Andrey L. Pankratov, Andrey S. Gavrilov
The computer simulations of the process of single pulse readout from the flux-biased phase qubit is performed in the frame of one-dimensional Schroedinger equation. It has been demonstrated that the readout error can be minimized by choosing the optimal pulse duration and the depth of a potential well, leading to the fidelity of 0.94 for 2ns and 0.965 for 12ns sinusoidal pulses.
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The duration and fidelity of qubit readout is a critical factor for applications in quantum information processing as it limits the fidelity of algorithms which reuse qubits after measurement or apply feedback based on the measurement result. Here we present fast multiplexed readout of five qubits in a single 1.2 GHz wide readout channel. Using a readout pulse length of 80 ns and populating readout resonators for less than 250 ns we find an average correct assignment probability for the five measured qubits to be $97\%$. The differences between the individual readout errors and those found whe…
High fidelity single shot qubit state readout is essential for many quantum information processing protocols. In superconducting quantum circuit, the qubit state is usually determined by detecting the dispersive frequency shift of a microwave cavity from either transmission or reflection. In this paper, we demonstrate the use of constructive interference between the transmitted and reflected signal to optimize the qubit state readout, with which we find a better resolved state discrimination and an improved qubit readout fidelity. As a simple and convenient approach, our scheme can be combined…
Modeling and Experimental Validation of the Intrinsic SNR in Spin Qubit Gate-Based Readout and Its Impacts on Readout Electronics
本研究针对半导体自旋量子比特的RF栅极读出场景，提出考虑量子力学特性的读出模型与理论分析框架，经实验验证有效，可优化读出链路，实现更快更低功耗的集成低温电子学读出系统。
A Simulation Methodology for Superconducting Qubit Readout Fidelity
本文提出一种基于Matlab和Ansys HFSS的超导量子比特读取保真度仿真方法，实现读取路径协同优化，预测结果与实验吻合，发现降低7dB功率或缩窄40%脉冲宽度仍能保持高保真度。
Readout-induced suppression and enhancement of superconducting qubit lifetimes
该研究揭示了超导量子比特读出期间寿命退化源于反Zeno效应，通过磁通可调量子比特和自洽主方程准确预测寿命变化，并可控展示了Zeno和反Zeno效应导致的寿命抑制与增强。
Measurement-induced state transitions in dispersive qubit readout schemes
本研究针对超导量子比特色散读出中高功率引发非计算态跃迁的问题，基于缀饰相干态提出两种确定无跃迁最大读出光子数的通用度量指标，经fluxonium验证不依赖特定量子比特近似，可用于电路优化与不同量子比特读出性能比较。
Balanced cross-Kerr coupling for superconducting qubit readout
本研究提出一种基于交叉凯尔耦合的结读出方法，通过非微扰相互作用提升超导量子比特读出的保真度和速度，克服多光子过程限制，提供更灵活的频率分配方案。
Dispersive coupling based on the Rabi model with large detuning is widely used for quantum nondemolition (QND) qubit readout in quantum computation. However, the measurement speed and fidelity are usually significantly limited by the Purcell effects, i.e.: Purcell decay, critical photon numbers, and qubit-dependent Kerr nonlinearity. To avoid these effects, we propose how to realize an ideal QND readout of a gradiometric flux qubit with a tunable gap via its non-perturbative dispersive coupling (NPDC) to a frequency-tunable measurement resonator. We show that this NPDC-based readout mechanism …
We describe single-shot readout of a trapped-ion multi-qubit register using space and time-resolved camera detection. For a single qubit we measure 0.9(3)x10^{-4} readout error in 400us exposure time, limited by the qubit's decay lifetime. For a four-qubit register (a "qunybble") we measure an additional error of only 0.1(1)x10^{-4} per qubit, despite the presence of 4% optical cross-talk between neighbouring qubits. A study of the cross-talk indicates that the method would scale with negligible loss of fidelity to ~10000 qubits at a density <~1 qubit/um^2, with a readout time ~1us/qubit.
We have realized a new interaction between superconducting qubits and a readout cavity that results in the displacement of a coherent state in the cavity, conditioned on the state of the qubit. This conditional state, when it reaches the cavity-following, phase-sensitive amplifier, matches its measured observable, namely the in-phase quadrature. In a setup where several qubits are coupled to the same readout resonator, we show it is possible to measure the state of a target qubit with minimal dephasing of the other qubits. Our results suggest novel directions for faster readout of superconduct…
Optimizing Pump Conditions of Parametric Amplifiers for Fast Multiplexed Readout of Superconducting Qubits
提出针对快速复用读取优化参量放大器泵浦条件的策略，通过最大化限制性量子比特的信噪比提升来最小化总复用读取时间，实验验证将读取时间减少320纳秒。
Non-demolition fluorescence readout and high-fidelity unconditional reset of a fluxonium qubit via dissipation engineering
本文提出无谐振器的耗散工程方案，利用平面滤波器和读出跃迁选择，实现 fluxonium 量子比特的非破坏荧光读出及快速、高保真、全微波无条件重置。
Readout-induced leakage of the fluxonium qubit
该研究通过实验探讨了磁通量子比特中的非QND效应，发现共振子光子会诱导量子比特状态的转变，并通过数值建模揭示了高激发态和外部偶发模式耦合的作用。
Ultrafast Superconducting Qubit Readout with the Quarton Coupler
本文提出利用 quarton 耦合器实现 transmon 量子比特与读出谐振器间大于 250 MHz 的交叉克尔耦合，将超导量子比特读出时间缩短至 5 ns，并保持超过 99% 的读出与 QND 保真度。
Enhancing qubit readout fidelity with two-mode squeezing of the coherent measurement signal
本文提出通过同时测量双模压缩态和相干组合提升量子比特读取保真度，适用于高频放大器和噪声抑制场景。
We demonstrate the application of pattern recognition algorithms via hidden Markov models (HMM) for qubit readout. This scheme provides a state-path trajectory approach capable of detecting qubit state transitions and makes for a robust classification scheme with higher starting state assignment fidelity than when compared to a multivariate Gaussian (MVG) or a support vector machine (SVM) scheme. Therefore, the method also eliminates the qubit-dependent readout time optimization requirement in current schemes. Using a HMM state discriminator we estimate fidelities reaching the ideal limit. Uns…
Transmon qubit readout fidelity at the threshold for quantum error correction without a quantum-limited amplifier
提出一种无量子极限放大器的超导量子比特读出方案，结合shelving技术与双音激发，并利用机器学习后处理，在140ns读出时间内实现了99.5%的双态和96.9%的三态分配保真度，达到量子纠错阈值。
The Gottesman-Kitaev-Preskill encoding of a qubit in a harmonic oscillator is a promising building block towards fault-tolerant quantum computation. Recently, this encoding was experimentally demonstrated for the first time in trapped-ion and superconducting circuit systems. However, these systems lack some of the Gaussian operations which are critical to efficiently manipulate the encoded qubits. In particular, homodyne detection, which is the preferred method for readout of the encoded qubit, is not readily available, heavily limiting the readout fidelity. Here, we present an alternative rea…
Qubit readouts enabled by qubit cloaking
本文基于量子比特隐身技术，提出两种互补的量子比特读取方案，在不改动标准电路QED架构的前提下，可在多种参数条件下实现优于标准色散读取的性能。
Qubit readout is an indispensable element of any quantum information processor. In this work, we experimentally demonstrate a non-perturbative cross-Kerr coupling between a transmon and a polariton mode which enables an improved quantum non-demolition (QND) readout for superconducting qubits. The new mechanism uses the same experimental techniques as the standard QND qubit readout in the dispersive approximation, but due to its non-perturbative nature, it maximizes the speed, the single-shot fidelity and the QND properties of the readout. In addition, it minimizes the effect of unwanted decay …
We analyze a single-shot readout for superconducting qubits via the controlled catch, dispersion, and release of a microwave field. A tunable coupler is used to decouple the microwave resonator from the transmission line during the dispersive qubit-resonator interaction, thus circumventing damping from the Purcell effect. We show that if the qubit frequency tuning is sufficiently adiabatic, a fast high-fidelity qubit readout is possible, even in the strongly nonlinear dispersive regime. Interestingly, the Jaynes-Cummings nonlinearity leads to the quadrature squeezing of the resonator field bel…
Multiplexed Readout of Superconducting Qubits Using a 3D Re-entrant Cavity Filter
提出一种3D重入腔滤波器用于超导量子比特频率多路复用读出，其平面外电容耦合设计可扩展至大规模阵列，兼具大线宽带通与固有Purcell滤波功能，并在四量子比特器件上完成验证。
Fast multiplexed superconducting qubit readout with intrinsic Purcell filtering
本文提出电容与电感同时耦合的紧凑陷波滤波电路，通过相消干涉实现本征Purcell滤波，在42 MHz线宽下完成四量子比特56 ns同时读出，平均赋值保真度达99.77%。
Syndrome-Resolved Decoding of Qubit Readout Errors
本文针对超导量子比特读出错误无法被传统指标解析的问题，提出读出错误解码器（RED），可识别限制读出性能的错误通道，为量子比特读出优化提供基础。
We describe a kinetic inductance traveling-wave (KIT) amplifier suitable for superconducting quantum information measurements and characterize its wideband scattering and noise properties. We use mechanical microwave switches to calibrate the four amplifier scattering parameters up to the device input and output connectors at the dilution refrigerator base temperature and a tunable temperature load to characterize the amplifier noise. Finally, we demonstrate the high fidelity simultaneous dispersive readout of two superconducting transmon qubits. The KIT amplifier provides low-noise amplificat…
We report a single-shot-based projective readout of a semiconductor hybrid qubit formed by three electrons in a GaAs double quantum dot. Voltage-controlled adiabatic transitions between the qubit operations and readout conditions allow high-fidelity mapping of quantum states. We show that a large ratio both in relaxation time vs. tunneling time (~ 50) and singlet-triplet splitting vs. thermal energy (~ 20) allow energy-selective tunneling-based spin-to-charge conversion with readout visibility ~ 92.6%. Combined with ac driving, we demonstrate high visibility coherent Rabi and Ramsey oscillatio…
We demonstrate single-shot qubit readout with fidelity sufficient for fault-tolerant quantum computation, for two types of qubit stored in single trapped calcium ions. For an optical qubit stored in the (4S_1/2, 3D_5/2) levels of 40Ca+ we achieve 99.991(1)% average readout fidelity in one million trials, using time-resolved photon counting. An adaptive measurement technique allows 99.99% fidelity to be reached in 145us average detection time. For a hyperfine qubit stored in the long-lived 4S_1/2 (F=3, F=4) sub-levels of 43Ca+ we propose and implement a simple and robust optical pumping scheme …
Common flux qubit readout schemes are qubit dominated, meaning they measure in the energy eigenbasis of the qubit. For various applications meausrements in a basis different from the energy eigenbasis are required. Here we present an indirect measurement protocol, which is detector dominated instead of qubit dominated, yielding a projective measurements in the persistent current basis for arbitrary bias points. We show that with our setup it is possible to perform a quantum nondemolition measurement (QND) in the persistent current basis at all flux bias points with fidelities reaching almost 1…
The Pound-Drever-Hall Method for Superconducting-Qubit Readout
提出基于多音自相位参考Pound-Drever-Hall技术的超稳定超导量子比特读取方法，通过室温外差检测实现单次读取，对微波相位漂移不敏感，相位稳定性达0.73度/2小时，且边带不会引起测量诱导态跃迁，信号增强至少14 dB。
Fast, high-fidelity Transmon readout with intrinsic Purcell protection via nonperturbative cross-Kerr coupling
通过非微扰交叉库珀耦合实现高保真度Transmon量子比特读取，无需外部滤波器，提升读取速度和保真度。
Readout-induced degradation of transmon lifetimes: interplay of TLSs and qubit spectral reshaping
研究揭示了读出过程通过TLSs和量子比特频谱重塑导致寿命退化，提出在共振器频率驱动下可优化读出协议。
High fidelity optical readout of a nuclear spin qubit in Silicon Carbide
本研究针对量子比特的量子态读出需求，基于重复读出技术实现了碳化硅中V2色心核自旋的高保真度读出（最高99.5%），完成核自旋初始化与振荡观测，并将其用作长寿命存储器应用于弱耦合自旋浴量子传感。
System Characterization of Dispersive Readout in Superconducting Qubits
本研究提出两种低校准需求的自动化协议，可表征超导量子比特色散读出的关键参数与系统效率，验证了基于提取参数的模型对54量子比特器件读出信噪比的预测误差在10%以内。
Readout errors are a significant source of noise for near term quantum computers. A variety of methods have been proposed to mitigate these errors using classical post processing. For a system with $n$ qubits, the entire readout error profile is specified by a $2^n\times 2^n$ matrix. Recent proposals to use sub-exponential approximations rely on small and/or short-ranged error correlations. In this paper, we introduce and demonstrate a methodology to categorize and quantify multiqubit readout error correlations. Two distinct types of error correlations are considered: sensitivity of the measur…
We study the quantitative characterization of the performance of qubit measurements in this paper. In particular, the back-action evading nature of quantum nondemolition (QND) readout of qubits is fully quantified by quantum trace distance. Only computational basis states are necessary to be taken into consideration. Most importantly, we propose an experimentally efficient method to evaluate the QND fidelity based on the classical trace distance, which uses an experimental scheme with two consecutive measurements. The three key quantifiers of a measurement, i.e., QND fidelity, readout fidelity…
Coupling a resonator to a superconducting qubit enables various operations on the qubit, including dispersive readout and unconditional reset. The speed of these operations is limited by the external decay rate of the resonator. However, increasing the decay rate also increases the rate of qubit decay via the resonator, limiting the qubit lifetime. Here, we demonstrate that the resonator-mediated qubit decay can be suppressed by utilizing the distributed-element, multi-mode nature of the resonator. We show that the suppression exceeds two orders of magnitude over a bandwidth of 600 MHz. We use…
Enhancing Dispersive Readout of Superconducting Qubits Through Dynamic Control of the Dispersive Shift: Experiment and Theory
该研究通过动态减小超导量子比特与读取谐振器失谐、增大色散频移并加倍读取模式线宽，在100 ns内实现0.25%的两态读取错误和近四倍信噪比提升。
High-fidelity qubit measurements play a crucial role in quantum computation, communication, and metrology. In recent experiments, it has been shown that readout fidelity may be improved by performing repeated quantum non-demolition (QND) readouts of a qubit's state through an ancilla. For a qubit encoded in a two-level system, the fidelity of such schemes is limited by the fact that a single error can destroy the information in the qubit. On the other hand, if a bosonic system is used, this fundamental limit could be overcome by utilizing higher levels such that a single error still leaves sta…
Photon-noise-tolerant dispersive readout of a superconducting qubit using a nonlinear Purcell filter
针对读出谐振腔残余噪声光子导致超导量子比特退相位的问题，提出非线性Purcell滤波器，在不牺牲读出性能的前提下将噪声容忍度提升3倍，结合分岔效应使测量速率再提升3倍，实现40ns脉冲下99.4%读出保真度与99.2%量子非破坏保真度。
The speed of quantum gates and measurements is a decisive factor for the overall fidelity of quantum protocols when performed on physical qubits with finite coherence time. Reducing the time required to distinguish qubit states with high fidelity is therefore a critical goal in quantum information science. The state-of-the-art readout of superconducting qubits is based on the dispersive interaction with a readout resonator. Here, we bring this technique to its current limit and demonstrate how the careful design of system parameters leads to fast and high-fidelity measurements without affectin…
Implementation and readout of maximally entangled two-qubit gates quantum circuits in a superconducting quantum processor
本文在基于 transmon 的 5 比特超导量子处理器上，比较了从单比特到最大纠缠 Bell 电路的性能，提出并验证了“乘积读出概率”和“条件读出概率”两种读出范式，以优化量子算法的输出解释。
Quantum computers are becoming increasingly accessible, and may soon outperform classical computers for useful tasks. However, qubit readout errors remain a significant hurdle to running quantum algorithms on current devices. We present a scheme to more efficiently mitigate these errors on quantum hardware and numerically show that our method consistently gives advantage over previous mitigation schemes. Our scheme removes biases in the readout errors allowing a general error model to be built with far fewer calibration measurements. Specifically, for reading out $n$-qubits we show a factor of…
We report high-fidelity, quantum nondemolition, single-shot readout of a superconducting transmon qubit using a DC-biased superconducting low-inductance undulatory galvanometer(SLUG) amplifier. The SLUG improves the system signal-to-noise ratio by 7 dB in a 20 MHz window compared with a bare HEMT amplifier. An optimal cavity drive pulse is chosen using a genetic search algorithm, leading to a maximum combined readout and preparation fidelity of 91.9% with a measurement time of Tmeas = 200ns. Using post-selection to remove preparation errors caused by heating, we realize a combined preparation …
Single-Shot Readout of a Superconducting Qubit Using a Thermal Detector
本研究利用超灵敏测辐射热计替代参量放大器，实验演示了超导量子比特单次读出，在13.9 μs读出时间下实现0.618保真度，为解决量子比特扩展性限制提供了新方案。
We reduce measurement errors in a quantum computer using machine learning techniques. We exploit a simple yet versatile neural network to classify multi-qubit quantum states, which is trained using experimental data. This flexible approach allows the incorporation of any number of features of the data with minimal modifications to the underlying network architecture. We experimentally illustrate this approach in the readout of trapped-ion qubits using additional spatial and temporal features in the data. Using this neural network classifier, we efficiently treat qubit readout crosstalk, result…
We present and demonstrate a general three-step method for extracting the quantum efficiency of dispersive qubit readout in circuit QED. We use active depletion of post-measurement photons and optimal integration weight functions on two quadratures to maximize the signal-to-noise ratio of the non-steady-state homodyne measurement. We derive analytically and demonstrate experimentally that the method robustly extracts the quantum efficiency for arbitrary readout conditions in the linear regime. We use the proven method to optimally bias a Josephson traveling-wave parametric amplifier and to qua…
Owing to their long excited state lifetimes, rare-earth ions in crystals are widely used in quantum applications. To allow optical readout of the qubit state of individual ions, we propose to dope the crystal with an additional nearby ancilla ion with a shorter radiative lifetime. We show how a Bayesian analysis exhausts the information about the state of the qubit from the optical signal of the ancilla ion. We study the effects of incoherent processes and propose ways to reduce their effect on the readout. Finally, we extend the architecture to ions residing in two remote cavities, and we sho…
The readout fidelity of superconducting transmon and Xmon qubits is partially limited by the qubit energy relaxation through the resonator into the transmission line, which is also known as the Purcell effect. One way to suppress this energy relaxation is to employ a filter which impedes microwave propagation at the qubit frequency. We present semiclassical and quantum analyses for the bandpass Purcell filter realized by E.\ Jeffrey \textit{et al}.\ [Phys.\ Rev.\ Lett.\ 112, 190504 (2014)]. For typical experimental parameters, the bandpass filter suppresses the qubit relaxation rate by up to t…
Directional emission of a readout resonator for qubit measurement
本研究提出基于全通谐振器的超导量子比特定向发射色散读出方案，解决传统方案故意失配带来的可扩展性问题，实验验证了低插入损耗与高读出保真度，为量子计算机可扩展架构提供新思路。
Noise reduction in qubit readout with a two-mode squeezed interferometer
针对相干光量子比特读出受真空涨落限制的问题，提出基于双JPC非平衡双模压缩干涉仪的测量方法，实现投影读出功率信噪比提升31%，并发现弱测量中非投影调谐可提高量子效率，为远程纠缠提供可能。
We study the dispersive readout of a qubit in the ultimate limit of a single-photon probe. The use of a single-photon probe avoids the errors due to nonorthogonality of coherent states. A photodetector is used in the scheme we consider. The dynamics of the system is studied using the Heisenberg-Langevin equations. We treat the counter-rotating terms in the Hamiltonian perturbatively, which leads to the Bloch-Siegert shift in the resonator frequency. It is shown how this can improve the readout. The theory of photon transport through the qubit and the resonator it couples to is provided while t…
Millimeter Wave Readout of a Superconducting Qubit
研究利用毫米波与transmon量子比特的相互作用，通过大频率差抑制驱动引起的非期望态跃迁，实现高保真度量子比特读取。
Qubit measurement is generally the most error-prone operation that degrades the performance of near-term quantum devices, and the exponential decay of readout fidelity severely impedes the development of large-scale quantum information processing. Given these disadvantages, we present a quantum state readout method, named \textit{compression readout}, that naturally avoids large multi-qubit measurement errors by compressing the quantum state into a single qubit for measurement. Our method generally outperforms direct measurements in terms of accuracy, and the advantage grows with the system si…
Flexible Readout and Unconditional Reset for Superconducting Multi-Qubit Processors with Tunable Purcell Filters
本文提出了一种利用可调普尔曼滤波器实现超导多量子比特处理器灵活读出与无条件重置的架构，实现了高保真度读出和快速重置，同时抑制光子噪声与退相干。
We consider a unitary transfer of an arbitrary state of a two-level atomic qubit in a cavity to the finite amplitude coherent state cavity field. Such transfer can be used to either provide an effective readout measurement on the atom by a subsequent measurement on the light field or as a method for initializing a fixed atomic state - a so-called "attractor state", studied previously for the case of an infinitely strong cavity field. We show that with a suitable adjustment of the coherent amplitude and evolution time the qubit transfers all its information to the field, attaining a selected st…
We propose a method to perform single-shot optical readout of a quantum bit (qubit) using cavity quantum electrodynamics. We selectively couple the optical transitions associated with different qubit basis states to the cavity, and utilize the change in cavity transmissivity to generate a qubit readout signal composed of many photons. We show that this approach enables single-shot optical readout even when the qubit does not have a good cycling transition required for standard resonance fluorescence measurements. We calculate the probability that the measurement detects the correct spin state …
The measurement of a quantum two-level system, or a qubit in modern terminology, often involves an electromagnetic field that interacts with the qubit, before the field is measured continuously and the qubit state is inferred from the noisy field measurement. During the measurement, the qubit may undergo spontaneous transitions, further obscuring the initial qubit state from the observer. Taking advantage of some well known techniques in stochastic detection theory, here we propose a novel signal processing protocol that can infer the initial qubit state optimally from the measurement in the p…
Flux-pulse-assisted Readout of a Fluxonium Qubit
本文提出利用fluxonium量子比特在大色散频移磁通偏置点进行通量脉冲辅助读出，理论模拟显示该方案在155 ns积分时间内将信噪比提升约5倍，并给出可实现该方案的合理能量参数。
Demonstrating a quantum computational advantage will require high-fidelity control and readout of multi-qubit systems. As system size increases, multiplexed qubit readout becomes a practical necessity to limit the growth of resource overhead. Many contemporary qubit-state discriminators presume single-qubit operating conditions or require considerable computational effort, limiting their potential extensibility. Here, we present multi-qubit readout using neural networks as state discriminators. We compare our approach to contemporary methods employed on a quantum device with five superconducti…
In this work, we introduce machine learning methods to implement readout of a single qubit on $^{171}\mathrm{Yb^{+}}$ trapped-ion system. Different machine learning methods including convolutional neural networks and fully-connected neural networks are compared with traditional methods in the tests. The results show that machine learning methods have higher fidelity, more robust readout results in relatively short time. To obtain a 99% readout fidelity, neural networks only take half of the detection time needed by traditional threshold or maximum likelihood methods. Furthermore, we implement …
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