arXiv subjects

Rikuto Fukumori

Publications and source records attributed to Rikuto Fukumori.

7 recordsLinked to original sources

arXiv2026

Cavity-mediated coherence protection and one-axis twisting for spins in solids

Long-range interactions between emitters give rise to collective phenomena, including superradiance, spin squeezing, and coherence protection, that are important to both fundamental physics and quantum technologies. Despite progress in cold atoms, coherent cavity-mediated all-to-all interactions have not yet been realized in a solid-state ensemble. Here we demonstrate such interactions in a $^{171}$Yb$^{3+}$:CaWO$_4$ crystal coupled to a microwave resonator, observing superradiant emission on resonance and unitary one-axis twisting dynamics in the dispersive regime. The same interaction also opens a many-body energy gap that suppresses inhomogeneous dephasing, extending the ensemble Ramsey coherence time from tens of microseconds to milliseconds without decoupling pulses. These results establish a solid-state platform for collective many-body physics with direct implications for quantum technologies. Specifically, the observed one-axis twisting dynamics opens a path towards spin squeezing for entanglement-enhanced quantum metrology, and the extended coherence due to gap-protection is relevant for both microwave photon storage and precision measurement.

quant-ph↗

arXiv2026

Observation of Tunable Superradiant Frequency Combs

Cavity quantum electrodynamics (QED) with quantum emitters coupled to resonators provides a powerful platform for engineering light-matter interactions and exploring collective phenomena. In particular, superradiance, arising from collective quantum interference among emitters, has been explored as a route to ultrastable continuous radiation. However, engineering superradiance in the time domain to realize periodic pulsed sources or frequency combs remains largely unexplored. Here, we investigate the non-equilibrium many-body dynamics of a driven spin ensemble coupled to an on-chip superconducting resonator and uncover a dynamical phase transition from continuous-wave to periodic pulsed superradiant emission. To quantitatively capture the observed dynamical phases, we introduce a driven-dissipative cavity-QED model that elucidates how the periodic pulsed superradiant phase emerges from collective, periodically repeating spin dynamics stabilized by the interplay of coherence growth, disorder, and dissipation. We also find that rare-earth ion spin systems exhibiting both optical and microwave transitions enable phase-synchronized, dual-rail superradiant frequency combs in both the microwave and optical domains. Our results not only open new avenues for dual-rail frequency-comb applications in quantum metrology and information processing, but also establish a fundamental connection between periodic pulsed superradiance and the emergence of a continuous time crystal as a novel nonequilibrium phase in driven open systems.

quant-ph↗

arXiv2024

Quantum thermalization and Floquet engineering in a spin ensemble with a clock transition

Studying and controlling quantum many-body interactions is fundamentally important for quantum science and related emerging technologies. Optically addressable solid-state spins offer a promising platform for exploring various quantum many-body phenomena due to their scalability to a large Hilbert space. However, it is often challenging to probe many-body dynamics in solid-state spin systems due to large on-site disorder and undesired coupling to the environment. Here, we investigate an optically addressable solid-state spin system comprising a strongly interacting ensemble of millions of ytterbium-171 ions in a crystal. Notably, this platform features a clock transition that gives rise to pure long-range spin-exchange interactions, termed the dipolar XY model. Leveraging this unique feature, we investigate quantum thermalization by varying the relative ratio of interaction strength to disorder, dynamically engineering the XY model into other many-body Hamiltonian models, and realizing a time-crystalline phase of matter through periodic driving. Our findings indicate that an ensemble of rare-earth ions serves as a versatile testbed for many-body physics and offers valuable insights for advancing quantum technologies.

quant-ph↗

arXiv2024

Scalable microwave-to-optical transducers at single photon level with spins

Microwave-to-optical transduction of single photons will play an essential role in interconnecting future superconducting quantum devices, with applications in distributed quantum computing and secure communications. Various transducers that couple microwave and optical modes via an optical drive have been developed, utilizing nonlinear phenomena such as the Pockels effect and a combination of electromechanical, piezoelectric, and optomechanical couplings. However, the limited strength of these nonlinearities, set by bulk material properties, requires the use of high quality factor resonators, often in conjunction with sophisticated nano-fabrication of suspended structures. Thus, an efficient and scalable transduction technology is still an outstanding goal. Rare-earth ion (REI) doped crystals provide high-quality atomic resonances that result in effective second-order nonlinearities stronger by many orders of magnitude compared to conventional materials. Here, we use ytterbium-171 ions doped in a YVO$_4$ crystal at 340 ppm with an effective resonant $χ^{(2)}$ nonlinearity of ~ 10$^7$ pm/V to implement an on-chip microwave-to-optical transducer. Without an engineered optical cavity, we achieve percent-level efficiencies with an added noise as low as 1.24(9) photons. To showcase scalability, we demonstrate the interference of photons originating from two simultaneously operated transducers, enabled by the inherent absolute frequencies of the atomic transitions. These results establish REI-based transducers as a highly competitive transduction platform, provide existing REI-based quantum technologies a native link to various leading quantum microwave platforms, and pave the way toward remote transducer-assisted entanglement of superconducting quantum machines.

quant-ph↗

arXiv2023

Many-body cavity quantum electrodynamics with driven inhomogeneous emitters

Quantum emitters coupled to optical resonators are quintessential systems for exploring fundamental phenomena in cavity quantum electrodynamics (cQED) and are commonly used in quantum devices acting as qubits, memories and transducers. Many previous experimental cQED studies have focused on regimes in which a small number of identical emitters interact with a weak external drive, such that the system can be described with simple, effective models. However, the dynamics of a disordered, many-body quantum system subject to a strong drive have not been fully explored, despite its importance and potential in quantum applications. Here we study how a large, inhomogeneously broadened ensemble of solid-state emitters coupled with high cooperativity to a nanophotonic resonator behaves under strong excitation. We discover a sharp, collectively induced transparency (CIT) in the cavity reflection spectrum, resulting from quantum interference and collective response induced by the interplay between driven inhomogeneous emitters and cavity photons. Furthermore, coherent excitation within the CIT window leads to highly nonlinear optical emission, spanning from fast superradiance to slow subradiance. These phenomena in the many-body cQED regime enable new mechanisms for achieving slow light and frequency referencing, pave a way towards solid-state superradiant lasers and inform the development of ensemble-based quantum interconnects.

quant-ph↗

arXiv2021

Origins of electronic bands in antiferromagnetic topological insulator MnBi$_2$Te$_4$

Despite the rapid progress in understanding the first intrinsic magnetic topological insulator MnBi$_2$Te$_4$, its electronic structure remains a topic under debates. Here we perform a thorough spectroscopic investigation into the electronic structure of MnBi$_2$Te$_4$ via laser-based angle-resolved photoemission spectroscopy. Through quantitative analysis, we estimate an upper bound of 3 meV for the gap size of the topological surface state. Furthermore, our circular dichroism measurements reveal band chiralities for both the topological surface state and quasi-2D bands, which can be well reproduced in a band hybridization model. A numerical simulation of energy-momentum dispersions based on a four-band model with an additional step potential near the surface provides a promising explanation for the origin of the quasi-2D bands. Our study represents a solid step forward in reconciling the existing controversies in the electronic structure of MnBi$_2$Te$_4$, and provides an important framework to understand the electronic structures of other relevant topological materials MnBi$_{2n}$Te$_{3n+1}$.

cond-mat.mtrl-sci↗

arXiv2020

Optical coherence of Er$^{3+}$:Y$_2$O$_3$ ceramics for telecommunication quantum technologies

We report an optical homogeneous linewidth of 580 $\pm$ 20 Hz of Er$^{3+}$:Y$_2$O$_3$ ceramics at millikelvin temperatures, narrowest so far in rare-earth doped ceramics. We show slow spectral diffusion of $\sim$2 kHz over a millisecond time scale. Temperature, field dependence of optical coherence and spectral diffusions reveal the remaining dephasing mechanism as elastic two-level systems in polycrystalline grain boundaries and superhyperfine interactions of Er$^{3+}$ with nuclear spins. In addition, we perform spectral holeburning and measure up to 5 s hole lifetimes. These spectroscopic results put Er$^{3+}$:Y$_2$O$_3$ ceramics as a promising candidate for telecommunication quantum memories and light-matter interfaces.

physics.app-ph↗