Session F01: Hybrid/Macroscopic Quantum Systems, Optomechanics, and Interfacing AMO with Solid State/Nano Systems I

Sensing and transduction with high-Q micromechanical membranes

I will present experiments probing micromechanical motion at quantum limits using SiN membranes. In particular I will discuss work in which we use ultra-high-Q patterned membranes in force sensing and in quantum transduction between microwave and optical signals.   

Improved optical cavity in microwave-mechanical-optical transducer

Linking quantum computational nodes to form a long-distance network in an architecture based on superconducting qubits faces an inherent challenge: the low excitation energy of microwave photons precludes quantum signal propagation at room temperature. A quantum coherent transducer between microwave and optical frequencies would overcome this difficulty. By coupling both a superconducting LC resonator and an optical Fabry-Perot cavity to the same MHz-frequency mode of a SiN membrane micromechanical oscillator, we realize a converter with 47% efficiency [1]. The process adds 38 photons of noise which prevent its quantum operation, with a significant contribution associated with the optical pump mediating the optomechanical interaction. By redesigning the optical cavity, we have reduced misalignment-induced optical loss and enabled greater optomechanical coupling, allowing reduced pump powers.

[1] Higginbotham, A. P., et. al. “Harnessing electro-optic correlations in an efficient mechanical converter,” Nature Physics 14, 1038-1042 (2018)   

Toward on-chip microwave to optical transduction using erbium doped crystals

Future quantum networks based on superconducting circuits that operate at microwave frequencies will strongly benefit from optical interconnects between the distant nodes. One promising platform to achieve bidirectional conversion between microwave and optical photons are ensembles of rare-earth ions (REIs) strongly coupled simultaneously to an optical and a microwave resonator. The strong interactions between the REIs and photons are critical to minimize the optical power required to mediate the conversion process and for potential operation at temperatures below 100 mK.
Here, we present our progress towards a REI-based transducer using superconducting microwave resonators and amorphous silicon photonic crystal resonators patterned on the surface of an erbium-doped yttrium orthovanadate crystal. Both resonators are designed to maintain high quality factors while preserving the mode overlap between the optical and microwave fields required for high conversion efficiency. We present initial results characterizing the device performance at cryogenic temperatures.   

Constructing Perfect Quantum Transducers Using Multi-Mode Imperfect Transducers

Quantum transducers can transfer quantum information between different bosonic physical platforms, which are crucial for hybrid quantum devices and quantum networks. In practice, however, transduction devices are not perfect, as there are systematic deviations and coupling to other bosonic modes. Recently, Lau and Clerk showed that combining finite single-mode squeezing and imperfect transduction operations we can implement desired perfect quantum transduction, but only restricted to two-mode . In this work, we generalize the previous results and show how to transform generic multi-mode imperfect transducers into perfect transducers requiring only finite-squeezing resources, based on a general protocol of decoupling irrelevant bosonic modes from the original system.
  

A compact trampoline-in-the-middle system for acoustic frequency quantum optomechanics

Strained nanomechanical resonators can have extremely low mechanical loss at acoustic frequencies, spurring recent proposals for ultra-sensitive force detection and quantum experiments at room temperature. We present a quasi-monolithic membrane-in-the-middle system that incorporates a 10 ng Si₃N₄ trampoline resonator with a fundamental frequency of 40 kHz into a 100 um Fabry-Perot cavity with a finesse of 30,000. Prior to cavity assembly, we record a mechanical loss rate of 0.8 mHz (a quality factor of 50 million) for the fundamental trampoline mode, corresponding to a force sensitivity of 30 aN/√Hz and a zero-point displacement spectral density of 0.3 pm/√Hz. Mounting results in significant added loss; however, the large optomechanical cooperativity still reveals itself as Brownian motion exceeding the cavity linewidth by an order of magnitude. We present a technique to “load” the resonator into the cavity by radiation pressure feedback cooling. Eliminating mounting loss would result in a vacuum cooperativity of 10⁴, accessing a regime currently targeted with levitated nanoparticles.   

Reducing added-noise in a micro-mechanically mediated electro-optic converter

Bidirectional transduction of microwave and optical fields would be a useful resource in the toolbox of quantum technology. We have demonstrated a mechanically mediated electro-optic converter by coupling a micromechanical oscillator to a microwave circuit and an optical cavity simultaneously. Operating this device at T < 100 mK, we recorded a conversion efficiency of 47% and added noise of 38 photons [1]. While we have realized high efficiency, the added noise is still an order of magnitude too large. Here, we present our efforts to lower the added noise by designing phononic shielding of the mechanics and replacing our superconducting metal with NbTiN.

[1] Higginbotham, A. P., et. al. “Harnessing electro-optic correlations in an efficient mechanical converter,” Nature Physics 14, 1038–1042 (2018)   

Towards quantum optomechanics using bulk acoustic wave resonators

Superconducting circuits are one of the most sophisticated architectures for quantum information processing to date. MW-to-optical conversion in the quantum regime could enhance their scalability and range of applications, since optical photons can be used as noise-free carriers of quantum information that connect circuits in different refrigerators. This requires conversion that is coherent, efficient, and with minimal added noise, which has not been demonstrated yet.
We present our advances in developing a cryogenic cavity optomechanical device based on a bulk-acoustic-wave (BAW) mechanical resonator, which can act as an essential part of a MW-to-optical transducer. Strong coupling to BAWs has been demonstrated both for microwave photons [1] and for optical photons [2]. Building on these works, we are developing an optomechanical system that is also compatible with coupling to superconducting circuits. We report on first experiments of BAW resonators cooled to millikelvin temperatures and coupled to optical cavity modes. As a first proof-of-principle, we discuss prospects for storage and retrieval of low-photon-number coherent optical states in the mechanical resonator.

[1] Chu et al., 2017, Science 358(6360).
[2] Kharel et al., 2018, Arxiv 1812.06202.   

Towards entanglement and interconversion of single mm-wave and optical photons in a hybrid cavity-QED system with Rydberg atoms

I will present our most recent progress towards entangling and interconverting single millimeter wave (mm-wave) and optical photons using Rydberg atoms as a transducer. Hybrid systems, which cross-couple optical and microwave regimes, can harness the unique strengths of optical systems for communication and microwave systems for quantum information processing, yielding a more powerful toolset for quantum information technology. The mm-wave band additionally offers access to single photon resolution at 1K, due to low thermal photon occupation at high frequencies. I will present our recent realization of a hybrid cavity with crossed mm-wave and optical modes, enabling smooth integration of cold atoms, laser beams and Rydberg excitations. The high-Q mm-wave cavity has a Q_tot = 3×10⁷ at 98GHz and mode volume of V =0.1λ³, which allows strong coupling with cooperativity of h=22000 between single mm-wave photon and a single Rydberg atom on the 36S → 35P transition. I will report our first observations of intra-cavity vacuum Rabi Splitting, Rydberg Electromagnetically Induced Transparency (EIT) and outline our path towards creating strong interactions between single mm-wave and optical photons.   

Large electromechanical coupling in inductively coupled electromechanics

Light-matter interaction in optomechanical systems is the foundation for ultra-sensitive detection schemes measuring displacements, accelerations, and forces. In addition, this interaction enables the generation of mechanical quantum states and entangled states of photons and phonons. Electromechanical systems implement the field of optomechanics using microwave circuits. Most implementations realize the coupling using a mechanically compliant capacitance, demonstrating a maximum interaction rate of 280 Hz [1]. However, it was realized early on that inductive coupling schemes promise much higher interaction strengths even with the potential to reach the strong vacuum coupling regime.
Here, we demonstrate electromechanical coupling based on a partly suspended SQUID combined with a coplanar microwave resonator exceeding a coupling rate of 1 kHz. This coupling enables force sensitivities on a sub-attonewton level using an single-photon ultra-low power readout. We discuss the performance of the concept including the tunability of this coupling with magnetic field.
[1] Reed et al., Nature Physics 13, 1163 (2017)   

Laser cooling to the zero-point energy of a nanomechanical oscillator

Silicon optomechanical crystals enable coupling of photons at telecommunication wavelengths to GHz mechanical modes, giving rise to optomechanical dynamics that can extend well into the resolved-sideband regime. Despite these promising characteristics, high-fidelity ground state preparation has to date only been achieved using passive cooling in a dilution refrigerator. Moreover, heating due to optical absorption has limited measurement protocols to short, low-energy optical pulses. Here, we demonstrate continuous-wave laser sideband cooling of a silicon optomechanical crystal to the zero-point energy, reaching a mean thermal occupancy of $0.09_{-0.01}^{+0.02}$ quanta, or 92\% ground state occupation, self-calibrated via motional sideband asymmetry. Our results overcome previous limitations due to optical absorption heating and highlight optomechanical crystals for quantum-enhanced continuous displacement measurements, low-added-noise quantum transducers, and integration with superconducting qubit technology.   

Strong magnetomechanical coupling Part 1

The possibility to operate mechanical systems close to the quantum regime has become central in both fundamental and applied science. A key parameter here is how strongly photons will impact the mechanics. The microwave regime offers the opportunity to achieve large couplings towards a regime where the coupling rate becomes the dominant time scale. In our ongoing experimental efforts to reach this single-photon strong coupling regime, we developed an approach based on a mechanical oscillator inductively coupled to a superconducting circuit. We place a NdFeB micromagnet on the tip of an AFM cantilever and align it on top of the SQUID of a tunable 3D microstrip resonator. When flux sensitive, the motion of the cantilever will result in a change in resonance frequency of the microstrip resonator, similar to typical cavity optomechanical setups. Varying the external flux allows us to tune the coupling strength from virtually zero to higher values, reaching single photon strong cooperativity, allowing for cooling with a single photon. Improving our current design, we hope to reach single photon strong coupling, allowing the quantum control of massive mechanical resonators.   

Strong magnetomechanical coupling Part 2

The possibility to operate mechanical systems close to the quantum regime has become central in both fundamental and applied science. A key parameter here is how strongly photons will impact the mechanics. The microwave regime offers the opportunity to achieve large couplings towards a regime where the coupling rate becomes the dominant time scale. In our ongoing experimental efforts to reach this single-photon strong coupling regime, we developed an approach based on a mechanical oscillator inductively coupled to a superconducting circuit. We place a NdFeB micromagnet on the tip of an AFM cantilever and align it on top of the SQUID of a tunable 3D microstrip resonator. When flux sensitive, the motion of the cantilever will result in a change in resonance frequency of the microstrip resonator, similar to typical cavity optomechanical setups. Varying the external flux allows us to tune the coupling strength from virtually zero to higher values, reaching single photon strong cooperativity -allowing for cooling with a single photon. Improving our current design, we hope to reach single photon strong coupling, allowing the quantum control of massive mechanical resonators.