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Phase-preserving amplification near the quantum limit with a Josephson ring modulator


Recent progress in solid-state quantum information processing1 has stimulated the search for amplifiers and frequency converters with quantum-limited performance in the microwave range. Depending on the gain applied to the quadratures of a single spatial and temporal mode of the electromagnetic field, linear amplifiers can be classified into two categories (phase sensitive and phase preserving) with fundamentally different noise properties2. Phase-sensitive amplifiers use squeezing to reduce the quantum noise, but are useful only in cases in which a reference phase is attached to the signal, such as in homodyne detection. A phase-preserving amplifier would be preferable in many applications, but such devices have not been available until now. Here we experimentally realize a proposal3 for an intrinsically phase-preserving, superconducting parametric amplifier of non-degenerate type. It is based on a Josephson ring modulator, which consists of four Josephson junctions in a Wheatstone bridge configuration. The device symmetry greatly enhances the purity of the amplification process and simplifies both its operation and its analysis. The measured characteristics of the amplifier in terms of gain and bandwidth are in good agreement with analytical predictions. Using a newly developed noise source, we show that the upper bound on the total system noise of our device under real operating conditions is three times the quantum limit. We foresee applications in the area of quantum analog signal processing, such as quantum non-demolition single-shot readout of qubits4, quantum feedback5 and the production of entangled microwave signal pairs6.

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Figure 1: The JPC and its microwave measurement set-up.
Figure 2: Gain of the JPC.
Figure 3: Tuning the bandwidth of the JPC.
Figure 4: Noise measurement of the JPC for a 30-dB gain.

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We thank B. Abdo for his reading of the manuscript. This work was supported by the US National Security Agency through the US Army Research Office grant W911NF-05-01-0365, the W. M. Keck Foundation, and the US National Science Foundation through grant DMR-032-5580. M.H.D. acknowledges partial support from the College de France and from the French Agence Nationale de la Recherche.

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Authors and Affiliations



N.B. and L.F. fabricated the device. N.B., assisted by F.S. and M.M., performed the measurements. N.B. and M.H.D. carried out the analysis of the results and wrote the paper. R.V., V.E.M., R.J.S. and S.M.G. contributed extensively to discussions of the results. D.E.P. suggested the hot-electron noise source for calibration. R.J.S. contributed through his knowledge of ultralow-noise microwave circuits and measurements.

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Correspondence to N. Bergeal or M. H. Devoret.

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Bergeal, N., Schackert, F., Metcalfe, M. et al. Phase-preserving amplification near the quantum limit with a Josephson ring modulator. Nature 465, 64–68 (2010).

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