David Olaya

Ultrasensitive hot-electron nanobolometers for terahertz astrophysics

The submillimetre or terahertz region of the electromagnetic spectrum contains approximately half of the total luminosity of the Universe and 98% of all the photons emitted since the Big Bang. This radiation is strongly absorbed in the Earths atmosphere, so space-based terahertz telescopes are crucial for exploring the evolution of the Universe. Thermal emission from the primary mirrors in these telescopes can be reduced below the level of the cosmic background by active cooling, which expands the range of faint objects that can be observed. However, it will also be necessary to develop bolometers-devices for measuring the energy of electromagnetic radiation-with sensitivities that are at least two orders of magnitude better than the present state of the art. To achieve this sensitivity without sacrificing operating speed, two conditions are required. First, the bolometer should be exceptionally well thermally isolated from the environment; second, its heat capacity should be sufficiently small. Here we demonstrate that these goals can be achieved by building a superconducting hot-electron nanobolometer. Its design eliminates the energy exchange between hot electrons and the leads by blocking electron outdiffusion and photon emission. The thermal conductance between hot electrons and the thermal bath, controlled by electron-phonon interactions, becomes very small at low temperatures ( approximately 1 x 10-16 W K-1 at 40 mK). These devices, with a heat capacity of approximately 1 x 10-19 J K-1, are sufficiently sensitive to detect single terahertz photons in submillimetre astronomy and other applications based on quantum calorimetry and photon counting.

1 V and 10 V SNS Programmable Voltage Standards for 70 GHz

Programmable Josephson voltage standards (PJVSs) in combination with fast switchable DC current sources have opened up new applications in the field of low-frequency AC metrology. The growing interest in output voltages of up to plusmn10 V initiated efforts by several National Metrological Institutes to realize 10 V PJVSs. Presently, only 10 V PJVSs from PTB based on SINIS junctions have been successfully incorporated into existing setups for AC metrology. However, the fabrication of 10 V SINIS arrays that are driven at 70 GHz suffers from very low yield. The recent technological progress made at NIST enabled the drop-in replacement of the low-yield SINIS arrays by more robust SNS arrays. The N-material is an amorphous NbxSi1-x alloy near the metal-insulator transition and is deposited by co-sputtering. For the first time, fully operational 1 V and 10 V PJVSs with SNS junctions that are suitable for a 70 GHz drive have been fabricated and tested. This work was done in close cooperation between NIST and PTB.

NIST 10 V Programmable Josephson Voltage Standard System

The National Institute of Standards and Technology has developed and implemented a new programmable Josephson voltage standard (PJVS) that operates at 10 V. This next-generation system is optimized for both dc metrology and stepwise-approximated ac voltage measurements for frequencies up to a few hundreds of hertz. The nonhysteretic Josephson junctions produce intrinsically stable voltages and are designed to operate in the 18-20 GHz frequency range. The most recent 10 V PJVS circuits have total output current ranges greater than 1 mA.

Superconducting nanocircuits for topologically protected qubits

For successful realization of a quantum computer, its building blocks—the individual qubits—should be simultaneously scalable and sufficiently protected from environmental noise. Recently, a novel approach to the protection of superconducting qubits has been proposed. The idea is to prevent errors at the hardware level, by building a fault-free logical qubit from ‘faulty’ physical qubits with properly engineered interactions between them. The decoupling of such a topologically protected logical qubit from local noises is expected to grow exponentially with the number of physical qubits. Here, we report on proof-of-concept experiments with a prototype device that consists of twelve physical qubits made of nanoscale Josephson junctions. We observed that owing to properly tuned quantum fluctuations, this qubit is protected against magnetic flux variations well beyond linear order, in agreement with theoretical predictions. These results suggest that topologically protected superconducting qubits are feasible. An array of superconducting nanocircuits has been designed that provides built-in protection from environmental noises. Such ‘topologically protected’ qubits could lead the way to a scalable architecture for practical quantum computation.

Record-Low NEP in Hot-Electron Titanium Nanobolometers

We are developing hot-electron superconducting transition-edge sensors (TES) capable of counting THz photons and operating at . We fabricated superconducting Ti nanosensors with Nb contacts with a volume of on planar Si substrates and have measured the thermal conductance in the material, G=4times10-3 W/K at 0.3 K, caused predominantly by the weak electron-phonon coupling. The corresponding phonon-noise NEP=3times10-19 W/Hz1/2 . Detection of single optical photons (1550 nm and 670 nm wavelength) has been demonstrated for larger devices and yielded the thermal time constants of 30 mus at 145 mK and of 25 mus at 190 mK. This hot-electron direct detector (HEDD) is expected to have a small enough energy fluctuation noise for detecting individual photons with v>THz where NEP~3times10-20 W/Hz1/2 is needed for spectroscopy in space.

10 Volt Programmable Josephson Voltage Standard Circuits Using NbSi-Barrier Junctions

Programmable Josephson voltage standard (PJVS) circuits were developed that operate at 16 GHz to 20 GHz with operating margins larger than 1 mA. Two circuit designs were demonstrated, each having a total of ~ 300,000 junctions, which were divided into either 16 or 32 sub-arrays. Triple-stacked junctions were used in order to fit the ~300,000 junctions on each circuit. The amorphous NbxSi1-x-barrier junction technology provided a high degree of uniformity of the barrier and junction electrical properties, which was necessary to achieve the 1 mA operating margin. Although the margins on both the 16-array and 32-array circuits were much greater than 1 mA, the circuit yield for the 16-array design was lower because the longer arrays are more sensitive to defects. The use of lumped-element microwave splitters and tapered arrays significantly reduced the microwave input power and increased the operating margins of these designs. In addition, the broadband microwave response of the designs allowed the PJVS output voltage to be continuously adjusted by using the microwave frequency while remaining on margins.

High-Speed Nb/Nb–Si/Nb Josephson Junctions for Superconductive Digital Electronics

Josephson junctions with cosputtered amorphous Nb-Si barriers are being developed at NIST for use in voltage standard circuits. These junctions have the potential for a wide range of applications beyond voltage standards because their electrical properties can be tuned by controlling both the composition and the thickness of the barrier. If the composition of the barrier is tuned so that the resistivity is close to the metal-insulator transition, the high resistivity allows junctions with a large characteristic voltage and reproducible critical-current densities, which should be ideal for high-speed digital superconductive device applications. Because these junctions are intrinsically shunted, there is no need for external shunt resistors, which could start to become a limitation as the development of devices leads to higher critical-current densities and greater circuit densities. Presently, the AlOx-barrier junctions used in digital superconducting electronics suffer from poor reproducibility, particularly for the high critical-current densities needed for high-speed applications. In this paper, amorphous Nb-Si barrier junctions with characteristic voltages on the order of 1 mV and characteristic frequencies on the order of hundreds of gigahertz are demonstrated. This junction technology looks promising for applications in high-speed digital electronics.

Electrical properties of La-doped strontium titanate thin films

We report on the properties of lanthanum-doped SrTiO3 thin films grown by off-axis laser ablation on LaAlO3 and SrTiO3 substrates in oxygen partial pressures ranging from 10−8 Torr to 55 mTorr. The La/Sr doping ratio of the ablation target was 1%. The resulting films have carrier densities measured in the range of 4.7–17.5×1019 cm−3 independent of temperature from room temperature to 4 K and low-temperature mobilities as high as 130 cm2/V s. These films are much more tolerant to the presence of oxygen during growth than were similar Nb-doped films reported previously.

Energy-resolved detection of single infrared photons with λ = 8 μm using a superconducting microbolometer

We report on the detection of single photons with λ = 8 μm using a superconducting hot-electron microbolometer. The sensing element is a titanium transition-edge sensor with a volume ∼0.1 μm3 fabricated on a silicon substrate. Poisson photon counting statistics including simultaneous detection of 3 photons was observed. The width of the photon-number peaks was 0.11 eV, 70% of the photon energy, at 50–100 mK. This achieved energy resolution is one of the best figures reported so far for superconducting devices. Such devices can be suitable for single-photon calorimetric spectroscopy throughout the mid-infrared and even the far-infrared.

Energy resolution of terahertz single-photon-sensitive bolometric detectors

We report measurements of the energy resolution of ultrasensitive superconducting bolometric detectors. The device is a superconducting titanium nanobridge with niobium contacts. A fast microwave pulse is used to simulate a single higher-frequency photon, where the absorbed energy of the pulse is equal to the photon energy. This technique allows precise calibration of the input coupling and avoids problems with unwanted background photons. Present devices have an intrinsic full-width at half-maximum energy resolution of approximately 23 THz, near the predicted value due to intrinsic thermal fluctuation noise.