Microwave photonics lends the advantages of fiber optics to electronic sensing and communication systems. In contrast to nonlinear optics, electro-optic devices so far require classical modulation fields whose variance is dominated by electronic or thermal noise rather than quantum fluctuations. Here we demonstrate bidirectional single-sideband conversion of X band microwave to C band telecom light with a microwave mode occupancy as low as 0.025±0.005 and an added output noise of less than or equal to 0.074 photons. This is facilitated by radiative cooling and a triply resonant ultra-low-loss transducer operating at millikelvin temperatures. The high bandwidth of 10.7MHz and total (internal) photon conversion efficiency of 0.03% (0.67%) combined with the extremely slow heating rate of 1.1 added output noise photons per second for the highest available pump power of 1.48 mW puts near-unity efficiency pulsed quantum transduction within reach. Together with the non-Gaussian resources of superconducting qubits this might provide the practical foundation to extend the range and scope of current quantum networks in analogy to electrical repeaters in classical fiber optic communication.
(a) Exploded-view rendering of the electro-optic converter. The WGM resonator (light blue disc) is clamped between two aluminum rings (blue shaded areas) belonging to the top and bottom parts of the aluminum microwave cavity, respectively. Two gradient index (GRIN) lenses are used to focus the optical input and output beams (red) on a diamond prism surface in close proximity to the optical resonator. The microwave tone is coupled in and out of the cavity using a coaxial pin coupler at the top of the cavity (gold). The prism, both lenses, and the microwave tuning cylinder (gold shaded area inside the lower ring) positions can be adjusted with eight linear piezo positioners. (b) Optical reflection spectrum of the WGM resonator at base temperature (approximately 7 mK). The optical pump mode at ωp /(2π) ≈ 193.5 THz (green) and the signal mode (blue) are critically coupled and separated by one free spectral range (FSR, dashed lines). On resonance 38% of the optical power is reflected without entering the WGM resonator due to imperfect optical mode overlap (horizontal dotted line). The lower sideband mode (red) is chosen to couple to a mode family of different polarization, which splits it and facilitates the singlesideband selectivity of the converter. (c) Reflection spectrum of the microwave cavity at base temperature (approximately 7 mK) for the tuning cylinder in its up position (blue line) and in its down position (red line). With a tuning range of approximately 0.5 GHz we can readily match the cavity frequency with that of the optical free spectral range FSR/(2π) = 8.818 GHz (dashed line).
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