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.
Figures at a glance
Figure 1: The JPC and its microwave measurement set-up. 
a, JPC sample (outlined in red) whose signal ports are connected to two attenuated input lines and two output lines using cryogenic circulators, and whose pump port is fed by a fifth line. The output signals are first amplified by high-electron-mobility transistor (HEMT) cryogenic amplifiers at the 4.2-K stage. Two isolators are placed at the 4.2-K and 1.6-K stages to minimize the back-action of the amplifier on the sample. At room temperature (300-K stage), the signals are further amplified by about 60 dB before being measured. b, Optical picture of the JPC. It consists of two coplanar-stripline aluminium resonators respectively 17 and 3.6 mm long coupled to the Josephson ring modulator on one side and to contact pads via input capacitors on the other side. A third coplanar stripline carries the pump signal. The input capacitors are built from two aluminium layers separated by a SiOx dielectric layer. c, Close-up of the connection between the Josephson ring modulator, the resonators and the pump line. The pump is weakly coupled to the resonators through the SiOx dielectric layer, which provides a small capacitance of order 20 fF. The crossing point of the two resonators is isolated by the same SiOx layer. d, Scanning electron microscope pictures of the Josephson ring modulator showing its four Al/AlOx/Al junctions. Each junction is surrounded by the two shadow electrodes produced by the Dolan bridge double-angle evaporation technique. The loop of the ring has an area of 3 μm × 17 μm and the junction area is 5 μm × 1 μm.
Figure 2: Gain of the JPC. 
a, Power cis-gain of the JPC as a function of the input signal frequency, for different values of the pump power, P, measured at port 1 (left) and port 2 (right). The solid lines correspond to the theoretical expressions for |r1|2 and |r2|2 (equation (3)) obtained for the indicated values of the fit parameter, |ρ|. Inset, curves obtained at higher gain. The fits correspond to |ρ| = 0.994 (left) and |ρ| = 0.992 (right). b, Photon trans-gain of the JPC as a function of the input signal frequency (bottom axis) and the converted frequency (top axis), measured between port 1 and port 2 (left) and between port 2 and port 1 (right) for different values of the pump power. The solid lines correspond to the theoretical expressions for |s1|2 and |s2|2 (equation (4)) obtained for the indicated values of |ρ|. c, Photon cis-gain of the JPC plotted (colour, dB) as a function of the drive frequencies and the pump power measured at port 1 (left) and port 2 (right). The data shown in a, b and c correspond to different runs.
Figure 3: Tuning the bandwidth of the JPC. 
Cis-gains |r1|2 (a) and |r2|2 (b) as functions of the corresponding drive frequency, for different values of the pump frequency, fp, as indicated. The pump amplitude has been adjusted at each frequency for optimal gain. The triangular symbols indicate the theoretical location of the centre frequency.
Figure 4: Noise measurement of the JPC for a 30-dB gain. 
a, Spectral noise power at the output of port 1 (colour) as a function of frequency and voltage across the resistor. b, Cuts of the spectral noise power corresponding to V = 0 μV and V = 85 μV. The noise floor obtained with the pump off is given as reference. c, Noise power at the output of port 1 (open dots; same units as in a), averaged over a 1-MHz band around fa, as a function of the voltage across the resistor. Red line, theoretical expression (equation (6) in Methods) with fitted added noise; green line, same as red line but assuming quantum shot noise for the resistor; black line, theoretical expression (6) plus an ideal quantum-limited amplifier. Inset, schematic of the resistor embedded in its thick and wide thermal reservoirs. d, Same as c but with the voltage axis converted into the effective temperature of the noise source, Teff. The dashed lines indicate asymptotes of the high-temperature variation. Also shown in this panel is the total added noise power of the system and the ideal case of the quantum limit (QL). Inset, temperature variation profile inside the resistor (Methods).
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