Abstract
Fiber optic communication is the backbone of our modern information society, offering high bandwidth, low loss, weight, size and cost, as well as an immunity to electromagnetic interference. Microwave photonics lends these advantages to electronic sensing and communication systems, but - unlike the field of 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 present a cavity electro-optic transceiver operating in a millikelvin environment with a mode occupancy as low as 0.025 $\pm$ 0.005 noise photons. Our system is based on a lithium niobate whispering gallery mode resonator, resonantly coupled to a superconducting microwave cavity via the Pockels effect. For the highest continuous wave pump power of 1.48 mW we demonstrate bidirectional single-sideband conversion of X band microwave to C band telecom light with a total (internal) efficiency of 0.03 % (0.7 %) and an added output conversion noise of 5.5 photons. The high bandwidth of 10.7 MHz combined with the observed very slow heating rate of 1.1 noise photons s$^{-1}$ puts quantum limited pulsed microwave-optics conversion within reach. The presented device is versatile and compatible with superconducting qubits, which might open the way for fast and deterministic entanglement distribution between microwave and optical fields, for optically mediated remote entanglement of superconducting qubits, and for new multiplexed cryogenic circuit control and readout strategies.
Highlights
The last three decades have witnessed the emergence of a great diversity of controllable quantum systems, and superconducting Josephson circuits are one of the most promising candidates for the realization of scalable quantum processors [1]
The electro-optic transducer consists of a z-cut lithium niobate (LiNbO3) whispering gallery mode (WGM) resonator, with major radius R = 2.5 mm, sidewall surface radius ρ ≈ 0.7 mm, and thickness d = 0.15 mm
The presented bidirectional microwave-optical interface operates in the quantum ground state Ne 1, as verified by measuring the minimal noise Nout 1 added to a converted microwave output signal
Summary
The last three decades have witnessed the emergence of a great diversity of controllable quantum systems, and superconducting Josephson circuits are one of the most promising candidates for the realization of scalable quantum processors [1]. Compared to the current benchmark for a general purpose quantum interface [13], we show close to ground-state operation, resulting in extremely low conversion noise of Nout ≤ 0.074 photons at the output and achieve that with a 102–103 times higher bandwidth This is at the expense of a lower efficiency, resulting in a significantly larger equivalent input noise Nin. the measurement of groundstate initialization is an important stepping stone in order to convert nonclassical states with high fidelity in the future. The large size and heat capacity allows for extremely slow thermalization times, which are about 107 times slower compared to state-ofthe-art microscopic microwave devices pulsed with about 103 times lower power [24] This is expected to result in a higher duty cycle at the same temperature and efficiency, which might lead to a significantly higher channel capacity in the context of pulsed conversion of quantum states
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