John Teufel

Quantum Nondemolition Measurement of a Nonclassical State of a Massive Object.

By coupling a macroscopic mechanical oscillator to two microwave cavities, we simultaneously prepare and monitor a nonclassical steady state of mechanical motion. In each cavity, correlated radiation pressure forces induced by two coherent drives engineer the coupling between the quadratures of light and motion. We, first, demonstrate the ability to perform a continuous quantum nondemolition measurement of a single mechanical quadrature at a rate that exceeds the mechanical decoherence rate, while avoiding measurement backaction by more than 13 dB. Second, we apply this measurement technique to independently verify the preparation of a squeezed state in the mechanical oscillator, resolving quadrature fluctuations 20% below the quantum noise.

Sideband cooling beyond the quantum backaction limit with squeezed light

Quantum fluctuations of the electromagnetic vacuum produce measurable physical effects such as Casimir forces and the Lamb shift. They also impose an observable limit—known as the quantum backaction limit—on the lowest temperatures that can be reached using conventional laser cooling techniques. As laser cooling experiments continue to bring massive mechanical systems to unprecedentedly low temperatures, this seemingly fundamental limit is increasingly important in the laboratory. Fortunately, vacuum fluctuations are not immutable and can be ‘squeezed’, reducing amplitude fluctuations at the expense of phase fluctuations. Here we propose and experimentally demonstrate that squeezed light can be used to cool the motion of a macroscopic mechanical object below the quantum backaction limit. We first cool a microwave cavity optomechanical system using a coherent state of light to within 15 per cent of this limit. We then cool the system to more than two decibels below the quantum backaction limit using a squeezed microwave field generated by a Josephson parametric amplifier. From heterodyne spectroscopy of the mechanical sidebands, we measure a minimum thermal occupancy of 0.19 ± 0.01 phonons. With our technique, even low-frequency mechanical oscillators can in principle be cooled arbitrarily close to the motional ground state, enabling the exploration of quantum physics in larger, more massive systems.

Demonstration of efficient nonreciprocity in a microwave optomechanical circuit

The ability to engineer nonreciprocal interactions is an essential tool in modern communication technology as well as a powerful resource for building quantum networks. Aside from large reverse isolation, a nonreciprocal device suitable for applications must also have high efficiency (low insertion loss) and low output noise. Recent theoretical and experimental studies have shown that nonreciprocal behavior can be achieved in optomechanical systems, but performance in these last two attributes has been limited. Here we demonstrate an efficient, frequency-converting microwave isolator based on the optomechanical interactions between electromagnetic fields and a mechanically compliant vacuum gap capacitor. We achieve simultaneous reverse isolation of more than 20 dB and insertion loss less than 1.5 dB over a bandwidth of 5 kHz. We characterize the nonreciprocal noise performance of the device, observing that the residual thermal noise from the mechanical environments is routed solely to the input of the isolator. Our measurements show quantitative agreement with a general coupled-mode theory. Unlike conventional isolators and circulators, these compact nonreciprocal devices do not require a static magnetic field, and they allow for dynamic control of the direction of isolation. With these advantages, similar devices could enable programmable, high-efficiency connections between disparate nodes of quantum networks, even efficiently bridging the microwave and optical domains.

Observation of strong radiation pressure forces from squeezed light on a mechanical oscillator

Non-classical states of light, such as squeezed states, are used in quantum metrology to improve the sensitivity of mechanical motion sensing, but conversely mechanical oscillations can enhance the measurement of squeezed light.

Ultrasensitive Quantum-Limited Far-Infrared STJ Detectors

We describe recent work at Yale on Superconducting Tunnel Junction (STJ) direct detectors that have been developed for submillimeter astronomy. To monitor the response of the detector with large readout bandwidth and excellent sensitivity, we use a novel readout based on radio frequency (RF) reflectometry, like the readout invented for the RF-SET. For calibration of the detector, we have developed an in-situ, on-chip, hot-cold photon source. This is a voltage biased gold microbridge. Noise emitted by the microbridge couples via a coplanar stripline to the detector. This provides a calibrated blackbody photon source with near unity coupling, fast chopping, and calculable output. We present recent detection results in the range 100-160 GHz. These demonstrate the expected good responsivity, high sensitivity, and fast response. This approach is easily used with a frequency-multiplexed readout, allowing economy of cold electronics. Ultimate sensitivity is in the range 1 times 10-19 W/(Hz)1/2.

Fabrication of tunnel junctions for direct detector arrays with single-electron transistor readout using electron-beam lithography

This paper describes the fabrication of small aluminum tunnel junctions for applications in astronomy. Antenna-coupled superconducting tunnel junctions with integrated single-electron transistor readout have the potential for photon-counting sensitivity at sub-mm wavelengths. The junctions for the detector and single-electron transistor can be made with electron-beam lithography and a standard self-aligned double-angle deposition process. However, high yield and uniformity of the junctions is required for large-format detector arrays. This paper describes how measurement and modification of the sensitivity ratio in the resist bilayer was used to greatly improve the reliability of forming devices with uniform, sub-micron size, low-leakage junctions.

Recent progress on photon-counting superconducting detectors for submillimeter astronomy

We are developing superconducting direct detectors for submillimeter astronomy that can in principle detect individual photons. These devices, Single Quasiparticle Photon Counter (SQPC), operate by measuring the quasiparticles generated when single Cooper-pairs are broken by absorption of a submillimeter photon. This photoconductive type of device could yield high quantum efficiency, large responsivity, microsecond response times, and sensitivities in the range of 10-20 Watts per root Hertz. The use of antenna coupling to a small absorber also suggests the potential for novel instrument designs and scalability to imaging or spectroscopic arrays. We will describe the device concept, recent results on fabrication and electrical characterization of these detectors, issues related to saturation and optimization of the device parameters. Finally, we have developed practical readout amplifiers for these high-impedance cryogenic detectors based on the Radio-Frequency Single-Electron Transistor (RF-SET). We will describe results of a demonstration of a transimpedance amplifier based on closed-loop operation of an RF-SET, and a demonstration of a wavelength-division multiplexing scheme for the RF-SET. These developments will be a key ingredient in scaling to large arrays of high-sensitivity detectors.

Measuring and Cooling the Motion of a Nanomechanical Oscillator with a Microwave Cavity Interferometer

By embedding a nanomechanical beam in a superconducting microwave cavity, we measure the beams motion near the standard quantum limit, we cool the beam with radiation pressure and, we realize an ultrasensitive force detector.