Superconducting acousto-optic phase modulator.

We report the development of a superconducting acousto-optic phase modulator fabricated on a lithium niobate substrate. A titanium-diffused optical waveguide is placed in a surface acoustic wave resonator, where the electrodes for mirrors and an interdigitated transducer are made of a superconducting niobium titanium nitride thin film. The device performance is evaluated as a substitute for the current electro-optic modulators, with the same fiber coupling scheme and comparable device size. Operating the device at a cryogenic temperature (T = 8 K), we observe the length-half-wave-voltage (length-Vπ) product of 1.78 V·cm. Numerical simulation is conducted to reproduce and extrapolate the performance of the device. An optical cavity with mirror coating on the input/output facets of the optical waveguide is tested for further enhancement of the modulation efficiency. A simple extension of the current device is estimated to achieve an efficient modulation with Vπ = 0.27 V.

Single-photon quantum regime of artificial radiation pressure on a surface acoustic wave resonator.

Electromagnetic fields carry momentum, which upon reflection on matter gives rise to the radiation pressure of photons. The radiation pressure has recently been utilized in cavity optomechanics for controlling mechanical motions of macroscopic objects at the quantum limit. However, because of the weakness of the interaction, attempts so far had to use a strong coherent drive to reach the quantum limit. Therefore, the single-photon quantum regime, where even the presence of a totally off-resonant single photon alters the quantum state of the mechanical mode significantly, is one of the next milestones in cavity optomechanics. Here we demonstrate an artificial realization of the radiation pressure of microwave photons acting on phonons in a surface acoustic wave resonator. The order-of-magnitude enhancement of the interaction strength originates in the well-tailored, strong, second-order nonlinearity of a superconducting Josephson junction circuit. The synthetic radiation pressure interaction adds a key element to the quantum optomechanical toolbox and can be applied to quantum information interfaces between electromagnetic and mechanical degrees of freedom.

Entanglement-based single-shot detection of a single magnon with a superconducting qubit.

The recent development of hybrid systems based on superconducting circuits provides the possibility of engineering quantum sensors that exploit different degrees of freedom. Quantum magnonics, which aims to control and read out quanta of collective spin excitations in magnetically ordered systems, provides opportunities for advances in both the study of magnetism and the development of quantum technologies. Using a superconducting qubit as a quantum sensor, we report the detection of a single magnon in a millimeter-sized ferrimagnetic crystal with a quantum efficiency of up to 0.71. The detection is based on the entanglement between a magnetostatic mode and the qubit, followed by a single-shot measurement of the qubit state. This proof-of-principle experiment establishes the single-photon detector counterpart for magnonics.

Qubit-Assisted Transduction for a Detection of Surface Acoustic Waves near the Quantum Limit.

We demonstrate ultrasensitive measurement of fluctuations in a surface-acoustic-wave (SAW) resonator using a hybrid quantum system consisting of the SAW resonator, a microwave (MW) resonator, and a superconducting qubit. The nonlinearity of the driven qubit induces parametric coupling, which up-converts the excitation in the SAW resonator to that in the MW resonator. Thermal fluctuations of the SAW resonator near the quantum limit are observed in the noise spectroscopy in the MW domain.

Resolving quanta of collective spin excitations in a millimeter-sized ferromagnet.

Combining different physical systems in hybrid quantum circuits opens up novel possibilities for quantum technologies. In quantum magnonics, quanta of collective excitation modes in a ferromagnet, called magnons, interact coherently with qubits to access quantum phenomena of magnonics. We use this architecture to probe the quanta of collective spin excitations in a millimeter-sized ferromagnetic crystal. More specifically, we resolve magnon number states through spectroscopic measurements of a superconducting qubit with the hybrid system in the strong dispersive regime. This enables us to detect a change in the magnetic moment of the ferromagnet equivalent to a single spin flipped among more than 1019 spins. Our demonstration highlights the strength of hybrid quantum systems to provide powerful tools for quantum sensing and quantum information processing.

All-optical observation and reconstruction of spin wave dispersion.

To know the properties of a particle or a wave, one should measure how its energy changes with its momentum. The relation between them is called the dispersion relation, which encodes essential information of the kinetics. In a magnet, the wave motion of atomic spins serves as an elementary excitation, called a spin wave, and behaves like a fictitious particle. Although the dispersion relation of spin waves governs many of the magnetic properties, observation of their entire dispersion is one of the challenges today. Spin waves whose dispersion is dominated by magnetostatic interaction are called pure-magnetostatic waves, which are still missing despite of their practical importance. Here, we report observation of the band dispersion relation of pure-magnetostatic waves by developing a table-top all-optical spectroscopy named spin-wave tomography. The result unmasks characteristics of pure-magnetostatic waves. We also demonstrate time-resolved measurements, which reveal coherent energy transfer between spin waves and lattice vibrations.

A Ku band pulsed electron paramagnetic resonance spectrometer using an arbitrary waveform generator for quantum control experiments at millikelvin temperatures.

We present a 17 GHz (Ku band) arbitrary waveform pulsed electron paramagnetic resonance spectrometer for experiments down to millikelvin temperatures. The spectrometer is located at room temperature, while the resonator is placed either in a room temperature magnet or inside a cryogen-free dilution refrigerator; the operating temperature range of the dilution unit is from ca. 10 mK to 8 K. This combination provides the opportunity to perform quantum control experiments on electron spins in the pure-state regime. At 0.6 T, spin echo experiments were carried out using γ-irradiated quartz glass from 1 K to 12.3 mK. With decreasing temperatures, we observed an increase in spin echo signal intensities due to increasing spin polarizations, in accordance with theoretical predictions. Through experimental data fitting, thermal spin polarization at 100 mK was estimated to be at least 99%, which was almost pure state. Next, to demonstrate the ability to create arbitrary waveform pulses, we generate a shaped pulse by superposing three Gaussian pulses of different frequencies. The resulting pulse was able to selectively and coherently excite three different spin packets simultaneously-a useful ability for analyzing multi-spin system and for controlling a multi-qubit quantum computer. By applying this pulse to the inhomogeneously broadened sample, we obtain three well-resolved excitations at 8 K, 1 K, and 14 mK.

QUANTUM INFORMATION. Coherent coupling between a ferromagnetic magnon and a superconducting qubit.

Rigidity of an ordered phase in condensed matter results in collective excitation modes spatially extending to macroscopic dimensions. A magnon is a quantum of such collective excitation modes in ordered spin systems. Here, we demonstrate the coherent coupling between a single-magnon excitation in a millimeter-sized ferromagnetic sphere and a superconducting qubit, with the interaction mediated by the virtual photon excitation in a microwave cavity. We obtain the coupling strength far exceeding the damping rates, thus bringing the hybrid system into the strong coupling regime. Furthermore, we use a parametric drive to realize a tunable magnon-qubit coupling scheme. Our approach provides a versatile tool for quantum control and measurement of the magnon excitations and may lead to advances in quantum information processing.

Hybridizing ferromagnetic magnons and microwave photons in the quantum limit.

We demonstrate large normal-mode splitting between a magnetostatic mode (the Kittel mode) in a ferromagnetic sphere of yttrium iron garnet and a microwave cavity mode. Strong coupling is achieved in the quantum regime where the average number of thermally or externally excited magnons and photons is less than one. We also confirm that the coupling strength is proportional to the square root of the number of spins. A nonmonotonic temperature dependence of the Kittel-mode linewidth is observed below 1 K and is attributed to the dissipation due to the coupling with a bath of two-level systems.

Strongly driven electron spins using a K(u) band stripline electron paramagnetic resonance resonator.

This article details our work to obtain strong excitation for electron paramagnetic resonance (EPR) experiments by improving the resonator's efficiency. The advantages and application of strong excitation are discussed. Two 17 GHz transmission-type, stripline resonators were designed, simulated and fabricated. Scattering parameter measurements were carried out and quality factor were measured to be around 160 and 85. Simulation results of the microwave's magnetic field distribution are also presented. To determine the excitation field at the sample, nutation experiments were carried out and power dependence were measured using two organic samples at room temperature. The highest recorded Rabi frequency was rated at 210 MHz with an input power of about 1 W, which corresponds to a π/2 pulse of about 1.2 ns.

Active compensation of rf-pulse transients.

A new approach to compensate rf-pulse transients is proposed. Based on the idea of the response theory of a linear system, a formula is derived to obtain the required excitation voltage profile back from the intended target rf-pulse shape. The validity of the formula is experimentally confirmed by monitoring the rf-field created inside the sample coil with a pickup coil. Since this approach realizes accurate rf-pulse shapes without reducing the Q-factor of the tank circuit of the probe, it can be used not only to suppress the transient tail of the rf-pulse, but also as a general concept for accurate rf-pulsing.