Low-Noise Amplification and Frequency Conversion with a Multiport Microwave Optomechanical Device

High-gain amplifiers of electromagnetic signals operating near the quantum limit are crucial for quantum information systems and ultrasensitive quantum measurements. However, the existing techniques have a limited gain-bandwidth product and only operate with weak input signals. Here, we demonstrate a two-port optomechanical scheme for amplification and routing of microwave signals, a system that simultaneously performs high-gain amplification and frequency conversion in the quantum regime. Our amplifier, implemented in a two-cavity microwave optomechanical device, shows 41 dB of gain and has a high dynamic range, handling input signals up to ${10}^{13}$ photons per second, 3 orders of magnitude more than corresponding Josephson parametric amplifiers. We show that although the active medium, the mechanical resonator, is at a high temperature far from the quantum limit, only 4.6 quanta of noise is added to the input signal. Our method can be readily applied to a wide variety of optomechanical systems, including hybrid optical-microwave systems, creating a universal hub for signals at the quantum level.

Popular Summary

Detecting a weak electromagnetic signal in quantum computers and ultrasensitive quantum measurements requires that the signal first be amplified. However, the laws of quantum mechanics dictate that it is impossible to make a perfect, amplified copy of the complete signal: A small amount of noise, corresponding to at least half a quantum of energy, must always be added. Reaching this so-called quantum limit is challenging, and existing techniques suffer from limitations, such as only operating with very weak input signals. Here, we demonstrate a new technique for boosting the strength of weak electromagnetic signals.

We propose a two-port optomechanical setup in which two electromagnetic cavities with different resonant frequencies are coupled to an aluminum mechanical resonator. Working at a base temperature of 7 mK, we show that microwave signals can be both amplified and frequency-converted. Our data reveal a peak gain of 41 dB (i.e., a factor of roughly 12,500). Our new microwave amplifier operates close to the quantum limit, but at the same time, it can handle very large signals. Even if the core element of our device—the mechanical resonator—lies in a relatively noisy environment, the amplifier design allows the output noise to be close to the limits imposed by quantum mechanics. The amplifier also acts as a converter between signals of different frequencies. This concept could be used to create a quantum hub, where signals of different, otherwise incompatible, quantum systems are connected together.

We expect that our findings will pave the way for future quantum networks and allow quantum computers to combine the best aspects of different systems.

Figure 1

Experimental setup. (a) Schematic of our device, showing two $LC$ microwave cavities (${\omega }_1$, ${\omega }_2$) both coupled to a central mechanical drum resonator (${\omega }_m$) as well as an individual feed line. Signals (${\omega }_{s1}$, ${\omega }_{s2}$) and pumps (${\omega }_{P+}$, ${\omega }_{P-}$) are fed to the cavities as shown. The output of cavity 1 is preamplified and measured with a signal analyzer. (b) Atomic force micrograph of the drum resonator. Scale bar is $10\mu \mathrm{m}$ long. (c) Conceptual two-port amplifier, exhibiting both direct ($A_d$) and cross ($A_x$) gain. (d) Schematic representation of the cavity modes and pump frequencies used to realize a two-port amplifier. As an example, an input signal with frequency ${\omega }_{s2}$ is injected to cavity 2, with the frequency-converted and amplified output emerging from cavity 1.

Figure 2

Two-mode amplifier performance. (a) Direct gain $|A_d{|}^2$ versus signal frequency for fixed $G_{-}$ and various values of $G_{+}$ (colored lines, legend shows $\mathcal{G}/2\pi$) together with theory fits (black lines). Inset: Amplifier configuration; the signal is input to cavity 1. (b) Example output spectrum of cavity 1 (blue line), showing high-gain frequency conversion of a weak signal injected in cavity 2 (narrow peak). The peak height at different frequencies (circles) agrees with the fitted model (red line). Inset: Amplifier configuration. (c) Effective input-referred noise for the same $G_{-}$,$G_{+}$ as in (a) (colored lines). The theory model (black line) is plotted only for the highest gain. The blue shaded area shows the input noise of one-half quanta at each input, and the red shaded area shows the modeled added noise $S_{\mathrm{add}}$.

Figure 3

Amplifier performance and oscillations. (a) Gain-bandwidth product for several values of $G_{-}$ (legends show $G_{-}/2\pi$) as a function of $\mathcal{G}$. (b) Peak direct gain and (c) effective input-referred noise (closed circles) and added amplifier noise $S_{\mathrm{add}}$ (open squares) on resonance, for the same parameters as (a). (d) Performance of direct amplification at large input powers, measured for $G_{-}\approx 2\pi \times 350\mathrm{kHz}$ and several values of $\mathcal{G}$ (colors). Legend shows the nominal gain for each setting. (e) Harmonic spectra of oscillations in the instability regime without (open squares) and with (closed circles) the red-detuned pump at ${\omega }_{P-}$ enabled. For the latter case, each harmonic component is measured separately and the horizontal line shows the measurement noise floor.

Figure 4

Frequency conversion without amplification. (a) Output spectrum $S({\omega }^{\prime })$ (arbitrary units, each curve offset by 10 dB) showing the frequency-converted output signal ${\omega }_s^{\prime }\approx {\omega }_1$ for several input frequencies ${\omega }_s\approx {\omega }_2$. (b) Pump and signal configuration. (c) Conversion efficiency on resonance (${\omega }_s={\omega }_2$) along with fitted theory (see text), for $P_{P2}\approx 4\mathrm{dBm}$ (yellow), 14 dBm (red), and 22 dBm (blue), in the same units as $P_{P1}$. Arrow indicates data shown in (a). (d) Coherence of conversion. Shown are spectra of the mixed-down pump tone ${\delta }_P$ (blue line) and the destructive interference of ${\delta }_P$ and ${\delta }_s$ (red line).