Session R45: Hybrid Quantum Systems III

Investigations of a transmon-coupled nanoresonator in a CPW cavity.

In this work, we describe our progress developing a qubit-coupled naonomechanical resonator(nmr), which has potential both for fundamental studies in quantum measurement and quantum thermodynamics and applications in quantum information. The hybrid system is composed of a superconducting charge-type transmon qubit and a ultra-high-frequency flexural nmr; both are embedded in, and measured through, a superconducting coplanar-wave-guide(CPW) resonator. Transmission measurements of the CPW cavity allow us to probe the state of transmon as it interacts resonantly with the NMR. In the talk, we'll present the latest measurements of this device at low NMR thermal occupation factors and discuss future prospects for developing this system for more advanced quantum measurements.   

Surface acoustic wave resonators in the quantum regime

Surface acoustic waves (SAWs) are mechanical modes confined to the surface of a piezoelectric crystal that can be excited and detected by electric circuits. These mechanical waves can be trapped between two reflectors producing a SAW resonator. In this talk, I will present an experimental study of SAW resonators at 10 mK [1], in which we find that internal quality factors $Q_{\mathrm{i}}$ approaching 0.5 million can be reached at 0.5 GHz and that $Q_{\mathrm{i}} > 10^{4}$ is achievable above 4 GHz, making SAW resonators promising devices for integration into quantum circuits. I will discuss the loss mechanisms that may be currently limiting these Q-factors, and report on our progress towards coupling these mechanical resonators to superconducting qubits. [1] R. Manenti et al., arXiv:1510.04965   

Cavity magnomechanics

Mechanical oscillators have been recently widely utilized to couple with optical and microwave photons in a variety of hybrid quantum systems, but they all lack the tunability. The magnetostrictive force provides an alternative mechanism to allow phonon to couple with a different type of information carrier–magnon, the collective excitation of magnetization whose frequency can be tuned by a bias magnetic field. Here, we demonstrate an intriguing hybrid system that consists of a magnonic, a mechanical, and a microwave resonator. The magnon-phonon interaction results in hallmark coherent phenomena such as magnomechanically induced transparency/absorption and magnomechanical parametric amplification. The magnetic field dependence of magnon provides our system with unprecedented tunability. Moreover, the great flexibility of our system allows us to achieve triple resonance among magnon, phonon and photon, which drastically enhances the magnomechanical interaction. Our work demonstrates the fundamental principle of cavity magnetomechanics, opening up great opportunities in various applications, such as tunable microwave filter and amplifier, long-lifetime quantum memories, microwave-to-optics conversion.   

Continuous dynamical decoupling of a single diamond nitrogen-vacancy center spin with a mechanical resonator

We use coherent interactions between a diamond mechanical resonator and a single nitrogen-vacancy (NV) center spin qubit to engineer a decoherence-protected spin basis. For solid state spin qubits such as the NV center, a dominant source of inhomogeneous dephasing is magnetic field fluctuations due to nearby paramagnetic impurities or instabilities in a magnetic bias field. By dressing the NV center spin states with a $581\pm2$ kHz mechanical Rabi field, we decrease the spin's sensitivity to magnetic fluctuations in a thermally isolated subspace, thus prolonging the Ramsey coherence time from $T_2^*=2.7\pm0.1$ $\mu$s to $15\pm1$ $\mu$s. We develop a model that quantitatively predicts the relationship between the mechanical Rabi field and the dephasing time. Our model shows that a combination of random magnetic field fluctuations and hyperfine coupling limits the protected coherence time over the range of mechanical dressing fields accessed in our experiment. Finally, we show that amplitude noise in the dressing field will dominate over magnetic noise for larger driving fields.   

Towards a highly efficient quantum spin-photon interface for an NV centre based quantum network

Nitrogen-vacancy (NV) centers in diamond recently emerged as promising candidates for realizing quantum information algorithms due to their remarkable versatility. The spin of these optically active defects can be entangled with their emitted photons, making them an excellent optical interface from the perspective of quantum communication.\\ Recently, we have demonstrated the first building blocks of such networks, performing kilometer scale entanglement of two NV centers and teleportation of quantum information.(1) However, our current protocols are inefficient due to the low emission of NV center’s resonant photons into the zero phonon line (ZPL).\\ Here we present our efforts of coupling a single NV center emitter in a diamond membrane to a fiber-based Fabry-Perot microcavity with high finesse $(F > 10^4)$ at cryogenic temperatures. This approach allows spectral tuning of the cavity resonance to the ZPL emission of the NV center, thereby significantly enhancing the resonant photon emission via Purcell effect. Furthermore, the bulk environment of the NV centers protects their spin properties against surface proximity effects, which is of crucial importance for quantum information processing applications. \\ (1) B.Hensen et al, \emph{Nature} \textbf{526}, 682 (2015)   

Investigating the positively charged nitrogen-vacancy center in diamond as a long lived quantum memory

The nitrogen-vacancy (NV) defect in diamond is one of the major candidates for a solid-state quantum processor. Its electron spin is readout and initialized optically. Proximal nuclear spins (e.g. $^{14}\text{N}$, $^{15}\text{N}$, $^{13}\text{C}$) serve as inherently robust qubits, their readout is facilitated via the electron spin in a QND measurement and they exhibit $\text{T}_1$ lifetimes of several minutes. However, for strongly coupled nuclear spins, the coherence time is limited by the electron spin's $\text{T}_1$ lifetime (˜ 5ms @ roomtemperature). In Si:P, this obstacle is overcome by ionizing the P donor into a spinless charge-state. In this work, we employ in-plane gate structures on the diamond surface for deterministic charge state switching of near-surface NVs from $\text{NV}^-$ over $\text{NV}^0$ to $\text{NV}^+$, while investigating the electron spin properties using the nitrogen nuclear spin as a probe. The positive charge state happens to have no unpaired electrons, therefore the nuclear spin coherence time is prolonged beyond the 5ms-limit imposed by the $\text{NV}^-$ electron spin. Proper charge state control removes an important roadblock for achieving minute-long coherence times at room-temperature and deterministic quantum system initialization.   

Coupling a single InAs quantum dot to mechanical motion of a photonic crystal membrane

Coupling quantum mechanical systems to mechanical motion is attractive for fundamental science, quantum information applications, and sensing. Semiconductor quantum dots (QDs) embedded in suspended photonic crystal structures provide a versatile system for advances in this area. Flexural modes of the suspended membrane as well as localized mechanical modes surrounding optical cavities couple to QDs through strain, with the photonic crystal used to maximize collection of photons from QDs. We have performed high resolution spectroscopy of InAs QDs embedded in photonic crystal structures while optically driving mechanical motion. Using time-correlated photon counting, the strain-induced shift of the QD optical transitions is measured as a function of time. For QDs at the center of the membrane (along the growth direction), the strain is minimum, and the optical transitions shift by only a few $\mu $eV. For QDs shifted 30 nm from the center, the strain induces larger shifts of \textpm 50$\mu $eV. Measurements in a magnetic field are being performed on charged QDs to determine the coupling of mechanical motion to electron and hole spin transitions.   

Inductive cooling in quantum magnetomechanics.

Coupling to light or microwave fields allows quantum control of the motion of a mechanical oscillator, and offers prospects for precision sensing, quantum information systems, and tests of fundamental physics. In cavity electromechanics ground state cooling has been achieved using resolved sideband cooling. Here we present an alternative approach based on a magnetomechanical system that inductively couples an $LC$ resonator to a mechanical oscillator. The experimental setup consists of a micro cantilever with a pyramidal magnetic tip attached at the end of the beam. The sharp end of the magnetic tip is positioned close to the planar microfabricated inductor of the $LC$ resonator. The displacement in the position of the end of the cantilever generates a change in flux through the coil inducing an electromotive force in the circuit. The current in the $LC$ resonator generates a magnetic field, and then a force between the tip and the coil. When they are strongly coupled and the mechanical resonance frequency $\omega_m$ exceeds the electrical decay rate of the resonator $\gamma_e$, resolved sideband cooling can be used to cool the mechanics. We present estimations for the coupling rates and the experimental parameters required for these experiments.   

Strain coupling of a mechanical resonator to a single quantum emitter in diamond

Hybrid quantum devices are central to the advancement of several emerging quantum technologies, including quantum information science and quantum-assisted sensing. Here, we present a hybrid quantum device in which strain fields associated with resonant vibrations of a diamond cantilever dynamically modulate the energy and polarization dependence of the optical transitions of a single nitrogen-vacancy defect center in diamond. With mechanical driving, we observe optomechanical couplings exceeding 10 GHz. Through resonant excitation spectroscopy, we quantitatively characterize the intrinsic strain environment of a single defect, and use this optomechanical coupling to tune the zero-phonon line of the defect. Through stroboscopic measurements, we show that we are able to match the frequency and polarization dependence of the optical zero-phonon lines of two separate NV centers. The experiments demonstrated here mark an important step toward realizing a monolithic hybrid quantum device capable of realizing and probing the dynamics of non-classical states of mechanical resonators, spin-systems, and photons.   

Microwave-frequency electromechanical resonators incorporating phononic crystals

Piezoelectric micromechanical resonators at gigahertz frequencies have been operated in the quantum limit, with quantum control and measurement achieved using superconducting qubits. However, experiments to date have been limited by mechanical dissipation, due to a combination of internal and radiative losses. In this talk, we explore the incorporation of phononic crystals into resonator designs. In phononic crystals, periodic patterning manipulates the acoustic band structure of the material. Through appropriately chosen geometries, these periodic patterns lead to full acoustic bandgaps which can be used to greatly reduce radiation losses from resonant structures. Alternatively, the crystal geometry can be manipulated to allow isolated modes within the bandgap, giving fine control over the spatial structure of the resonator modes. In this talk, we will describe the design, fabrication, and measurement of resonators with phononic crystals.   

Superconducting-circuit quantum heat engine with frequency resolved thermal baths

The study of quantum heat engines promises to unravel deep, fundamental concepts in quantum thermodynamics. With this in mind, we propose a novel, realistic device that efficiently converts heat into work while maintaining reasonably large output powers. The key concept in our proposal is a highly peaked spectral density in both the thermal baths as well as the working fluid. This allows for a complete separation of the heat current from the working fluid. In our setup, Cooper pairs tunnelling across a Josephson junction serve as the the working fluid, while two resonant cavities coupled to the junction act as frequency-resolved thermal baths. The device is operated such that a heat flux carried entirely by the photons induces an electrical current against a voltage bias, providing work.   

Strong nonlinearity of mesoscopic vibrational modes induced by electron-phonon coupling

We show that the electron-phonon coupling can lead to a strong nonlinearity of vibrational modes in semiconductor nano- and micro-resonators. For typical mode frequencies, the electron distribution adiabatically follows lattice strain. Therefore strain leads to redistribution of the electron density over the valleys of the conduction band. It also leads to the onset of a spatial charge. The parameter that controls the distribution is the ratio of the deformation potential to the electron chemical potential or temperature. It is $\sim 10^2$ for many semiconductors of interest even when they are heavily doped. Therefore the change of the electron distribution is strongly nonlinear in the strain. As a consequence, the stress induced by the electron-phonon coupling is also strongly nonlinear. We have found the vibration nonlinearity parameters for $n$-doped Si and calculated the amplitude dependence of the frequencies of several low-lying Si resonator modes with account taken of their spatial structure. The results are compared with the recent experimental data that shows strong effect of doping on the vibration nonlinearity.   

\textbf{Novel High Cooperativity Photon-Magnon Cavity QED}

Novel microwave cavities are presented, which couple photons and magnons in YIG spheres in a super- and ultra-strong way at around 20 mK in temperature. Few/Single photon couplings (or normal mode splitting, 2g) of more than 6 GHz at microwave frequencies are obtained. Types of cavities include multiple post reentrant cavities, which co-couple photons at different frequencies with a coupling greater that the free spectral range, as well as spherical loaded dielectric cavity resonators. In such cavities we show that the bare dielectric properties can be obtained by polarizing all magnon modes to high energy using a 7 Tesla magnet. We also show that at zero-field, collective effects of the spins significantly perturb the photon modes. Other effects like time-reversal symmetry breaking are observed.   

Cavity QED with ferromagnetic magnons in a small YIG sphere

Hybridizing collective spin excitations in ferromagnetic crystals and a cavity with high cooperativity provides a new research subject in the field of cavity quantum electrodynamics and can also have potential applications to quantum information. In contrast to spin ensembles based on dilute paramagnetic impurities, these spins are strongly exchange-coupled and have a much higher density. Here we report a direct observation of the strong coupling between magnons and microwave photons at both cryogenic and room temperatures by using the same small yttrium-iron-garnet (YIG) ferromagnetic sphere in a 3D copper cavity. We observed strong couplings of the same cavity mode to both ferromagnetic-resonance (FMR, uniform precession) mode and a magnetostatic (MS, non-uniform precession) mode in the quantum limit at 22 mK. Then, at room temperature, we observed a strong coupling of the cavity mode to the FMR mode with slightly increased damping rate. This reveals the robustness of the FMR mode against temperature. However, the coupling to MS mode disappears at room temperature and numerically simulations show that this is due to a drastic increase of the damping rate of the MS mode. Our work unveils quantum-coherence properties of the magnons.