arXiv subjects

C. P. Sun

Publications and source records attributed to C. P. Sun.

At least 37 records · Page 2Linked to original sources

arXiv2024

On Quantum Reliability Characterizing Systematic Errors in Quantum Sensing

Quantum sensing utilize quantum effects, such as entanglement and coherence, to measure physical signals. The performance of a sensing process is characterized by error which requires comparison to a true value. However, in practice, such a true value might be inaccessible. In this study, we utilize quantum reliability as a metric to evaluate quantum sensor's performance based solely on the apparatus itself, without any prior knowledge of true value. We derive a general relationship among reliability, sensitivity, and systematic error, and demonstrate this relationship using a typical quantum sensing process. That is to measure magnetic fields (as a signal) by a spin-$1/2$ particle and using the Stern-Gerlach apparatus to read out the signal information. Our findings illustrate the application of quantum reliability in quantum sensing, opening new perspectives for reliability analysis in quantum systems.

quant-ph↗

arXiv2024

Feynman Paradox about the Josephson effect and a sawtooth current in the double junction

We revisit the Feynman approach to the Josephson effect, which employs a pair of linear coupling equations for its modeling. It is found that while the exact solutions can account for the AC Josephson effect when the coupling strength is significantly less than the voltage, they fail to produce the DC Josephson effect in any practical scenario. To address this fundamental discrepancy, we derive the coupled Ginzburg-Landau (GL) equations for two interconnected superconductors based on BCS theory. These equations reveal that the nonlinear coupling, which is overlooked in the Feynman method, is crucial in describing the spontaneous symmetry breaking in superconductors, a critical factor for achieving the DC Josephson effect. When the coupled GL equations are applied to a double junction, a sawtooth current pattern emerges, a result unattainable via the Feynman approach.

cond-mat.supr-con↗

arXiv2024

Dressed Majorana fermion in a hybrid nanowire

The low-energy theory of hybrid nanowire systems fails to define Majorana fermion (MF) in the strong tunneling and magnetic field strength. To address this limitation, we propose a holistic approach to define MF in which the quasi-excitation in nanowire and superconductor constitutes together its own ``antiparticles''. This definition is general, beyond the constraint presented in the low-energy theory. It reveals that the Majorana phase depends not only on the chemical potential and Zeeman energy in nanowire but also on those of superconductor, and that the mismatch of chemical potential leads not to observe MF. Such a broader perspective provides more specific experimental guidance under various conditions

cond-mat.mes-hall↗

arXiv2023

Quantum Reliability

Quantum technology has led to increasingly sophisticated and complex quantum devices. Assessing their reliability (quantum reliability) is an important issue. Although reliability theory for classical devices has been well developed in industry and technology, a suitable metric on quantum reliability and its loss has not been systematically investigated. Since reliability-loss depends on the process, quantum fidelity does not always fully depict it. This study provides a metric of quantum reliability by shifting the focus from state-distinguishing to trajectory-distinguishing. In contrast to the conventional notion of classical reliability, which is evaluated using probabilistic measurements of binary logical variables, quantum reliability is grounded in the quantum probability amplitude or wave function. This research provides a universal framework for reliability theory encompassing both classical and quantum devices. It offers a new perspective on quantum engineering by elucidating how intensely the real quantum process a device undergoes influences its performance.

quant-ph↗

arXiv2023

Coherence filtration under strictly incoherent operations

We study the task of coherence filtration under strictly incoherent operations in this paper. The aim of this task is to transform a given state $ρ$ into another one $ρ^\prime$ whose fidelity with the maximally coherent state is maximal by using stochastic strictly incoherent operations. We find that the maximal fidelity between $ρ^\prime$ and the maximally coherent state is given by a multiple of the $Δ$ robustness of coherence $R(ρ\|Δρ):=\min\{\uplambda|ρ\leq\uplambdaΔρ\}$, which provides $R(ρ\|Δρ)$ an operational interpretation. Finally, we provide a coherence measure based on the task of coherence filtration.

quant-ph↗

arXiv2023

Controlling superconducting transistor by coherent light

The Josephson junction is typically tuned by a magnetic field or electrostatic gates to realize a superconducting transistor, which manipulates the supercurrent in integrated superconducting circuits. However, this tunable method does not achieve simultaneous control for the supercurrent phase (phase difference between two superconductors) and magnitude. Here, we propose a novel scheme for the light-controlled superconducting transistor, which is composed of two superconductor leads linked by a coherent light-driven quantum dot. We discover a Josephson-like relation for supercurrent $I_{\mathrm{s}}=I_{c}(Φ)\,\sinΦ$, where both supercurrent phase $Φ$ and magnitude $I_{c}$ could be entirely controlled by the phase, intensity, and detuning of the driving light. Additionally, the supercurrent magnitude displays a Fano profile with the increase of the driving light intensity, which is clearly understood by comparing the level splitting of the quantum dot under light driving and the superconducting gap. Moreover, when two such superconducting transistors form a loop, they make up a light-controlled superconducting quantum interference device (SQUID). Such a light-controlled SQUID could demonstrate the Josephson diode effect, and the optimized non-reciprocal efficiency achieves up to $54\%$, surpassing the maximum record reported in recent literature. Thus, our feasible scheme delivers a promising platform to perform diverse and flexible manipulations in superconducting circuits.

cond-mat.mes-hall↗

arXiv2023

Refined Majorana phase diagram in topological insulator-superconductor hybrid system

The edge state of the topological insulator coupled to a superconductor system is able to simulate the Majorana fermion in zero energy mode since the Kitaev-type pairing is induced by exchanging quasi-excitations in electron tunneling. However, the present study has revealed that this physical simulation is not valid for a larger surface gap, which is the energy gap of the insulator's surface states. To address this issue, a refined pairing term that depends on the surface gap has been obtained as a second-order effect of the proximity effect, whereas the lowest order produces a constant pairing strength. By carefully considering the dependence of pairing strength on the surface gap, the Majorana phase diagram is re-achieved and a significant difference from previous work is observed, where the pairing strength was assumed to be independent of the surface gap and resulted in a conical phase boundary.

cond-mat.mes-hall↗

arXiv2023

Finite-Time Optimization of Quantum Szilard heat engine

We propose a finite-time quantum Szilard engine (QSE) with a quantum particle with spin as the working substance (WS) to accelerate the operation of information engines. We introduce a Maxwell's demon (MD) to probe the spin state within a finite measurement time $t_{\rm M}$ to capture the which-way information of the particle, quantified by the mutual information $I(t_{\rm{M}})$ between WS and MD. We establish that the efficiency $η$ of QSE is bounded by $η\leq1-(1-η_{\rm{C}}){\rm ln}2/I(t_{\rm M})$, where $I(t_{\rm M})/\rm{ln}2$ characterizes the ideality of quantum measurement, and approaches $1$ for the Carnot efficiency reached under ideal measurement in quasi-static regime. We find that the power of QSE scales as $P\propto t_{\rm M}^{3}$ in the short-time regime and as $P\propto t_{\rm M}^{-1}$ in the long-time regime. Additionally, considering the energy cost for erasing the MD's memory required by Landauer's principle, there exists a threshold time that guarantees QSE to output positive work.

quant-ph↗

arXiv2022

Geodesic path for the optimal nonequilibrium transition: Momentum-independent protocol

Accelerating controlled thermodynamic processes requires an auxiliary Hamiltonian to steer the system into instantaneous equilibrium states. An extra energy cost is inevitably needed in such finite-time operation. We recently develop a geodesic approach to minimize such energy cost for the shortcut to isothermal process. The auxiliary control typically contains momentum-dependent terms, which are hard to be experimentally implemented due to the requirement of constantly monitoring the speed. In this work, we employ a variational auxiliary control without the momentum-dependent force to approximate the exact control. Following the geometric approach, we obtain the optimal control protocol with variational minimum energy cost. We demonstrate the construction of such protocol via an example of Brownian motion with a controllable harmonic potential.

cond-mat.stat-mech↗

arXiv2022

Geodesic bound of the minimum energy expense to achieve membrane separation within finite time

To accomplish a task within limited operation time typically requires an excess expense of energy, whose minimum is of practical importance for the optimal design in various applications, especially in the industrial separation of mixtures for purification of components. Technological progress has been made to achieve better purification with lower energy expense, yet little is known about the fundamental limit on the least excess energy expense in finite operation time. We derive such a limit and show its proportionality to the square of a geometric distance between the initial and final states and inverse proportionality to the operation time $τ$. Our result demonstrates that optimizing the separation protocol is equivalent to finding the geodesic curve in a geometric space. Interestingly, we show the optimal control with the minimum energy expense is achieved by a symmetry-breaking protocol, where the two membranes are moved toward each other with different speeds.

math.OC↗

arXiv2022

Experimental implementation of finite-time Carnot cycle

The Carnot cycle is a prototype of ideal heat engine to draw mechanical energy from the heat flux between two thermal baths with the maximum efficiency, dubbed as the Carnot efficiency $η_{\mathrm{C}}$. Such efficiency can only be reached by thermodynamical equilibrium processes with infinite time, accompanied unavoidably with vanishing power - energy output per unit time. In real-world applications, the quest to acquire high power leads to an open question whether a fundamental maximum efficiency exists for finite-time heat engines with given power. We experimentally implement a finite-time Carnot cycle with sealed dry air as working substance and verify the existence of a tradeoff relation between power and efficiency. Efficiency up to $(0.524\pm0.034)η_{\mathrm{C}}$ is reached for the engine to generate the maximum power, consistent with the theoretical prediction $η_{\mathrm{C}}/2$. Our results shall provide a new platform for studying finite-time thermodynamics consisting of nonequilibrium processes.

cond-mat.stat-mech↗

arXiv2022

Approximate distillation of quantum coherence

Coherence distillation is a basic information-theoretic task in the resource theory of coherence. In this paper, we develop the framework of the approximate coherence distillation under strictly incoherent operations. This protocol considers the situation that we cannot transform an initial state $ρ$ into a target state $ψ$ with certainty; instead, we aim to deterministically transform the initial state $ρ$ into an intermediate state $ϕ$ that is most approximate to the target state $ψ$ in terms of fidelity. We also present the explicit conversion strategy of the approximate coherence distillation.

quant-ph↗

arXiv2022

Many-body systems with SU(1,1) dynamical symmetry: from dynamics to thermodynamics based on the trace formula

For a quantum (many-body) system with dynamical symmetry described by a given Lie group, we study the trace of exponential operators with complex coefficients in one of the irreducible subspaces in terms of the boson realization. By using this approach, for compact groups, we obtain the result of the trace that is consistent with the well-known Weyl character formula. For non-compact groups (with SU(1,1) as an application), convergent condition of the trace is also obtained. This approach may be a powerful tool to study the thermodynamics of quantum (many-body) systems in equilibrium state or nonequilibrium processes.

math-ph↗

arXiv2022

Criterion for a state to be distillable via stochastic incoherent operations

Coherence distillation is a basic information-theoretic task in the resource theory of coherence. In this paper, we present the necessary and sufficient conditions under which a mixed state can be distilled into a pure coherent state via stochastic incoherent operations (sIOs). With the help of this result, we further show the following: (i) Any $2$-dimensional coherent state is distillable via sIOs if and only if it is a pure coherent state; (ii) a state $ρ$ is n-distillable via sIOs if and only if it is 1-distillable; and (iii) the set of distillable states via stochastic maximally incoherent operations is identical to the set of distillable states via sIOs. Finally, we analyze the reason why sIO is stronger than stochastic strictly incoherent operations when we use them to distill a coherent state.

quant-ph↗

arXiv2021

Optimal probabilistic distillation of quantum coherence

We complete the task of optimal probabilistic coherence distillation protocol, whose aim is to transform a general state into a set of n-level maximally coherent states via strictly incoherent operations (SIO). Specifically, we present the necessary and sufficient condition for the transformation from a mixed state into a pure-state ensemble via SIO. With the aid of this condition and the simplex algorithm, we can accomplish the probabilistic distillation protocol by presenting an analytical expression of the maximal average distillable coherence of a general state and constructing the corresponding operation achieving this bound. Our protocol is a universal protocol since it can be applied to any coherence measure.

quant-ph↗

arXiv2021

Do Majorana zero modes emerge in the hybrid nanowire under a strong magnetic field?

The hybrid nanowire consisting of semiconductor with proximity to superconductor is expected to serve as an experimental platform to display Majorana zero modes. By rederiving its effective Kitaev model with spins, we discover a novel topological phase diagram, which assigns a more precise constraint on the magnetic field strength for the emergence of Majorana zero modes. It then turns out the effective pairing strength dressed by the proximity effect exhibits a significant dependence on the magnetic field, and thus the topological phase region is refined as a closed triangle in the phase diagram with chemical potential vs. Zeeman energy(which is obviously different from the open hyperbolic region known before). This prediction is confirmed again by an exact calculation of quantum transport, where the zero bias peak of $2e^{2}/h$ in the differential conductance spectrum, as the necessary evidence for the Majorana zero modes, disappears when the magnetic field grows too strong. For illustrations with practical hybrid systems, in the InSb nanowire coupled to NbTiN, the accessible magnetic field range is around 0.1--1.5T; when coupled to aluminum shell, the accessible magnetic field range should be smaller than 0.12T. These predictions obviously clarify the current controversial issues about some experiments of Majorana zero modes with hybrid nonawire.

cond-mat.mes-hall↗

arXiv2021

Minimal Energy Cost to Initialize a Quantum Bit with Tolerable Error

Landauer's principle imposes a fundamental limit on the energy cost to perfectly initialize a classical bit, which is only reached under the ideal operation with infinite-long time. The question on the cost in the practical operation for a quantum bit (qubit) has been posted under the constraint by the finiteness of operation time. We discover a raise-up of energy cost by $\mathcal{L}^{2}(ε)/τ$ from the Landaeur's limit ($k_{B}T\ln2$) for a finite-time $τ$ initialization with an error probability $ε$. The thermodynamic length $\mathcal{L}(ε)$ between the states before and after initializing in the parametric space increases monotonously as the error decreases. For example, in the constant dissipation coefficient ($γ_{0}$) case, the minimal additional cost is $0.997k_{B}T/(γ_{0}τ)$ for $ε=1\%$ and $1.288k_{B}T/(γ_{0}τ)$ for $ε=0.1\%$. Furthermore, the optimal protocol to reach the bound of minimal energy cost is proposed for the qubit initialization realized via a finite-time isothermal process.

quant-ph↗

arXiv2021

Optimizing Thermodynamic Cycles with Two Finite-Sized Reservoirs

We study the non-equilibrium thermodynamics of a heat engine operating between two finite-sized reservoirs with well-defined temperatures. Within the linear response regime, it is found that the uniform temperature of the two reservoirs at final time $τ$ is bounded from below by the entropy production $σ_{\mathrm{min}}\propto1/τ$. We discover a general power-efficiency trade-off depending on the ratio of heat capacities ($γ$) of the reservoirs for the engine. And a universal efficiency at maximum average power of the engine for arbitrary $γ$ is obtained. For practical purposes, the operation protocol of an ideal gas heat engine to achieve the optimal performance associated with $σ_{\mathrm{min}}$ is demonstrated. Our findings can be used to develop an general optimization scenario for thermodynamic cycles with finite-sized reservoirs in real-world circumstances.

cond-mat.stat-mech↗