Session N67: Hybrid Quantum Systems II

Gatemon qubits with superconductor-semiconductor-superconductor junctions: transition from high transparency to low transparency

We consider a superconductor-semiconductor-superconductor Josephson junction embedded in a general superconducting circuit environment, whose properties can be tuned (via conduction channel transparency and Andreev-level separation) with an electrostatic gate. Our work is based on a self-consistent approach, which focuses on the Andreev-level physics of a semiconductor quantum dot coupled via tunneling to superconducting leads, and we aim to establish a rigorous physical correspondence with conventional Josephson junction physics at low transparency. Using our approach, we investigate the qubit's full level structure, selection rules, and coherence properties. Gatemon manipulation and measurement via a capacitively coupled superconducting resonator are also discussed.   

A parametric coupling scheme for mitigating piezoelectric loss in quantum acoustics devices

Quantum states of mechanical motion offer the potential to realize long lived multimodal quantum memories as well as quantum transduction between disparate quantum systems.  Coupling this motion to superconducting qubits through piezoelectricity provides the toolkit of circuit quantum electrodynamics for quantum control over phonons.  However, this interface is also a source of loss due to radiation into spurious phonon modes.  This results in superconducting qubit lifetimes around two orders of magnitude smaller than those on the best dielectrics.  Here we present a 3D cavity architecture to mitigate the deleterious effect of this extra loss by removing the qubit entirely from the piezoelectric substrate and instead parametrically coupling the qubit to the acoustic modes utilizing a 3D superconducting nonlinear asymmetric inductive element (SNAIL).  This design allocates the loss primarily to the SNAIL coupler, and so operating with small qubit participation in this SNAIL mode will allow for longer qubit lifetimes.  We demonstrate strong coupling between our SNAIL coupler and the longitudinal modes of a surface acoustic wave resonator and characterize its 3-wave mixing nonlinearity.

    

Epitaxially-grown Barium-Titanate-Silicon Heterostructures for Piezo-acoustic Quantum Devices

Piezo-acoustic cavities in quantum limits have technological advantages in processing and storage of quantum information. Coupling piezo-acoustic cavities to superconducting quantum devices can also lead to hybrid circuits that act as quantum memories or electromechanical transducers. However, this demands on chip integration of piezoelectric and superconducting materials with disparate physical and chemical properties. Here, we propose a path for on-chip integration of piezoelectric thin films via heteroepitaxy of barium titanate (BTO) on Si substrates. BTO is an ideal candidate for this application due to large piezoelectric and electro-optic coefficients that are maintained even at cryogenic temperatures. We demonstrate growth of highly crystalline BTO thin film on Si via pulsed laser deposition method. By changing the buffer layer composition we tune the crystalline orientation of the BTO thin films between (100) and (101). By performing low-temperature X-ray diffractometry, we confirm the absence phase transitions down to 12 K. We will complement these results with cryogenic dielectric constant measurements and high-resolution transmission electron microscopy.   

Surface-magnon mediated self-interaction of nitrogen-vacancy centers in diamond

Hybrid quantum systems consisting of nitrogen-vacancy (NV) centers in diamond and magnons in ferrimagnets have recently attracted much attention as a platform for on-chip long-distance entanglement, interfacing quantum information science with magnonics [1,2]. Here, we experimentally determine the magnon-induced self-interaction of this hybrid system by combining longitudinal (T_1) relaxometry measurements with the fluctuation-dissipation and Kramers-Kronig relations. This self-interaction is a function of the NV-magnon coupling strength and thereby provides an estimate of the magnon-mediated two-qubit interaction. Our results, including the enhanced T_1 relaxation rates caused by magnetostatic surface magnons are quantitatively consistent with a model in which the NV center is coupled to magnons by the magnetic dipole interactions. These findings help build a foundation for the hybrid quantum architecture of spin qubits coupled to magnons.

[1] D. R. Candido, et al., Mater. Quantum. Technol. 1, 011001 (2021)

[2] M. Fukami, et al., PRX Quantum 2, 040314 (2021)    

AlN-on-Diamond FBAR for Coherent Acoustic Control of NV Centers

Spin systems in negatively charged Nitrogen-Vacancy (NV) centers in diamond have been shown to be promising platform for quantum information science and sensing at room temperature. Spin-phonon interactions have been demonstrated to drive magnetically forbidden transitions coherently using high overtone bulk acoustic resonators (HBARs) with planar and solid immersion lens (SIL) geometry for improved strain coupling to the NV spin manifolds. In this work we demonstrate surface micromachining of diamond integrated with AlN transducers to realize thin film bulk acoustic wave resonators (FBARs) that are free standing mechanical resonators for enhanced phonon mode confinement. The AlN/Diamond FBARs are fabricated thinner than HBARs while retaining uniform in plane stress profile for a given depth location. Our microfabrication technique with double side processing on diamond allows desired thickness and lateral apodization of diamond plates that require no ion implantation. We also confirm that the spin dephasing time (T₂*) is not affected by the microfabrication processing. The impedance of the packaged diamond resonator is designed to match the 50 Ω RF source for maximum power transfer strain generating and microwave driving. We provide efficient strain coupling to spin systems in diamond NV centers via acoustic impedance matching of transducer layers to the thin released diamond membrane while generating stress wave confined in the diamond layer by utilizing 2^nd order harmonic of the resonator.   

Development of fresnel-type solid immersion lens and bulk acoustic resonators for spin-mechanical hybrid system in diamond

Quantum defects in solids are actively studied as promising resources in research fields such as quantum information, quantum computing, quantum networks and quantum sensing. Nanostructuring is essential to efficiently manupulate and measure defect spins, and various structures including solid immersion lens and mechanical resonators have been developed. In this presentation, we demonstrate a method for milling fresnel lens using focused ion beam, which can enhance process yield by reducing milling time (1/3 compared to hemispherical structure) while retaining high photon collection efficiency (>2 compared to flat surface). Including shallow fresnel-type lens, we further discuss the development of beam-shapred bulk acoustic resonators toward mechanical manupulation of spin states. We discuss a new design of mechanical resonator system, reflection bessel beam system, which is promissing for covering wide frequency bandwidth and high photon collection efficiency.   

Quantum Spin Squeezing in a Squeezed Environment

We apply the stochastic Schrödinger equation approach to study the non-Markovian dynamics of open quantum systems when coupled to the environment that is initially prepared in a squeezed state. After deriving the non-Markovian quantum diffusion equation and the associated master equation, we study the nonequilibrium dynamics of general models in a pure quantum context. Particularly, we compare the quantum spin squeezing resulting from the environment, which is prepared as the squeezed states, and the coherent states, respectively. In addition, we numerically show how the squeezing of the environment is transferred into the system. Besides the spin squeezing model discussed, our method can be extended to generic open quantum systems coupled with a finite temperature environment.   

Itinerant phonons as carriers of quantum information

Heavily used in classical signal processing, surface acoustic waves (SAWs) have also been proposed as a means to coherently couple distant solid-state quantum systems. Several groups have already reported the coherent coupling of standing SAWs modes to superconducting qubits [1-4], opening the door to the control and detection of quantum phonon states [5]. In this talk, I will describe our work in coupling superconducting qubits to propagating SAWs. We can controllably release and capture individual itinerant photons, demonstrating that quantum state transfer as well as remote entanglement generation between superconducting qubits using phonons can be realized [6]. Going a step further, I will show how two-phonon entanglement can be generated and used to realize a fundamental quantum optics experiment, quantum erasure [7], using phonons [8].

[1] M. V. Gustafsson, et al, Science, 346, 207-211, 2014

[2] R. Manenti, et al, Phys. Rev. B, 93, 041411, 2016

[3] B. A. Moores, Phys. Rev. Lett., 120, 2018

[4] A. N. Bolgar, et al, Phys. Rev. Lett., 120,  2018

[5] K. J. Satzinger, et al, Nature 563, 60-61, 2018

[6] A. Bienfait, et al, Science, 364, 368-371, 2019

[7] M. O. Scully and K. Drühl, Phys. Rev. A 25, 2208, 1982

[8] A. Bienfait et al., Phys. Rev X 10, 021055, 2020

    

Laser Cooling of Nuclear Magnons

The initialization of nuclear spin to its ground state is challenging due to its small energy scale compared with thermal energy even at cryogenic temperature. In this Letter, we propose an opto-nuclear quadrupolar effect, whereby two-color optical photons can efficiently interact with nuclear spins. Leveraging such an optical interface, we demonstrate that nuclear magnons, the collective excitations of nuclear spin ensemble, can be cooled down optically. Under feasible experimental conditions, laser cooling can suppress the population and entropy of nuclear magnons by more than two orders of magnitude, which could facilitate the application of nuclear spins in e.g., quantum information science.

    

Deterministic Construction of Arbitrary States in Permutationally-Invariant Spin Ensemble

Spin ensemble systems are capable of hosting long-lived qubits, making them promising platforms to implement quantum technologies such as sensors, repeaters, and processors. To achieve the quantum advantages of these applications, specific states are required to be prepared, but a systematic way to prepare arbitrary spin-ensemble state is missing in the literature. In this work, we propose a scheme to fill this gap. Our scheme first employs engineered dissipation to deterministically prepare the spin ensemble in any angular momentum subspace. Arbitrary superpositions of excitation can then be encoded through interacting with an auxilary qubit. As an example of application, we show that our scheme can encode a qubit into a subspace that is more robust against collective errors than in the conventional Dicke subspace.   

Universal adaptive Gaussian quantum operations

Gaussian operation on hybrid bosonic modes is a critical technology for quantum information processing and communication. For example, SWAP of two hybrid bosonic modes enables quantum transduction, which is a fundamental building block for scalable quantum computing; and two-mode squeezing operations are widely applied to quantum signal amplification, crucial to bettering quantum measurement. However, different Gaussian operations are usually achieved with setups and procedures with distinct physical configurations, limiting the flexibility and feasibility of these protocols. Here, we propose a scheme based on a randomly given immutable hybrid Gaussian unitary operation and single-mode quantum operations and classical controls. More specifically, our scheme utilizes single-mode squeezing, single-mode homodyne measurements and adaptive displacement operations to transform the given hybrid Gaussian operation to one of a different form. Moreover, we also demonstrate that the generated Gaussian operation can be of an arbitrary desired form by composing <!-- ?? -->the given hybrid operation with carefully designed single-mode phase-shifters. Our scheme only requires minimal modification of the experimental setup—tuning the phase-shifters—to yield universal hybrid Gaussian operations and hence can be hardware-efficient in experimental settings.