arXiv subjects

Donhee Ham

Publications and source records attributed to Donhee Ham.

8 recordsLinked to original sources

arXiv2025

Impedance-tuned microwave loop for fast, homogeneous Rabi oscillations of a dense ensemble of NV-diamond electronic spins

Obtaining a high Rabi oscillation frequency homogeneously across a spatially-extended population of nitrogen-vacancy (NV) center electronic spins in diamond is useful for efficient spin-state manipulation of the NV ensemble and in using NVs to detect ensembles of other spin species. Here, we achieve a high, homogeneous Rabi frequency for a dense NV ensemble by enhancing the microwave magnetic fields in the center region of a diamond-coupled planar metallic loop via systematic engineering that increases the microwave current driving of the loop, while avoiding off-center proximity to the loop that gives strong but inhomogeneous microwave fields. With such enhanced microwave fields at 2.55 GHz, we achieve a 136.3 MHz NV Rabi frequency with 1.5% inhomogeneity over a 40 $\times$ 40 $\mu m^{2}$ diamond area; and use the NV ensemble to detect a ~30-MHz magnetic signal, similar to a nuclear magnetic resonance signal at a tesla-scale bias magnetic field, with Hz-scale spectral resolution.

quant-ph↗

arXiv2025

Microwave-regime demonstration of plasmonic non-reciprocity in a flowing two-dimensional electron gas

The speed of a plasmonic wave in the presence of electron drift in a conductor depends on the wave's propagation direction, with the wave traveling along the drift (`forward wave') faster than the wave traveling against the drift (`backward wave'). Phenomena related to this plasmonic non-reciprocity -- which is relatively more pronounced in two-dimensional conductors than in bulk conductors and could lead to solid-state device applications -- have been studied in THz and optical spectral regimes. Here we demonstrate the plasmonic non-reciprocity at microwave frequencies (10 $\sim$ 50 GHz). Concretely, we conduct, at 4K, a microwave network analysis on a gated GaAs two-dimensional electron gas with electron drift (i.e., DC current), directly measuring out forward and backward wave speeds via their propagation phase delays. We resolve, for example, forward and backward wave speeds of $4.26 \times 10^{-3} \pm 8.97 \times 10^{-6}$ (normalized to the speed of light). Sufficient consistency between the electron drift speed obtained from the microwave measurement and that alternatively estimated by a DC transport theory further confirms the non-reciprocity. We conclude this paper with a discussion on how to enhance the non-reciprocity for real-world applications, where degeneracy pressure would play an important role.

cond-mat.mes-hall↗

arXiv2022

Quantum Logic Enhanced Sensing in Solid-State Spin Ensembles

We demonstrate quantum logic enhanced sensitivity for a macroscopic ensemble of solid-state, hybrid two-qubit sensors. We achieve a factor of 30 improvement in signal-to-noise ratio, translating to a sensitivity enhancement exceeding an order of magnitude. Using the electronic spins of nitrogen vacancy (NV) centers in diamond as sensors, we leverage the on-site nitrogen nuclear spins of the NV centers as memory qubits, in combination with homogeneous bias and control fields, ensuring that all of the ${\sim}10^9$ two-qubit sensors are sufficiently identical to permit global control of the NV ensemble spin states. We find quantum logic sensitivity enhancement for multiple measurement protocols with varying optimal sensing intervals, including XY8 dynamical decoupling and correlation spectroscopy, using a synthetic AC magnetic field. The results are independent of the nature of the target signal and broadly applicable to metrology using NV centers and other solid-state ensembles. This work provides a benchmark for macroscopic ensembles of quantum sensors that employ quantum logic or quantum error correction algorithms for enhanced sensitivity.

quant-ph↗

arXiv2014

Massive thermal fluctuation of massless graphene electrons

Whereas thermal current noise $\langle I^2 \rangle$ in typical conductors is proportional to temperature $T$, $\langle I^2 \rangle$ in graphene exhibits a nonlinear $T$ dependence due to the massless nature of individual electrons. This unique $\langle I^2 \rangle$ arising from individually massless electrons is intimately linked to the non-zero collective mass of graphene electrons; namely, $\langle I^2 \rangle$ is set by the equipartition theorem applied to the collective mass's kinetic energy, with the nonlinear $T$-dependence arising from the $T$-dependence of the collective mass. This link between thermal fluctuation and collective dynamics unifies $\langle I^2 \rangle$ in graphene and typical conductors, while elucidating the uniqueness of the former at the same time.

cond-mat.mes-hall↗

arXiv2014

Measurement of Collective Dynamical Mass of Dirac Fermions in Graphene

Individual electrons in graphene behave as massless quasiparticles. In surprising twist, it is inferred from plasmonic investigations that collectively excited graphene electrons must exhibit non-zero mass and its inertial acceleration is essential for graphene plasmonics. Despite such importance, this collective mass has defied direct unequivocal measurement. It may be directly measured by accelerating it with a time-varying voltage and quantifying the phase delay of the resulting current; this voltage-current phase relation would manifest as kinetic inductance, representing the collective inertia's reluctance to accelerate. However, at optical (infrared) frequencies phase measurement of current is generally difficult and at microwave frequencies the inertial phase delay has been buried under electron scattering. Here we directly, precisely measure the kinetic inductance, thus, collective mass, by combining innovative device engineering that reduces electron scattering and delicate microwave phase measurements. Particularly, encapsulation of graphene between hexagonal-boron-nitride layers, one-dimensional edge contacts, and a proximate top gate configured as microwave ground together enable resolving the inertial phase delay from the electron scattering. Beside the fundamental importance, the kinetic inductance demonstrated here to be orders-of-magnitude larger than magnetic inductance can dramatically miniaturize radio-frequency integrated circuits. Moreover, its bias-dependency heralds a solid-state voltage-controlled inductor to complement the prevalent voltage-controlled capacitor.

cond-mat.mes-hall↗

arXiv2012

Two-Path Solid-State Interferometry Using Ultra-Subwavelength 2D Plasmonic Waves

We report an on-chip solid-state Mach-Zehnder interferometer operating on two-dimensional (2D) plasmonic waves at microwave frequencies. Two plasmonic paths are defined with GaAs/AlGaAs 2D electron gas 80 nm below a metallic gate. The gated 2D plasmonic waves achieve a velocity of ~c/300 (c: free-space light speed). Due to this ultra-subwavelength confinement, the resolution of the 2D plasmonic interferometer is two orders of magnitude higher than that of its electromagnetic counterpart at a given frequency. This GHz proof-of-concept at cryogenic temperatures can be scaled to the THz IR range for room temperature operation, while maintaining the benefits of the ultra-subwavelength confinement.

cond-mat.mes-hall↗

arXiv2010

Phase Diffusion and Lamb-Shift-Like Spectrum Shift in Classical Oscillators

The phase diffusion in a self-sustained oscillator, which produces oscillator's spectral linewidth, is inherently governed by a nonlinear Langevin equation. Over past 40 years, the equation has been treated with linear approximation, rendering the nonlinearity's effects unknown. Here we solve the nonlinear Langevin equation using the perturbation method borrowed from quantum mechanics, and reveal the physics of the nonlinearity: slower phase diffusion (linewidth narrowing) and a surprising oscillation frequency shift that formally corresponds to the Lamb shift in quantum electrodynamics.

cond-mat.stat-mech↗

arXiv2006

Integrated Cell Manipulation Systems

A new type of microfluidic system for biological cell manipulation, a CMOS/microfluidic hybrid, is demonstrated. The hybrid system starts with a custom-designed CMOS (complementary metal-oxide semiconductor) chip fabricated in a semiconductor foundry using standard integration circuit technology. A microfluidic channel is post-fabricated on top of the CMOS chip to provide biocompatible environment. The motion of individual biological cells that are tagged with magnetic beads is directly controlled by the CMOS chip that generates localized magnetic filed patterns using an on-chip array of micro-electromagnets. The speed and the programmability of the CMOS chip further allow for the dynamic reconfiguration of the magnetic fields, substantially increasing the manipulation capability of the hybrid system. The concept of a hybrid system is verified by simultaneously manipulating individual biological cells with microscopic resolution. A new operation protocol that exploits the fast speed of electronics to trap and move a large number of cells with less power consumption is also demonstrated. Combining the advantages of microelectronics, the CMOS/microfluidic hybrid approach presents a new model for a multifunctional lab-on-a chip for biological and medical applications.

q-bio.QM↗