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arXiv:1008.0046 · Applied Physics Letters

Strongly quadrature-dependent noise in superconducting microresonators measured at the vacuum-noise limit

J. Gao, Leila R. Vale, John A. B. Mates, Dan R. Schmidt, Gene C Hilton, K. D. Irwin, F. Mallet, Manuel A. Castellanos-Beltran, Konrad W. Lehnert, Jonas Stasys Zmuidzinas, Henry George LeDuc

Published Jun 6, 2011• 33 citations
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Abstract

We measure frequency- and dissipation-quadrature noise in superconducting microresonators with sensitivity near the vacuum noise level using a Josephson parametric amplifier. At an excitation power of 100 nW, frequency noise rises orders of magnitude above the vacuum noise, but no excess dissipation-quadrature noise is observed above the vacuum noise level. Our results suggest that using quantum amplifiers in dissipation measurement may greatly improve the sensitivity of microresonator readout, which has important implications for applications such as detectors, qubits, and nanomechanical oscillators.

DOI
10.1063/1.3597156

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We measure frequency- and dissipation-quadrature noise in superconducting microresonators with sensitivity near the vacuum noise level using a Josephson parametric amplifier. At an excitation power of 100 nW, frequency noise rises orders of magnitude above the vacuum noise, but no excess dissipation-quadrature noise is observed above the vacuum noise level. Our results suggest that using quantum amplifiers in dissipation measurement may greatly improve the sensitivity of microresonator readout, which has important implications for applications such as detectors, qubits, and nanomechanical oscillators.
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We present measurements of the temperature and power dependence of the resonance frequency and frequency noise of superconducting niobium thin-film coplanar waveguide resonators carried out at temperatures well below the superconducting transition (Tc=9.2K). The noise decreases by nearly two orders of magnitude as the temperature is increased from 120to1200mK, while the variation of the resonance frequency with temperature over this range agrees well with the standard two-level system (TLS) model for amorphous dielectrics. These results support the hypothesis that TLSs are responsible for the noise in superconducting microresonators and have important implications for resonator applications such as qubits and photon detectors.
We are engaged in an experimental effort to generate and detect squeezed vacuum fluctuations at microwave frequencies using a Josephson parametric amplifier. We have demonstrated 42% deamplification of 4.2-K thermal noise at 19.4 GHz. At this frequency the vacuum fluctuation noise is an order of magnitude smaller than 4.2-K thermal noise. We have installed cryogenic HEMT amplifiers in our apparatus, thereby increasing the detector sensitivity by an order of magnitude. Josephson parametric amplifiers have been notorious for exhibiting excess noise whose origin is still controversial. We have performed a systematic study of the noise performance of our device both below and above threshold in a variety of modes of operation. The experiments indicate that the excess noise in our device arises from low-frequency noise (0-20 MHz) propagating down dc bias lines. When an intense coherent oscillation is present in the signal passband, such as the pump when the amplifier is in the four-photon mode or the period doubled oscillations when the amplifier is operated in the three-photon mode above threshold, the low- frequency noise mixes with the coherent oscillation to generate noise sidebands in the amplifier's passband. Nevertheless, in suitable conditions, extremely low noise and high gains were achieved.
The measurement of a quantum system is often performed by encoding its state in a single observable of a light field. The measurement efficiency of this observable can be reduced by loss or excess noise on the way to the detector. Even a quantum-limited detector that simultaneously measures a second noncommuting observable would double the output noise, therefore limiting the efficiency to $50\mathrm{%}$. At microwave frequencies, an ideal measurement efficiency can be achieved by noiselessly amplifying the information-carrying quadrature of the light field but this has remained an experimental challenge. Indeed, while state-of-the-art Josephson-junction-based parametric amplifiers can perform an ideal single-quadrature measurement, they require lossy ferrite circulators in the signal path, drastically decreasing the overall efficiency. In this paper, we present a nonreciprocal parametric amplifier that combines single-quadrature measurement and directionality without the use of strong external magnetic fields. We extract a measurement efficiency of ${62}_{\ensuremath{-}9}^{+17}\mathrm{%}$ that exceeds the quantum limit and that is not limited by fundamental factors. The amplifier can be readily integrated with superconducting devices, creating a path for ideal measurements of quantum bits and mechanical oscillators.
We introduce a new and quantitative theoretical framework for noise spectral analysis using a threshold detector, which is then applied to a superconducting device: the Josephson bifurcation amplifier. We show that this new framework provides direct access to the environmental noise spectrum with a sensitivity approaching the standard quantum limit of weak continuous measurements. In addition, the accessible frequency range of the spectrum is, in principle, limited only by the ring down time of the resonant mode. This on-chip noise detector is non-dissipative and works with low probing powers, allowing it to be operated at low temperatures ( mK). We exploit this technique for measuring the frequency fluctuations of our device and find a low frequency noise with an amplitude and spectrum compatible with a dielectric origin.