arXiv subjects

Jianshe Liu

Publications and source records attributed to Jianshe Liu.

9 recordsLinked to original sources

arXiv2026

Oval-shaped resonance distortion as a signature of quasiparticle heating effect in a niobium superconducting resonator

We investigate the nonlinear behavior of a superconducting microwave resonator subjected to a dissipative mechanism where the associated quality factor (Q factor) decreases with increasing dissipated power, leading to a dissipative feedback effect. By modifying the Rothwarf-Taylor equations, we establish a macroscopic quasiparticle heating (QPH) model that directly links the quality factor to the microwave readout power. The key finding is the identification of a distinctive oval-shaped distortion in the resonance circle in the complex plane. This distortion serves as a practical experimental signature for identifying the readout power regime in which QPH dominates the loss, under conditions where other nonlinear mechanisms are sufficiently weak. To validate the model, we design and fabricate a niobium (Nb) half-wavelength coplanar waveguide (CPW) resonator and conduct systematic bath temperature and readout power sweeps. The model provides a well fit to the observed oval-shaped resonance circle distortion across a wide range of operating conditions, confirming the QPH mechanism as the primary source of the dissipative non-linearity in the parameter space investigated.

cond-mat.supr-con↗

arXiv2025

Dictionary-based Block Encoding of Sparse Matrices with Low Subnormalization and Circuit Depth

Block encoding severs as an important data input model in quantum algorithms, enabling quantum computers to simulate non-unitary operators effectively. In this paper, we propose an efficient block-encoding protocol for sparse matrices based on a novel data structure, called the dictionary data structure, which classifies all non-zero elements according to their values and indices. Non-zero elements with the same values, lacking common column and row indices, belong to the same classification in our block-encoding protocol's dictionary. When compiled into the \{\rm U(2), CNOT\} gate set, the protocol queries a $2^n \times 2^n$ sparse matrix with $s$ non-zero elements at a circuit depth of $\mathcal{O}(\log(ns))$, utilizing $\mathcal{O}(n^2s)$ ancillary qubits. This offers an exponential improvement in circuit depth relative to the number of system qubits, compared to existing methods~\cite{clader2022quantum,zhang2024circuit} with a circuit depth of $\mathcal{O}(n)$. Moreover, in our protocol, the subnormalization, a scaled factor that influences the measurement probability of ancillary qubits, is minimized to $\sum_{l=0}^{s_0}\vert A_l\vert$, where $s_0$ denotes the number of classifications in the dictionary and $A_l$ represents the value of the $l$-th classification. Furthermore, we show that our protocol connects to linear combinations of unitaries (LCU) and the sparse access input model (SAIM). To demonstrate the practical utility of our approach, we provide several applications, including Laplacian matrices in graph problems and discrete differential operators.

quant-ph↗

arXiv2023

Qubit Energy Tuner Based on Single Flux Quantum Circuits

A device called qubit energy tuner (QET) based on single flux quantum (SFQ) circuits is proposed for Z control of superconducting qubits. Created from the improvement of flux digital-to-analog converters (flux DACs), a QET is able to set the energy levels or the frequencies of qubits, especially flux-tunable transmons, and perform gate operations requiring Z control. The circuit structure of QET is elucidated, which consists of an inductor loop and flux bias units for coarse tuning or fine tuning. The key feature of a QET is analyzed to understand how SFQ pulses change the inductor loop current, which provides external flux for qubits. To verify the functionality of the QET, three simulations are carried out. The first one verifies the responses of the inductor loop current to SFQ pulses. The results show that there is about 4.2% relative deviation between analytical solutions of the inductor loop current and the solutions from WRSpice time-domain simulation. The second and the third simulations with QuTip show how a Z gate and an iSWAP gate can be performed by this QET, respectively, with corresponding fidelities 99.99884% and 99.93906% for only once gate operation to specific initial states. These simulations indicate that the SFQ-based QET could act as an efficient component of SFQ-based quantum-classical interfaces for digital Z control of large-scale superconducting quantum computers.

quant-ph↗

arXiv2016

Measurement Of Energy Resolution For TES Microcalorimeter With Optical Pulses

Energy resolution is an important figure of merit for TES microcalorimeter. We propose a laser system to measure the energy resolution of TES microcalorimeter with a 1550 nm laser source. Compared to method that characterizes the performance by irradiating the detector using X-ray photons from a radioactive source placed inside the refrigerator, our system is safer and more convenient. The feasibility of this system has been demonstrated in the measurement of an Al/Ti bilayer TES microcalorimeter. In this experiment, the tested detector showed a energy resolution of 72 eV in the energy range from 0.2 keV to 0.9 keV

physics.ins-det↗

arXiv2016

Quantum coherence between cavity and artificial atom in a superconducting circuit QED ladder system

We have created a quantum three-level ladder system with the cavity dispersive energy level in a superconducting circuit quantum electrodynamics system consisting of a transmon qubit and a cavity, and have directly observed the Autler-Townes splitting effect instead of representing it by the probability of the qubit being at each level. A coupler tone is applied on the transition between the second excited state of transmon and cavity dispersive level, while the cavity spectrum is probed. A doublet transmission and anormalous dispersion spectrum of the cavity level is clearly shown. The inverse Fourier transform of cavity spectrum indicates that there is a quantum coherence Rabi oscillation of the populations between cavity and qubit.

quant-ph↗

arXiv2013

Charge-SQUID and Tunable Phase-slip Flux Qubit

A phase-slip flux qubit, exactly dual to a charge qubit, is composed of a superconducting loop interrupted by a phase-slip junction. Here we propose a tunable phase-slip flux qubit by replacing the phase-slip junction with a charge-related superconducting quantum interference device (SQUID) consisting of two phase-slip junctions connected in series with a superconducting island. This charge-SQUID acts as an effective phase-slip junction controlled by the applied gate voltage and can be used to tune the energy-level splitting of the qubit. Also, we show that a large inductance inserted in the loop can reduce the inductance energy and consequently suppress the dominating flux noise of the phase-slip flux qubit. This enhanced phase-slip flux qubit is exactly dual to a transmon qubit.

cond-mat.supr-con↗

arXiv2009

Optimal effective current operator for flux qubit accounting for inductive effects

An optimal effective current operator for flux qubit has been investigated by taking account of the inductive effects of the circuit loop. The whole system is treated as two interacting subsystems: one is the inductance-free flux qubit consisting of three Josephson junctions and the other a high frequency LC-oscillator. As the composite system hardly affords one excessively high energy LC photon, an effective theory for the inductive flux qubit providing its physical variable operators has been achieved, which can take account of the inductive effects but does not include the additional degree of freedom for the LC-oscillator. Considering the trade-off between simplicity and accuracy, it has been revealed that the optimal effective current operator resulting in an error only on the order of $L^{3/2}$ provides an approximation of high accuracy, which is also verified numerically.

cond-mat.supr-con↗

arXiv2006

Numerical simulation of leakage effect for quantum NOT operation on three-Josephson-junction flux qubit

Superconducting flux qubits with three Josephson junctions are promising candidates for the building blocks of a quantum computer. We have applied the imaginary time evolution method to study the model of this qubit accurately by calculating its wave functions and eigenenergies. Because such qubits are manipulated with magnetic flux microwave pulses they might be irradiated into non-computational states which is called the leakage effect. Through the evolution of the density matrix of the qubit under either hard-shaped pulse or Gaussian-shaped pulse to carry out quantum NOT operation, we show that the leakage effect is not a serious problem for a flux qubit.

cond-mat.supr-con↗

arXiv2005

Diagonalizing the Hamiltonian of capacitively coupled superconducting phase qubits

Coupling qubits together towards large-scale integration is a key point for realizing a quantum computer. We study the capacitively coupled superconducting phase qubits using two diagonalization methods, which are very efficient to obtain the wave functions and energies of the bound states of such two-qubit system. The first diagonalization method is based on two-dimensional cubic approximation for the coupled system with wave functions of the eigenstates for harmonic oscillators as the bases of diagonalization, and also reveals the physics underlying it. The other one utilizes the Fast Fourier Transform to perform diagonalization with plane waves as the bases, and it can be easily extended to other problems with even high dimension.

cond-mat.supr-con↗