Zhi-Hao Ma, Fu-Lin Zhang, Zhi-Hua Chen, Jing-Ling Chen
We report a new metric of quantum states. This metric is build up from super-fidelity, which has deep connection with the Uhlmann-Jozsa fidelity and plays an important role in quantifying entanglement. We find that the new metric possess some interesting properties.
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Fidelity plays an important role in quantum information theory. In this letter, we introduce new metric of quantum states induced by fidelity, and connect it with the well-known trace metric, Sine metric and Bures metric for the qubit case. The metric character is also presented for the qudit (i.e., $d$-dimensional system) case. The CPT contractive property and joint convex property of the metric are also studied.
We show that the recently introduced operator fidelity metric provides a natural tool to investigate the cross-over to quantum chaotic behaviour. This metric is an information-theoretic measure of the global stability of a unitary evolution against perturbations. We use random matrix theory arguments to conjecture that the operator fidelity metric can be used as an "order parameter" to discriminates phases with regular behaviour from quantum chaotic ones. A numerical study of the onset of chaotic in the Dicke model is given in order to support the conjecture
We apply the fidelity metric approach to analyze two recently introduced models that exhibit a quantum phase transition to a topologically ordered phase. These quantum models have a known connection to classical statistical mechanical models; we exploit this mapping to obtain the scaling of the fidelity metric tensor near criticality. The topological phase transitions manifest themselves in divergences of the fidelity metric across the phase boundaries. These results provide evidence that the fidelity approach is a valuable tool to investigate novel phases lacking a clear characterization in t…
The fidelity metric has recently been proposed as a useful and elegant approach to identify and characterize both quantum and classical phase transitions. We study this metric on the manifold of thermal states for the Lipkin-Meshkov-Glick (LMG) model. For the isotropic LMG model, we find that the metric reduces to a Fisher-Rao metric, reflecting an underlying classical probability distribution. Furthermore, this metric can be expressed in terms of derivatives of the free energy, indicating a relation to Ruppeiner geometry. This allows us to obtain exact expressions for the (suitably rescaled) …
Numerous quantum mechanical schemes have been proposed that are intended to improve the sensitivity to rotation provided by the classical Sagnac effect in gyroscopes. A general metric is needed that can compare the performance of the new quantum systems with the classical systems. The fidelity (Shannon mutual information between the measurement and the rotation rate) is proposed as a metric that is capable of this comparison. A theoretical upper bound is derived for the fidelity of an ideal classical Sagnac gyroscope. This upper bound for the classical Sagnac gyroscope should be used as a benc…
A geometric interpretation for the A-fidelity between two states of a qubit system is presented, which leads to an upper bound of the Bures fidelity. The metrics defined based on the A-fidelity are studied by numerical method. An alternative generalization of the A-fidelity, which has the same geometric picture, to a $N$-state quantum system is also discussed.
The notion of fidelity susceptibility, introduced within the context of quantum metric tensor, has been an important quantity to characterize the criticality near quantum phase transitions. We demonstrate that for topological phase transitions in Dirac models, provided the momentum space is treated as the manifold of the quantum metric, the fidelity susceptibility coincides with the curvature function whose integration gives the topological invariant. Thus the quantum criticality of the curvature function near a topological phase transition also describes the criticality of the fidelity suscep…
We study finite-temperature Dynamical Quantum Phase Transitions (DQPTs) by means of the fidelity and the interferometric Loschmidt Echo (LE) induced metrics. We analyse the associated dynamical susceptibilities (Riemannian metrics), and derive analytic expressions for the case of two-band Hamiltonians. At zero temperature the two quantities are identical, nevertheless, at finite temperatures they behave very differently. Using the fidelity LE, the zero temperature DQPTs are gradually washed away with temperature, while the interferometric counterpart exhibits finite-temperature Phase Transitio…
We derive several bounds on fidelity between quantum states. In particular we show that fidelity is bounded from above by a simple to compute quantity we call super--fidelity. It is analogous to another quantity called sub--fidelity. For any two states of a two--dimensional quantum system (N=2) all three quantities coincide. We demonstrate that sub-- and super--fidelity are concave functions. We also show that super--fidelity is super--multiplicative while sub--fidelity is sub--multiplicative and design feasible schemes to measure these quantities in an experiment. Super--fidelity can be used …
We show that nonorthogonal states achieve a higher level of entanglement when one party undergoes uniform acceleration. We also show that in this regime non-orthogonal states achieve a higher fidelity for teleportation. A quantum field as observed by an observer confined to a Rindler wedge acts as an open system. When one traces over the field modes in the causally disconnected region of space time, one finds that, for a qubit derived from the modes of a scalar field, the effective state space of a qubit is deformed. In order to show this we numerically calculate the Bures angle between states…
We use the fidelity approach to quantum critical points to study the zero temperature phase diagram of the one-dimensional Hubbard model. Using a variety of analytical and numerical techniques, we analyze the fidelity metric in various regions of the phase diagram, with particular care to the critical points. Specifically we show that close to the Mott transition, taking place at on-site repulsion U=0 and electron density n=1, the fidelity metric satisfies an hyper-scaling form which we calculate. This implies that in general, as one approaches the critical point U=0, n=1, the fidelity metric …
Measurement induced nonlocality (MIN) captures global nonlocal effect of bipartite quantum state due to locally invariant projective measurements. In this paper, we propose a new version of MIN using fidelity induced metric,andthe same is calculated for pure and mixed states. For mixed state, the upper bound is obtained from eigenvalues of correlation matrix. Further, dynamics of MIN and fidelity based MIN under various noisy quantum channels show that they are more robust than entanglement.
We propose a novel quantum transduction hybrid system based on the coupling of long-coherence time superconducting cavities with electro-optic resonators to achieve high-efficiency and high-fidelity in quantum communication protocols and quantum sensing. ©2022 The Author(s)
A geometric approach to formulate the uncertainty principle between quantum observables acting on an $N$-dimensional Hilbert space is proposed. We consider the fidelity between a density operator associated with a quantum system and a projector associated with an observable, and interpret it as the probability of obtaining the outcome corresponding to that projector. We make use of fidelity-based metrics such as angle, Bures and root-infidelity ones, to propose a measure of uncertainty. The triangle inequality allows us to derive a family of uncertainty relations. In the case of the angle metr…
We analyze uncertainty relations on finite dimensional Hilbert spaces expressed in terms of classical fidelity, which are stronger then metric uncertainty relations introduced by Fawzi, Hayden and Sen. We establish validity of fidelity uncertainty relations for random unitary matrices with optimal parameters (up to universal constants) which improves upon known results for the weaker notion of metric uncertainty. This result is then applied to locking classical information in quantum states and allows to obtain optimal locking in Hellinger distance, improving upon previous results on locking i…
Comparing and correcting robustness metrics for quantum optimal control
本文比较并修正了量子最优控制中鲁棒性度量方法，通过系统研究两种近似方法的差异，并改进了toggle-frame鲁棒性估计器，以获得更精确的鲁棒性评估。
Enhancing the teleportation fidelity of a quantum network using purification
本文分析并比较了不同拓扑结构量子网络在两种纠缠分发方案下的资源性，提出算法评估平均最大隐形传态保真度，证明基于纯化的方案能显著提升网络隐形传态能力。
It is shown that a large class of weak disturbances on macroscopic quantum superpositions can be canceled by a probabilistic reversing operation on the system. We illustrate this for spin systems undergoing an Ising-type interaction with the environment and demonstrate that both the fidelity to the original state and the purity of the amended state can simultaneously be increased by the reversing operation. A possible experimental scheme to implement our scheme is discussed.
In this paper we propose measurement induced nonlocality (MIN) using a metric based on fidelity to capture global nonlocal effect of a quantum state due to locally invariant projective measurements. This quantity is a remedy for local ancilla problem in the original definition of MIN. We present an analytical expression of the proposed version of MIN for pure bipartite state and 2Xn dimensional mixed state. We also provide an upper bound of the MIN for general mixed state. Finally, we compare this quantity with MINs based on Hilbert-Schmidt norm and skew information for Werner and isotropic st…
A plug-and-play superconducting quantum controller at millikelvin temperatures enables exceeding 99.9% average gate fidelity
该研究开发了一种在毫开尔文温度下实现高保真量子操作的超导控制器，通过直接芯片间连接和全数字操控，实现了99.9%的平均Clifford保真度和低能耗的门操作。
High-quality two-qubit gate operations are crucial for scalable quantum information processing. Often, the gate fidelity is compromised when the system becomes more integrated. Therefore, a low-error-rate, easy-to-scale two-qubit gate scheme is highly desirable. Here, we experimentally demonstrate a new two-qubit gate scheme that exploits fixed-frequency qubits and a tunable coupler in a superconducting quantum circuit. The scheme requires less control lines, reduces crosstalk effect, simplifies calibration procedures, yet produces a controlled-Z gate in 30ns with a high fidelity of 99.5%, der…
Based on a `shortcut-to-adiabaticity' (STA) scheme, we theoretically design and experimentally realize a set of high-fidelity single-qubit quantum gates in a superconducting Xmon qubit system. Through a precise microwave control, the qubit is driven to follow a fast `adiabatic' trajectory with the assistance of a counter-diabatic field and the correction of derivative removal by adiabatic gates. The experimental measurements of quantum process tomography and interleaved randomized benchmarking show that the process fidelities of our STA quantum gates are higher than 94.9% and the gate fideliti…
We introduce a method for high-fidelity quantum state transduction between a superconducting microwave qubit and the ground state spin system of a solid-state artificial atom, mediated via an acoustic bus connected by piezoelectric transducers. Applied to present-day experimental parameters for superconducting circuit qubits and diamond silicon vacancy centers in an optimized phononic cavity, we estimate quantum state transduction with fidelity exceeding 99\% at a MHz-scale bandwidth. By combining the complementary strengths of superconducting circuit quantum computing and artificial atoms, th…
Geometric Latent Space Tomography with Metric-Preserving Autoencoders
本文提出几何潜在空间断层扫描方法，结合经典编码器与量子解码器，通过度量保持损失保留量子度量结构，实现高保真量子态重建与误差缓解。
The leading approach to fault tolerant quantum computing requires a continual supply of magic states. When a new magic state is first encoded, its initial fidelity will be too poor for use in the computation. This necessitates a resource-intensive distillation process that occupies the majority of the computer's hardware; creating magic states with a high initial fidelity minimises this cost and is therefore crucial for practical quantum computing. Here we present the surprising and encouraging result that raw magic states can have a fidelity significantly better than that of the two-qubit gat…
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…
Arrays of neutral-atom qubits in optical tweezers are a promising platform for quantum computation. Despite experimental progress, a major roadblock for realizing neutral atom quantum computation is the qubit initialization. Here we propose that supersymmetry -- a theoretical framework developed in particle physics -- can be used for ultra-high fidelity initialization of neutral-atom qubits. We show that a single atom can be deterministically prepared in the vibrational ground state of an optical tweezer by adiabatically extracting all excited atoms to a supersymmetric auxiliary tweezer. The s…
Geometric phases are robust against certain types of local noises, and thus provide a promising way towards high-fidelity quantum gates. However, comparing with the dynamical ones, previous implementations of nonadiabatic geometric quantum gates usually require longer evolution time, due to the needed longer evolution path. Here, we propose a scheme to realize nonadiabatic geometric quantum gates with short paths based on simple pulse control techniques, instead of deliberated pulse control in previous investigations, which can thus further suppress the influence from the environment induced n…
High-fidelity two-qubits gates are essential for the realization of large-scale quantum computation and simulation. Tunable coupler design is used to reduce the problem of parasitic coupling and frequency crowding in many-qubit systems and thus thought to be advantageous. Here we design a extensible 5-qubit system in which center transmon qubit can couple to every four near-neighbor qubit via a capacitive tunable coupler and experimentally demonstrate high-fidelity controlled-phase (CZ) gate by manipulating center qubit and one near-neighbor qubit. Speckle purity benchmarking (SPB) and cross e…
We propose an alternative fidelity measure (namely, a measure of the degree of similarity) between quantum states and benchmark it against a number of properties of the standard Uhlmann-Jozsa fidelity. This measure is a simple function of the linear entropy and the Hilbert-Schmidt inner product between the given states and is thus, in comparison, not as computationally demanding. It also features several remarkable properties such as being jointly concave and satisfying all of "Jozsa's axioms". The trade-off, however, is that it is supermultiplicative and does not behave monotonically under qu…
We experimentally demonstrate a high-fidelity visible-to-telecommunication wavelength conversion of a photon by using a solid-state-based difference frequency generation. In the experiment, one half of a pico-second visible entangled photon pair at 780 nm is converted to a 1522-nm photon, resulting in the entangled photon pair between 780 nm and 1522 nm. Using superconducting single-photon detectors with low dark count rates and small timing jitters, we selectively observed well-defined temporal modes containing the two photons. We achieved a fidelity of $0.93 \pm 0.04$ after the wavelength co…
Here we develop two quantum-computational models for supervised and unsupervised classification tasks in quantum world. Presuming that the states of a set of given quantum systems (or objects) belong to one of two known classes, the objective here is to decide to which of these classes each system belongs -- without knowing its state. The supervised binary classification algorithm is based on having a training sample of quantum systems whose class memberships are already known. The unsupervised binary classification algorithm, however, uses a quantum oracle which knows the class memberships of…
High-fidelity three-qubit iToffoli gate for fixed-frequency superconducting qubits
研究固定频率超导量子比特中高保真三比特 iToffoli 门的实现，基于交叉共振效应并同步施加微波脉冲，达到 98.26% 的过程保真度，为多比特门开发开辟了新路径。
Quantum Fidelity-per-Cost: A Metric for Evaluation of Quantum Computing Systems
本文针对云量子计算异构计费下的成本感知比较难题，开展跨提供商量子电路保真度测量，提出整合KL散度、shots数与货币成本的QFC指标，发现成本感知排名与纯保真度排名存在差异，计费模型影响QFC随shots数的变化。
Fidelity-Age-Aware Scheduling in Quantum Repeater Networks
本文提出保真度-年龄指标，通过轻量级调度器优化量子中继网络中的纠缠分发，减少极端年龄事件并提升可靠性。
Measurement for qubits plays a key role in quantum computation. Current methods for classifying states of single qubit in a superconducting multi-qubit system produce fidelities lower than expected due to the existence of crosstalk, especially in case of frequency crowding. Here, We make the digital signal processing (DSP) system used in measurement into a shallow neural network and train it to be an optimal classifier to reduce the impact of crosstalk. The experiment result shows the crosstalk-induced readout error deceased by 100% after a 3-second optimization applied on the 6-qubit supercon…
It is well established that quantum criticality is one of the most intriguing phenomena which signals the presence of new states of matter. Without prior knowledge of the local order parameter, the quantum information metric (or fidelity susceptibility) can indicate the presence of a phase transition as well as it measures distance between quantum states. In this work, we calculate the distance between quantum states which is equal to the fidelity susceptibility in quantum model for a time-dependent system describing a two-level atom coupled to a time-driven external field. As inspired by the …
SPulseGen: Succinct pulse generator architecture maximizing gate fidelity for superconducting quantum computers
本文提出SPulseGen，一种高性价比的射频脉冲发生器架构，通过将电路和波形简化为成本感知的方波脉冲并优化波形，在大幅降低存储和模拟电路要求的同时，实现了与理想射频脉冲相当的门保真度，消除了可扩展量子计算机的AWG障碍。
Quantum computers must be able to function in the presence of decoherence. The simplest strategy for decoherence reduction is dynamical decoupling (DD), which requires no encoding overhead and works by converting quantum gates into decoupling pulses. Here, using the IBM and Rigetti platforms, we demonstrate that the DD method is suitable for implementation in today's relatively noisy and small-scale cloud based quantum computers. Using DD, we achieve substantial fidelity gains relative to unprotected, free evolution of individual superconducting transmon qubits. To a lesser degree, DD is also …
High-Precision Calibration Workflow Achieves Above $99.9\%$ CZ Gate Fidelity on a Scalable Superconducting Processor
提出闭环校准流程，利用回波泄漏误差放大和上下文感知保真度估计电路，在84量子比特处理器上首次实现CZ门保真度超过99.9%，相干误差抑制至0.007%，并实现自动化校准与稳定性提升。
Collateral coupling between superconducting resonators: Fast and high fidelity generation of qudit-qudit entanglement
本文提出利用超导谐振器间经量子比特电容耦合产生的距离无关相互作用，实现快速高保真的量子比特-量子比特纠缠，并以更少操作生成高保真NOON态，方案还可推广至光学腔原子晶格系统。
Enhancing Quantum Dense Coding Robustness Using Information Entropy-Based Metrics
针对量子密集编码受噪声和退相干影响的问题，提出集成五量子比特完美码与全局自适应纯化的协议，通过监测量子失协QD和形成纠缠EoF提升鲁棒性和吞吐量，为高容量量子互联网提供路径。
To certify that an experimentally implemented quantum transformation is a certain unitary operation U on a d-dimensional Hilbert space, it suffices to determine fidelities of output states for d+1 suitably chosen pure input states [Reich et al., Phys. Rev. A 88, 042309 (2013)]. The set of these d+1 probe states can consist of d orthogonal states that form a basis and one additional state which is a balanced superposition of all d basis states. Here we provide an analytical lower bound on quantum process fidelity for two-qubit quantum gates which results from the knowledge of average state fide…
Calibrating quantum gates up to 52 qubits in a superconducting processor
本研究利用浅层电路中的特征平均基准测试协议，在超导处理器上成功对高达52量子比特的量子门进行基准测试，通过全局保真度量化门间相关性与串扰误差，并应用于门优化以显著提升保真度，实验结果与复合噪声模型高度吻合。
Random circuit sampling has become a popular means for demonstrating the superiority of quantum computers over classical supercomputers. While quantum chips are evolving rapidly, classical sampling algorithms are also getting better and better. The major challenge is to generate bitstrings exhibiting an XEB fidelity above that of the quantum chips. Here we present a classical sampling algorithm for producing the probability distribution of any given random quantum circuit, where the fidelity can be rigorously bounded. Specifically, our algorithm performs rejection sampling after the introduced…
Data and code for collision-model Dicke-state preparation: depth-fidelity frontiers, circuit costs, and superconducting-processor measurements
本研究提供基于碰撞的量子协议制备Dicke态的开放数据集与配套代码，涵盖多规模系统及实验测量，为相关权衡研究提供可复用基准。
High-fidelity quantum entanglement is a key resource for quantum communication and distributed quantum computing, enabling quantum state teleportation, dense coding, and quantum encryption. Any sources of decoherence in the communication channel however degrade entanglement fidelity, thereby increasing the error rates of entangled state protocols. Entanglement purification provides a method to alleviate these non-idealities, by distilling impure states into higher-fidelity entangled states. Here we demonstrate the entanglement purification of Bell pairs shared between two remote superconductin…
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 …
A unified fidelity-based framework for quantum speed limits in open quantum systems
本文提出了一种基于保真度的统一框架，用于推导量子开放系统中的量子速度限制，并展示了该框架在不同保真度度量下的不变性。
99.9%-fidelity in measuring a superconducting qubit
针对超导量子比特测量速度与保真度落后于门操作的问题，本文设计纵向耦合架构消除横向耦合并引入非线性，实现202纳秒内99.8%测量保真度，扣除制备误差后达99.9%以上，兼容多路复用读出并可用于量子纠错。
Heisenberg limited quantum algorithm for estimating the fidelity susceptibility
针对经典计算保真率受限于希尔伯特空间指数增长的问题，提出基于预解式重构、结合量子奇异值变换与振幅估计的海森堡极限量子算法，实现最优精度标度，为量子临界性研究提供可扩展工具。
Performance optimization of continuous variable quantum teleportation with generalized photon-varying non-Gaussian operations
本研究构建多模态光子可变非高斯操作框架，提出适用于任意隐形传态输入态的性能指标，经评估发现近似特定形式的操作组合对量子隐形传态性能提升效果最佳，为相关实际应用提供参考。
Robust, fast and high-fidelity composite single-qubit gates for superconducting transmon qubits
针对超导 transmon 量子比特的相干误差问题，提出基于复合脉冲的控制方法，实现鲁棒快速的高保真单量子比特门，并经仿真和四种独立技术验证。
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…
We define and study a fidelity criterion for quantum channels, which we term the minimax fidelity, through a noncommutative generalization of maximal Hellinger distance between two positive kernels in classical probability theory. Like other known fidelities for quantum channels, the minimax fidelity is well-defined for channels between finite-dimensional algebras, but it also applies to a certain class of channels between infinite-dimensional algebras (explicitly, those channels that possess an operator-valued Radon--Nikodym density with respect to the trace in the sense of Belavkin--Staszews…
Transition Probability (fidelity) for pairs of density operators can be defined as "functor" in the hierarchy of "all" quantum systems and also within any quantum system. The introduction of "amplitudes" for density operators allows for a more intuitive treatment of these quantities, also pointing to a natural parallel transport. The latter is governed by a remarkable gauge theory with strong relations to the Riemann-Bures metric.
A fidelity metric for quantum annealing benchmarked by extreme scaling quantum Monte-Carlo simulations
提出量子退火的保真度度量，通过两种量子蒙特卡洛方法在Rydberg原子系统中计算精度，验证了在10^{-2}-10^{-3}精度下量子退火与经典热退火不可区分。
Single-Shot Fidelity Reveals Hard and Soft Limits: A Universal Yardstick for Photon-Number-Resolving Detectors
该研究提出通用单次射击保真度度量，可直接量化单次测量中正确识别光子数的概率，能区分光子损失与误识别，实现不同架构光子数分辨探测器的定量比较，为光子量子计算相关探测器评估提供统一基准。
We consider the problem of teleporting superposed coherent state using non-maximally entangled coherent state as quantum channel and study the effect of entanglement over quality of teleportation. We show that if maximally entangled coherent state (MECS) used by Van Enk and Hirota [Phys. Rev. A 64, 022313 (2001)] and by H Prakash et al. [Phys. Rev. A 75, 044305 (2007)] is replaced by a particular non-maximally entangled coherent state (NMECS), average fidelity of quantum teleportation increases appreciably at small coherent amplitudes. Since it is very challenging to produce superposed coheren…
Q-fid: Quantum Circuit Fidelity Improvement with LSTM Networks
针对量子电路噪声性能难评估的问题，本研究提出基于LSTM的Q-fid系统，将保真度预测转化为时间序列预测问题，配套新型度量指标，可动态适配硬件变化，预测准确率远超现有工具，助力优化量子电路实现。
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