Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark.
Department of Micro- and Nanotechnology, Technical University of Denmark, DTU Nanotech, 2800 Kongens Lyngby, Denmark.
Nature. 2014 Mar 6;507(7490):81-5. doi: 10.1038/nature13029.
Low-loss transmission and sensitive recovery of weak radio-frequency and microwave signals is a ubiquitous challenge, crucial in radio astronomy, medical imaging, navigation, and classical and quantum communication. Efficient up-conversion of radio-frequency signals to an optical carrier would enable their transmission through optical fibres instead of through copper wires, drastically reducing losses, and would give access to the set of established quantum optical techniques that are routinely used in quantum-limited signal detection. Research in cavity optomechanics has shown that nanomechanical oscillators can couple strongly to either microwave or optical fields. Here we demonstrate a room-temperature optoelectromechanical transducer with both these functionalities, following a recent proposal using a high-quality nanomembrane. A voltage bias of less than 10 V is sufficient to induce strong coupling between the voltage fluctuations in a radio-frequency resonance circuit and the membrane's displacement, which is simultaneously coupled to light reflected off its surface. The radio-frequency signals are detected as an optical phase shift with quantum-limited sensitivity. The corresponding half-wave voltage is in the microvolt range, orders of magnitude less than that of standard optical modulators. The noise of the transducer--beyond the measured 800 pV Hz-1/2 Johnson noise of the resonant circuit--consists of the quantum noise of light and thermal fluctuations of the membrane, dominating the noise floor in potential applications in radio astronomy and nuclear magnetic imaging. Each of these contributions is inferred to be 60 pV Hz-1/2 when balanced by choosing an electromechanical cooperativity of ~150 with an optical power of 1 mW. The noise temperature of the membrane is divided by the cooperativity. For the highest observed cooperativity of 6,800, this leads to a projected noise temperature of 40 mK and a sensitivity limit of 5 pV Hz-1/2. Our approach to all-optical, ultralow-noise detection of classical electronic signals sets the stage for coherent up-conversion of low-frequency quantum signals to the optical domain.
弱射频和微波信号的低损耗传输和灵敏恢复是一个普遍存在的挑战，在射电天文学、医学成像、导航以及经典和量子通信中至关重要。将射频信号高效地转换到光载波上，将使它们能够通过光纤传输，而不是通过铜线传输，从而大大降低损耗，并使人们能够使用在量子限幅信号检测中常规使用的一整套已建立的量子光学技术。腔光机械学的研究表明，纳米机械振荡器可以与微波或光学场强烈耦合。在这里，我们展示了一种具有这两种功能的室温光电机械换能器，这是继最近使用高质量纳米膜提出的方案之后。小于 10 V 的电压偏置足以在射频共振电路中的电压波动与膜的位移之间诱导强耦合，而膜的位移同时与从其表面反射的光耦合。射频信号被检测为具有量子限幅灵敏度的光相位移动。相应的半波电压处于微伏范围，比标准光学调制器小几个数量级。换能器的噪声--除了测量到的射频共振电路的 800 pV Hz-1/2 约翰逊噪声之外--包括光的量子噪声和膜的热噪声，在射电天文学和核磁共振成像等潜在应用中主导噪声底。当通过选择 1 mW 的光功率和大约 150 的机电协同作用来平衡时，这些贡献中的每一个都被推断为 60 pV Hz-1/2。膜的噪声温度除以协同作用。对于观察到的最高协同作用 6800，这导致预计噪声温度为 40 mK，灵敏度极限为 5 pV Hz-1/2。我们的方法用于对经典电子信号进行全光学、超低噪声检测，为将低频量子信号相干地转换到光域奠定了基础。
