用硅光子纳米力学界面转换微波和电信光子

Practical quantum networks require low-loss and noise-resilient optical interconnects as well as non-Gaussian resources for entanglement distillation and distributed quantum computation. The latter could be provided by superconducting circuits but existing solutions to interface the microwave and optical domains lack either scalability or effificiency, and in most cases the conversion noise is not known. In this work we utilize the unique opportunities of silicon photonics, cavity optomechanics and superconducting circuits to demonstrate a fully integrated, coherent transducer interfacing the microwave X and the telecom S bands with a total (internal) bidirectional transduction effificiency of 1.2% (135%) at millikelvin temperatures. The coupling relies solely on the radiation pressure interaction mediated by the femtometer-scale motion of two silicon nanobeams reaching a Vπ as low as 16 μV for subnanowatt pump powers. Without the associated optomechanical gain, we achieve a total (internal) pure conversion effificiency of up to 0.019% (1.6%), relevant for future noise-free operation on this qubit-compatible platform.

Large scale quantum networks will facilitate the next level in quantum information technology, such as the internet did for classical communication, enabling, e.g., secure communication and distributed quantum computation. Some of the most promising platforms to process quantum information locally, such as superconducting circuits, spins in solids, and quantum dots, operate naturally in the gigahertz frequency range, but the long-distance transmission of gigahertz radiation is relatively lossy and not resilient to ambient noise. This limits the length of supercooled microwave waveguides in a realistic scenario to tens of meters5. In contrast, the transport of quantum information over distances of about 100 km is nowadays routinely achieved by sending optical photons at telecom frequency through optical fifibers.

We realize conversion by connecting an optomechanical photonic crystal zipper cavity with two aluminum coated and mechanically compliant silicon nanostrings as shown in Fig. 1c. The mechanical coupling between these two components is carefully designed (see Supplementary Note 2), leading to a hybridization of their in-plane vibrational modes into symmetric and antisymmetric supermodes. In case of the antisymmetric mode that is used in this experiment, the strings and the photonic crystal beams vibrate 180° out of phase as shown by the fifinite-element method simulation in Fig. 1d. The photonic crystal cavity features two resonances at telecom frequencies with similar optomechanical coupling strength. The simulated spatial distribution of the electric fifield component Ey(x, y) of the higher frequency mode with lower loss rate used in the experiment is shown in Fig. 1e. The lumped element microwave resonator consists of an ultra-low stray capacitance planar spiral coil inductor and two mechanically compliant capacitors with a vacuum gap of size of ~70 nm. This resonator is inductively coupled to a shorted coplanar waveguide, which is used to send and retrieve microwave signals from the device. The sample is fabricated using a robust multi-step recipe including electron beam lithography, silicon etching, aluminum thin-fifilm deposition, and hydroflfluoric vapor acid etching, as described in detail in ref. 

Fig1(d)

Figure 2b shows the total transduction effificiency for different pump power combinations with microwave and optical pump powers ranging from 30 to 953 pW and 48 to 1561 pW, respectively. Figure 2c, d shows the effificiency versus Po (Pe) for fifixed microwave (optical) pump power Pe = 601 (Po = 625) pW. As expected, the transduction effificiency rises with increasing pump powers and reaches a maximum of ζ = 1.2%. The internal transduction effificiency is signifificantly higher (ζ/(ηoηe) ≤ 135%) because both the microwave resonator as well as the optical cavity are highly undercoupled with coupling ratios of ηo = 0.11 and ηe ranging between 0.07 and 0.18 when both pumps are on. The increase in the intrinsic loss rate of microwave κin,e and mechanical resonator γm at higher pump powers are shown in Fig. 2e and f caused by considerable heating related to (especially optical) photon absorption. This results in the degradation of the microwave and mechanical quality factors and consequently reduces the waveguide coupling effificiency, the cooperativities and the total transduction effificiency (see Supplementary Note 5).

In conclusion, we demonstrated an effificient bidirectional and chip-scale microwave-to-optics transducer using pump powers orders of magnitude lower than comparable all-integrated approaches. Low pump powers are desired to limit the heat load of the cryostat and to minimize on-chip heating, which is particularly important for integrated devices because of their limited heat dissipation at millikelvin temperatures. Due to the standard material choice involving only silicon and aluminum, our device can be easily integrated with other elements of superconducting circuits as well as silicon photonic and phononic devices in the future.