arXiv subjects

S. -L. Su

Publications and source records attributed to S. -L. Su.

13 recordsLinked to original sources

arXiv2026

Recovering Readout-Limited Fisher Information in Superconducting-Qubit Magnetometry with Squeezed Microwaves

The performance of superconducting-qubit magnetometers depends not only on magnetic-field encoding during Ramsey interrogation, but also on how efficiently the encoded information is recovered during readout. Here we quantify how squeezed-microwave-assisted dispersive readout can recover magnetic-field information lost during qubit-state assignment. We develop an effective detected-mode framework linking projected quadrature noise, state-assignment error, and the classical Fisher information accessible from binary readout outcomes. A finite mismatch between the squeezed quadrature and the discrimination axis produces an optimal squeezing strength through the competition between squeezed and anti-squeezed fluctuations. For representative parameters, squeezed readout reduces the readout-limited magnetic-field sensitivity bound by $27.3\%$. This improvement arises from recovering information lost in the readout stage rather than from increasing the information encoded during Ramsey interrogation. These results may provide a practical route for mitigating measurement-stage information loss in superconducting quantum sensing.

quant-ph↗

arXiv2026

Experimental Demonstration of a Brachistochrone Nonadiabatic Holonomic Quantum-Gate Scheme in a Trapped Ion

Nonadiabatic holonomic quantum computation (NHQC) offers intrinsic resilience to certain control imperfections. However, conventional nonadiabatic holonomic protocols are constrained by the fixed-pulse-area condition, which limits flexibility and prolongs duration of small-angle gates. Here we experimentally demonstrate a universal brachistochrone nonadiabatic holonomic quantum gate scheme in a trapped 40Ca+ ion, and realized the construction of pX gate under the conventional NHQC, brachistochrone NHQC (BNHQC) and composite BNHQC (CBNHQC) protocols. By characterizing the performance of gate performance in the presence of dissipation, Rabi-frequency errors and detuning errors, we show that BNHQC and CBNHQC outperform conventional NHQC, and BNHQC can offer a favorable balance between operation speed and robustness. It further shows that keeping high fidelity and strong robustness need decrease the accumulated population of excited state in the evolution process. These results highlight nonadiabatic holonomic computation as a practical route toward fast and robust quantum gates in trapped-ion platforms.

quant-ph↗

arXiv2025

Time Crystal in the Nonlinear Phonon Mode of the Trapped Ions

Time crystals constitute a novel phase of matter defined by the spontaneous breaking of timetranslation symmetry. Here we present a scheme to realize a continuous-time crystal of the vibrational phonon in the normal mode of two coupled ultra-cold ions. By utilizing two addressable standing-wave lasers and adiabatic elimination method, we generate a controllable nonlinear phonon mode with the well-designed efficient linear gain and nonlinear damping. By controlling these parameters to satisfy the phase transition conditions of Hopf bifurcation and limit cycle phase, it behaves as a stable dissipative dynamics over timescales significantly longer than the oscillation period, indicating the emergence of discrete time-translation symmetry breaking in the phonon mode, i.e., a phonon time crystal. We further numerically simulate this phonon time crystal by using accessible experimental parameters and also demonstrate a robustness to the initial thermal state and thermalization of phonon mode, spin dephasing, and the control errors of Rabi frequencies. These results provide a practical scheme for observing a time crystal in a nonlinear phonon mode and will advance the research of time crystals.

quant-ph↗

arXiv2025

Optomechanically Induced Transparency on Exceptional Surfaces

Exceptional points (EPs) are singularities in non-Hermitian systems, where the system transmission spectrum varies significantly at the phase transition point. Here, we propose a practical scheme to study the changes of the optomechanically induced transparency (OMIT) spectrum on the exceptional surface (ES), which is formed by designing the structure of the waveguide in a non-Hermitian cavity optomechanical system. By comparing the transmission spectra of the system at different normal points, EPs on the same or different ESs, and exceptional derived points, we find that the peak-valley conversion of the system transmission spectra is obtained at the phase transition point and the arbitrary manipulation of the system transmission spectrum can be realized by moving the system on the same or different ESs. Furthermore, the phenomena of conversion and enhancement of the fast-slow light in the system transmission spectra have also been discovered in our researches. Different from the isolated EP, our proposal can discuss the system properties at different EPs, can find a richer transmission spectrum, and can provide more convenient options for experimental implementation, which paves the way for studying the nature of non-Hermitian systems in a higher dimension.

quant-ph↗

arXiv2024

Beyond the Carnot Limit in the Internal Cycles of a Quantum Heat Engine under Finite Heat Reservoirs

We investigate, in an analytical fashion, quantum Carnot cycles of a microscopic heat engine coupled to two nite heat reservoirs, whose internal cycles could own higher e ciency than the standard Carnot limit without consuming extra quantum resources, e.g., coherence or squeezing properties. The engine runs time-dependently, involving both the internal and external cycles to collaboratively accomplish a complete Carnot cycle, and the e ciency of the engine depends on the reservoirs heat capacities and the working substance. Our analytical results of the maximum efficiency and the maximum power output clarify the mechanism behind the high performance of the microscopic engines, displaying the key roles played by the nite-sized heat reservoirs. Our proposal is generally valid for any microscopic thermodynamic system and fully feasible under current laboratory conditions.

cond-mat.stat-mech↗

arXiv2024

Experimental Verification of Demon-Involved Fluctuation Theorems

The limit of energy saving in the control of small systems has recently attracted much interest due to the concept refinement of the Maxwell demon. Inspired by a newly proposed set of fluctuation theorems, we report the first experimental verification of these equalities and inequalities in a ultracold 40Ca ion system, confirming the intrinsic nonequilibrium in the system due to involvement of the demon. Based on elaborately designed demon-involved control protocols, such as the Szilard engine protocol, we provide experimentally quantitative evidence of the dissipative information, and observe tighter bounds of both the extracted work and the demon's efficacy than the limits predicted by the Sagawa-Ueda theorem. Our results substantiate a close connection between the physical nature of information and nonequilibrium processes at the microscale, which help further understanding the thermodynamic characteristics of information and the optimal design of nanoscale and smaller systems.

quant-ph↗

arXiv2023

Thermal-dephasing-tolerant generation of Schrödinger cat states with Rydberg dressed blockade

Multipartite entangled states involving non-locality are one of the most fascinating characteristics of quantum mechanics. In this work, we propose a thermal-dephasing-tolerant generation of mesoscopic entangled states with Rydberg dressed atoms. We encode logical state on dressed states rather than Rydberg states. Such treatment can increase the lifetime of multipartite entanglement coherence to around 3 times compared to the Rydberg-state-coding one at the same system size, and therefore induce solid fidelities of mesoscopic superposition states generation. The current work theoretically verifies the advantages of using Rydberg dressed states in many-body quantum entanglement, which is helpful for large-scale quantum computation and many-body Rydberg quantum simulation.

quant-ph↗

arXiv2023

Enhancement of Quantum Sensing in a Cavity Optomechanical System around Quantum Critical Point

The precision of quantum sensing could be improved by exploiting quantum phase transitions, where the physical quantity tends to diverge when the system approaches the quantum critical point. This critical enhancement phenomenon has been applied to the quantum Rabi model in a dynamic framework, showing a promising sensing enhancement without the need for complex initial state preparation. In this work, we present a quantum phase transition in the coupling cavity-mechanical oscillator system when the coupling strength crosses a critical point, determined by the effective detuning of cavity and frequency of mechanical mode. By utilizing this critical phenomenon, we obtain a prominent enhancement of quantum sensing, such as the position and momentum of the mechanical oscillator. This result provides an alternative method to enhance the quantum sensing of some physical quantities, such as mass, charge, and weak force, in a large mass system.

quant-ph↗

arXiv2023

Quantum computation in silicon-vacancy centers based on nonadiabatic geometric gates protected by dynamical decoupling

Due to strong zero-phonon line emission, narrow inhomogeneous broadening, and stable optical transition frequencies, the quantum system consisting of negatively charged silicon-vacancy (SiV) centers in diamond is highly expected to develop universal quantum computation. We propose to implement quantum computation for the first time using SiV centers placed in a one-dimensional phononic waveguide, for which quantum gates are realized in a nonadiabatic geometric way and protected by dynamical decoupling (DD). The scheme has the feature of geometric quantum computation that is robust to control errors and the advantage of DD that is insensitive to environmental impact. Furthermore, the encoding of qubits in long-lifetime ground states of silicon-vacancy centers can reduce the effect of spontaneous emission. Numerical simulations demonstrate the practicability of the SiV center system for quantum computation and the robustness improvement of quantum gates by DD pulses. This scheme may provide a promising path toward high-fidelity geometric quantum computation in solid-state systems.

quant-ph↗

arXiv2023

Rabi-error and Blockade-error-resilient All-Geometric Rydberg Quantum Gates

We propose a nontrivial two-qubit gate scheme in which Rydberg atoms are subject to designed pulses resulting from geometric evolution processes. By utilizing a hybrid robust non-adiabatic and adiabatic geometric operations on the control atom and target atom, respectively, we improve the robustness of two-qubit Rydberg gate against Rabi control errors as well as blockade errors in comparison with the conventional two-qubit blockade gate. Numerical results with the current state-of-the-art experimental parameters corroborates the above mentioned robustness. We also evaluated the influence induced by the motion-induced dephasing and the dipole-dipole interaction and imperfection excitation induced leakage errors, which both could decrease the gate fidelity. Our scheme provides a promising route towards systematic control error (Rabi error) as well as blockade error tolerant geometric quantum computation on neutral atom system.

quant-ph↗

arXiv2022

Dynamical Control of Quantum Heat Engines Using Exceptional Points

A quantum thermal machine is an open quantum system coupled to hot and cold thermal baths. Thus, its dynamics can be well understood using the concepts and tools from non-Hermitian quantum systems. A hallmark of non-Hermiticity is the existence of exceptional points where the eigenvalues of a non-Hermitian Hamiltonian or an Liouvillian superoperator and their associated eigenvectors coalesce. Here, we report the experimental realisation of a single-ion heat engine and demonstrate the effect of the Liouvillian exceptional points on the dynamics and the performance of a quantum heat engine. Our experiments have revealed that operating the engine in the exact- and broken-phases, separated by a Liouvillian exceptional point, respectively during the isochoric heating and cooling strokes of an Otto cycle produces more work and output power and achieves higher efficiency than executing the Otto cycle completely in the exact phase where the system has an oscillatory dynamics and higher coherence. This result opens interesting possibilities for the control of quantum heat engines and will be of interest to other research areas that are concerned with the role of coherence and exceptional points in quantum processes and in work extraction by thermal machines.

quant-ph↗

arXiv2021

Single-atom verification of the noise-resilient and fast characteristics of universal nonadiabatic noncyclic geometric quantum gates

Quantum gates induced by geometric phases are intrinsically robust against noise due to their global properties of the evolution paths. Compared to conventional nonadiabatic geometric quantum computation (NGQC), the recently proposed nonadiabatic noncyclic geometric quantum computation (NNGQC) works in a faster fashion, while still remaining the robust feature of the geometric operations. Here, we experimentally implement the NNGQC in a single trapped ultracold $^{40}$Ca$^{+}$ ion for verifying the noise-resilient and fast feature. By performing unitary operations under imperfect conditions, we witness the advantages of the NNGQC with measured fidelities by quantum process tomography in comparison with other two quantum gates by conventional NGQC and by straightforwardly dynamical evolution. Our results provide the first evidence confirming the possibility of accelerated quantum information processing with limited systematic errors even in the imperfect situation.

quant-ph↗

arXiv2020

Single-Atom Verification of the Information-Theoretical Bound of Irreversibility at the Quantum Level

Quantitative measure of disorder or randomness based on the entropy production characterizes thermodynamical irreversibility, which is relevant to the conventional second law of thermodynamics. Here we report, in a quantum mechanical fashion, the first theoretical prediction and experimental exploration of an information-theoretical bound on the entropy production. Our theoretical model consists of a simplest two-level dissipative system driven by a purely classical field, and under the Markovian dissipation, we find that such an information-theoretical bound, not fully validating quantum relaxation processes, strongly depends on the drive-to-decay ratio and the initial state. Furthermore, we carry out experimental verification of this information-theoretical bound by means of a single spin embedded in an ultracold trapped $^{40}$Ca$^{+}$ ion. Our finding, based on a two-level model, is fundamental to any quantum thermodynamical process and indicates much difference and complexity in quantum thermodynamics with respect to the conventionally classical counterpart.

quant-ph↗