHz cut-off. We measure the state of the system at 20 mK by sending a short current pulse to the device and monitoring whether it switches to the voltage state. The s-curves for the ground state (0) and excited state (1) look much as expected. However, unlike previous results on phase qubits, when the device is pumped to a superposition state, the resulting s-curve is not a weighted sum of the s-curves for the 0 and 1 state. Instead, the s-curve appears to shift to lower pulse current with increasing amplitude to be in the 1 state. We examine this behavior and discuss implications for measurement of quantum superposition states in this system.

We report spectroscopic measurements of discrete two-level systems (TLSs) coupled to a dc superconducting quantum interference device phase qubit with a 16 mu m(2) area Al/AlO(x)/Al junction. Applying microwaves in the 10-11 GHz range, we found eight avoided level crossings with splitting sizes from 10 to 200 MHz and spectroscopic lifetimes from 4 to 160 ns. Assuming the transitions are from the ground state of the composite system to an excited state of the qubit or an excited state of one of the TLS states, we fit the location and spectral width to get the energy levels, splitting sizes, and spectroscopic coherence times of the phase qubit and TLSs. The distribution of splittings is consistent with noninteracting individual charged ions tunneling between random locations in the tunnel barrier and the distribution of lifetimes is consistent with the AlO(x) in the junction barrier having a frequency-independent loss tangent. To check that the charge of each TLS couples independently to the voltage across the junction, we also measured the spectrum in the 20-22 GHz range and found tilted avoided level crossings due to the second excited state of the junction and states in which both the junction and a TLS were excited.