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Moonjoo Lee

Publications and source records attributed to Moonjoo Lee.

At least 19 recordsLinked to original sources
arXiv2025

In-situ tuning of cavity dissipation and a topological transition in an atom-nanotip-cavity system

We theoretically investigate a method for tuning a dissipation rate in an atom-cavity system. By inserting a nanotip into the cavity mode, we estimate that the cavity dissipation rate increases by a factor of approximately 20, due to the scattering loss at the tip. As applications of our in-situ technique, we demonstrate that an exceptional line can be obtained when a single atom or ion is coupled to the resonator. Moreover, the position of an exceptional point is tuned by adjusting the decay rate, enabling a topological transition through dissipation control alone.

quant-ph↗
arXiv2024

A photonic quantum engine driven by superradiance

Performance of nano- and micro-scale heat engines can be improved with a help from quantum mechanical phenomena. Recently, heat reservoirs with quantum coherence have been proposed to enhance engine performance beyond the Carnot limit even with a single reservoir. However, no physical realizations have been achieved so far. Here, we report the first proof-of-principle experimental demonstration of a photonic quantum engine driven by superradiance employing a single heat reservoir composed of atoms and photonic vacuum. Reservoir atoms prepared in a quantum coherent superposition state underwent superradiance while traversing the cavity. This led to about 40-fold increase of the effective engine temperature, resulting in a near-unity engine efficiency. Moreover, the observed engine output power grew quadratically with respect to the atomic injection rate. Our work can be utilized in quantum mechanical heat transfer as well as in boosting engine powers, opening a pathway to development of photomechanical devices that run on quantum coherence embedded in heat baths.

quant-ph↗
arXiv2024

Comparative Study of Quantum-Circuit Scalability in a Financial Problem

Quantum computer is extensively used in solving financial problems. Quantum amplitude estimation, an algorithm that aims to estimate the amplitude of a given quantum state, can be utilized to determine the expectation value of bonds as the logic introduced in quantum risk analysis. As the number of the evaluation qubit increases, the more accurate the precise the outcome expectation value is. This augmentation in qubits, however, also leads to a varied escalation in circuit complexity, contingent upon the type of quantum computing device. By analyzing the number of two-qubit gates in the superconducting circuit and ion-trap quantum system, this study examines that the native gates and connectivity nature of the ion-trap system lead to less complicated quantum circuits. Across a range of experiments conducted with one to nineteen qubits, the examination reveals that the ion-trap system exhibits a two to three factor reduction in the number of required two-qubit gates when compared to the superconducting circuit system.

quant-ph↗
arXiv2024

Pushing single atoms near an optical cavity

Optical scattering force is used to reduce the loading time of single atoms to a cavity mode. Releasing a cold atomic ensemble above the resonator, we apply a push beam along the direction of gravity, offering fast atomic transport with narrow velocity distribution. We also observe in real time that, when the push beam is illuminated against gravity, single atoms slow down and even turn around in the mode, through the cavity-transmission measurement. Our method can be employed to make atom-cavity experiments more efficient.

quant-ph↗
arXiv2023

Tolerance and breakdown of topological protection in a disordered waveguide

We consider a disordered waveguide consisting of trivial dielectric and non-trivial magnetically anisotropic material. A topologically-protected edge mode appears owing to the broken time-reversal symmetry of the non-trivial lattice. While the edge mode maintains under other position and radius disorders, the protection is immediately broken by applying a radius disorder to the non-trivial lattice. This breakdown originates from donor and acceptor modes occupying the topological bandgap. Furthermore, via the calculation of the Bott index, we show that Anderson localization occurs as a metal conducting gap changes to a topological gap along with increasing disorders.

cond-mat.dis-nn↗
arXiv2023

Third-order exceptional point in an ion-cavity system

We investigate a scheme for observing the third-order exceptional point (EP3) in an ion-cavity setting. In the lambda-type level configuration, the ion is driven by a pump field, and the resonator is probed with another weak laser field. We exploit the highly asymmetric branching ratio of an ion's excited state to satisfy the weak-excitation limit, which allows us to construct the non-Hermitian Hamiltonian $(H_{\textrm{nH}})$. Via fitting the cavity-transmission spectrum, the eigenvalues of $H_{\textrm{nH}}$ are obtained. The EP3 appears at a point where the Rabi frequency of the pump laser and the atom-cavity coupling constant balance the loss rates of the system. Feasible experimental parameters are provided.

quant-ph↗
arXiv2023

Realization of superabsorption by time reversal of superradiance

Emission and absorption of light lie at the heart of light-matter interaction. Although emission and absorption rates are regarded as intrinsic properties of atoms and molecules, various ways to modify these rates have been sought in applications such as quantum information processing, metrology and light-energy harvesting. One promising approach is to utilize collective behaviour of emitters in the same way as in superradiance5. Although superradiance has been observed in diverse systems, its conceptual counterpart in absorption has never been realized11 until now. Here we demonstrate enhanced cooperative absorption - superabsorption - by implementing a time-reversal process of superradiance. The observed superabsorption rate is much higher than that of ordinary absorption, with the number of absorbed photons scaling with the square of the number of atoms, exhibiting the cooperative nature of superabsorption. The present superabsorption - which performs beyond the limitations of conventional absorption - can facilitate weak-signal sensing, light-energy harvesting and light-matter quantum interfaces

quant-ph↗
arXiv2022

Numerical investigation of a segmented-blade ion trap with biasing rods

We report a numerical study of a linear ion trap that has segmented blades and biasing rods. Our system consists of radio frequency (rf) blades, dc blades with ten separate electrodes, and two biasing rods for compensating the ions' micromotion. After calculating the optical access for the ions, we find rf and dc voltages that result in a stable trapping configuration of $^{171}$Yb$^{+}$ ions. We also explore the micromotion compensation with the biasing rods, and calculate the influence of blade misalignment to the trap potential. Our work offers quantitative understanding of the trap architecture, assisting reliable operation of an ion-trap quantum computer.

quant-ph↗
arXiv2022

Ion trap with gold-plated alumina: substrate and surface characterization

We describe a complete development process of a segmented-blade linear ion trap. Alumina substrate is characterized with an X-ray diffraction and loss-tangent measurement. The blade is laser-micromachined and polished, followed by the sputtering and gold electroplating. Surface roughness is examined at each step of the fabrication via both electron and optical microscopies. On the gold-plated facet, we obtain a height deviation of tens of nanometers in the vicinity of the ion position. Trapping of laser-cooled $^{174}$Yb$^{+}$ ions is demonstrated.

quant-ph↗
arXiv2022

Precise and extensive characterization of an optical resonator for cavity-based quantum networks

Cavity-based quantum node is a competitive platform for distributed quantum networks. Here, we characterize a high-finesse Fabry-Perot optical resonator for coupling single or few atomic quantum registers. Our cavity consists of two mirrors with different reflectivities: One has minimal optical loss, and the other high transmission loss where more than 90% of the intracavity photons would be emitted. Cavity finesse, birefringent effects, and mechanical resonances are measured using the lasers at 780, 782, and 795 nm. In order to obtain cavity geometric parameters, we drive the adjacent longitudinal or transverse modes with two lasers simultaneously, and measure those frequencies using a precision wavelength meter (WLM). A major novelty of this method is that the parameters' uncertainties are solely determined by the resolution of the WLM, eliminating all of the temporal environment fluctuations. Our scheme makes it possible to quantify the atom-cavity coupling constant up to four significant figures, the most precise and accurate estimation so far, which would become a key ingredient for benchmarking a cavity-based quantum node. Furthermore, the distortion of polarized photonic qubits would be minimized owing to the small birefringent splitting, below 4.9% of the cavity linewidth. Our system should operate in the intermediate atom-cavity coupling regime that would allow us to implement various quantum network protocols.

quant-ph↗
arXiv2021

Locking Multi-laser Frequencies to a Precision Wavelength Meter: Application to Cold Atoms

We herein report a simultaneous frequency stabilization of two 780-nm external cavity diode lasers using a precision wavelength meter (WLM). The laser lock performance is characterized by the Allan deviation measurement in which we find $σ_{y}=10^{-12}$ at an averaging time of 1000 s. We also obtain spectral profiles through a heterodyne spectroscopy, identifying the contribution of white and flicker noises to the laser linewidth. The frequency drift of the WLM is measured to be about 2.0(4) MHz over 36 hours. Utilizing the two lasers as a cooling and repumping field, we demonstrate a magneto-optical trap of $^{87}$Rb atoms near a high-finesse optical cavity. Our laser stabilization technique operates at broad wavelength range without a radio frequency element.

quant-ph↗
arXiv2020

Microelectromechanical-System-Based Design of a High-Finesse Fiber Cavity Integrated with an Ion Trap

We present a numerical study of a MEMS-based design of a fiber cavity integrated with an ion trap system. Each fiber mirror is supported by a microactuator that controls the mirror's position in three dimensions. The mechanical stability is investigated by a feasibility analysis showing that the actuator offers a stable support of the fiber. The actuators move the fibers' positions continuously with a stroke of more than 10 $μ$m, with mechanical resonance frequencies on the order of kHz. A calculation of the trapping potential shows that a separation between ion and fiber consistent with strong ion-cavity coupling is feasible. Our miniaturized ion-photon interface constitutes a viable approach to integrated hardware for quantum information.

quant-ph↗
arXiv2020

Probing surface charge densities on optical fibers with a trapped ion

We describe a novel method to measure the surface charge densities on optical fibers placed in the vicinity of a trapped ion, where the ion itself acts as the probe. Surface charges distort the trapping potential, and when the fibers are displaced, the ion's equilibrium position and secular motional frequencies are altered. We measure the latter quantities for different positions of the fibers and compare these measurements to simulations in which unknown charge densities on the fibers are adjustable parameters. Values ranging from $-10$ to $+50$ e/$μ$m$^2$ were determined. Our results will benefit the design and simulation of miniaturized experimental systems combining ion traps and integrated optics, for example, in the fields of quantum computation, communication and metrology. Furthermore, our method can be applied to any setup in which a dielectric element can be displaced relative to a trapped charge-sensitive particle.

quant-ph↗
arXiv2018

Ion-based nondestructive sensor for cavity photon numbers

We dispersively couple a single trapped ion to an optical cavity to extract information about the cavity photon-number distribution in a nondestructive way. The photon-number-dependent AC-Stark shift experienced by the ion is measured via Ramsey spectroscopy. We use these measurements first to obtain the ion-cavity interaction strength. Next, we reconstruct the cavity photon-number distribution for coherent states and for a state with mixed thermal-coherent statistics, finding overlaps above 99% with the calibrated states.

quant-ph↗
arXiv2016

Generation of an optical Schrödinger-cat-like state in a non-ideal cavity by injecting opposite-phase atomic dipoles

We propose a method to generate an optical Schrödinger-cat-like state in a cavity in a substantial decoherence regime. Even when the cavity decay rate is considerably large, a cat-like state can be generated in a laser-like setting if the gain for the field is larger than the loss. Under the condition that opposite-phase atomic dipoles repeatedly traverse the cavity, the cavity field converges to a squeezed vacuum state in a steady state. A Schrödinger-cat-like state is then generated when a single photon decay occurs. The phase-space distribution of the cat state can be revealed in homodyne detection by using the decaying photon as a herald event. Quantum trajectory simulation was used to identify the conditions for the Schrödinger-cat-like state formation as well as to analyze the properties of those states. Based on these simulations, possible experiments are proposed within the reach of the current technology.

quant-ph↗
arXiv2014

Optical transport and manipulation of an ultracold atomic cloud using focus-tunable lenses

We present an optical setup with focus-tunable lenses to dynamically control the waist and focus position of a laser beam, in which we transport a trapped ultracold cloud of 87-Rb over a distance of 28 cm. The scheme allows us to shift the focus position at constant waist, providing uniform trapping conditions over the full transport length. The fraction of atoms that are transported over the entire distance comes near to unity, while the heating of the cloud is in the range of a few microkelvin. We characterize the position stability of the focus and show that residual drift rates in focus position can be compensated for by counteracting with the tunable lenses. Beyond being a compact and robust scheme to transport ultracold atoms, the reported control of laser beams makes dynamic tailoring of trapping potentials possible. As an example, we steer the size of the atomic cloud by changing the waist size of the dipole beam.

cond-mat.quant-gas↗
arXiv2014

Three-dimensional imaging of cavity vacuum with single atoms localized by a nanohole array

Zero-point electromagnetic fields were first introduced to explain the origin of atomic spontaneous emission. Vacuum fluctuations associated with the zero-point energy in cavities are now utilized in quantum devices such as single-photon sources, quantum memories, switches and network nodes. Here we present three-dimensional (3D) imaging of vacuum fluctuations in a high-Q cavity based on the measurement of position-dependent emission of single atoms. Atomic position localization is achieved by using a nanoscale atomic beam aperture scannable in front of the cavity mode. The 3D structure of the cavity vacuum is reconstructed from the cavity output. The root mean squared amplitude of the vacuum field at the antinode is also measured to be 0.92+-0.07V/cm. The present work utilizing a single atom as a probe for sub-wavelength imaging demonstrates the utility of nanometre-scale technology in cavity quantum electrodynamics.

quant-ph↗