Pumping up the quantum

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Science  16 Oct 2015:
Vol. 350, Issue 6258, pp. 280
DOI: 10.1126/science.aad0858

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The detection of quantum signals from electronic devices requires exquisite instrumentation: The energy of a single photon from a cell phone, for example, is more than five orders of magnitude smaller than that of a visible-light photon. However, experiments on superconducting and semiconducting quantum bits require this level of sensitivity, as these quantum systems operate in the same band of frequencies as cell phones and microwave ovens. On page 307 of this issue, Macklin et al. (1) demonstrate a new kind of microwave amplifier that achieves the requisite sensitivity: a traveling-wave parametric amplifier, which uses a chain of superconducting amplifying elements, linked together in a nonlinear transmission line. This device can amplify microwave signals over a broad range of frequencies, while adding an extremely small amount of noise—close to the minimum amount of noise as set by quantum mechanics. This innovation represents an improvement over the previous amplifiers, which while achieving roughly equivalent performance in terms of quantum-limited noise (2), only work over a narrow range of frequencies and for very small microwave powers. The latter two limitations have now been substantially lifted.