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Josephson amplifier for qubit readout
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View: Figures


Image of FIG. 1.
FIG. 1.

(Color online) (a) A circuit representation of the J-amp device based on the JPC consisting of two half-wave transmission line resonators (λS /2 and λI /2) supporting two eigenmodes of the JRM, denoted as signal (S) and idler (I). The idler is fed through the difference port (Δ) of a 180° hybrid, while the third eigenmode denoted as pump (P) is non-resonant and fed to the device via the sum port (Σ). The flux bias applied to the ring is half a flux-quantum. (b) Optical microscope image of the J-amp device, consisting of two microstrip resonators and a JRM at the middle. The upper insets show zoom-in SEM micrographs of the JRM (left inset) and one Josephson junction of the ring (right inset). The bottom inset shows one of the gap capacitors which couple the feedlines to the resonators and determine the total quality factor of the resonators.

Image of FIG. 2.
FIG. 2.

(Color online) Gain curves of the signal mode versus frequency. The center frequency of the amplifier is tuned by varying the pump frequency. The data show a frequency range of about 60 MHz in which the signal gain exceeds 10 dB. The arrows plotted on top of the curves indicate the locations of the calculated signal frequencies.

Image of FIG. 3.
FIG. 3.

(Color online) The bandwidth of the amplifier B versus . The red (squares) and blue (circles) correspond to idler and signal data respectively. The linear fits drawn as red and blue dashed lines satisfy a relation of the form . The black solid line depicts the theoretical expression for the bandwidth B of the amplifier in the high gain limit (see text). The two diamond-shaped points red (top) and blue (bottom) drawn at unity gain correspond to the bandwidths of the bare resonators I and S, respectively.

Image of FIG. 4.
FIG. 4.

(Color online) A 1 dB compression point measurement of the J-amp as a function of signal gain. The dashed green line connects 1 dB compression points measured for different gains G 0. The black curve depicts the upper limit on the signal power predicted by theory. The solid red curve corresponds to a Microwave Office transmission-line simulation of the device in the range of measured gains.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Josephson amplifier for qubit readout