We propose a family of microwave-activated entangling gates on two capacitively coupled fluxonium qubits. A microwave pulse applied to either qubit at a frequency near the half-frequency of the $|00\rangle - |11\rangle$ transition induces two-photon Rabi oscillations with a negligible leakage outside the computational subspace, owing to the strong anharmonicity of fluxoniums. By adjusting the drive frequency, amplitude, and duration, we obtain the gate family that is locally equivalent to the fermionic-simulation gates such as $\sqrt{\rm SWAP}$-like and controlled-phase gates. The gate error can be tuned below $10^{-4}$ for a pulse duration under 100 ns without excessive circuit parameter matching. Given that the fluxonium coherence time can exceed 1 ms, our gate scheme is promising for large-scale quantum processors.
We analyze a high-fidelity two-qubit gate using fast flux pulses on superconducting fluxonium qubits. The gate is realized by temporarily detuning magnetic flux through fluxonium loop away from the half flux quantum sweet spot. We simulate dynamics of two capacitively coupled fluxoniums during the flux pulses and optimize the pulse parameters to obtain a highly accurate $\sqrt{i\mathrm{swap}}$-like entangling gate. We also evaluate the effect of the flux noise and qubit relaxation on the gate fidelity. Our results demonstrate that the gate error remains below $10^{-4}$ for currently achievable…
Fast entangling gates on fluxoniums via parametric modulation of plasmon interaction
Quantum Science Center of Guangdong-Hong Kong-Macao Greater Bay Area（粤港澳大湾区量子科学中心）；Nanjing University of Posts and Telecommunications（南京邮电大学）；Shanxi University（山西大学）；South China Normal University（华南师范大学）
Long coherence times, large anharmonicity and robust charge-noise insensitivity render fluxonium qubits an interesting alternative to transmons. Recent experiments have demonstrated record coherence times for low-frequency fluxonia. Here, we propose a galvanic-coupling scheme with flux-tunable $\textit{XX}$ coupling. To implement a high-fidelity entangling $\sqrt{i\mathrm{SWAP}}$ gate, we modulate the strength of this coupling and devise variable-time identity gates to synchronize required single-qubit operations. Both types of gates are implemented using strong ac flux drives, lasting for onl…
Fast CZ gate in hybrid fluxonium-transmon systems with tunable couplers
Nanjing University of Posts and Telecommunications（南京邮电大学）；Nanjing University（南京大学）；Shanxi University（山西大学）；Quantum Science Center of Guangdong-Hong Kong-Macao Greater Bay Area（粤港澳大湾区量子科学中心）
The technological development of hardware heading toward universal fault-tolerant quantum computation requires a large-scale processing unit with high performance. While fluxonium qubits are promising with high coherence and large anharmonicity, their scalability has not been systematically explored. In this work, we propose a superconducting quantum information processor based on compact high-coherence fluxoniums with suppressed crosstalk, reduced design complexity, improved operational efficiency, high-fidelity gates, and resistance to parameter fluctuations. In this architecture, the qubits…
Fast microwave-driven two-qubit gates between fluxonium qubits with a transmon coupler
QuTech and Kavli Institute of Nanoscience, Delft University of Technology（代尔夫特理工大学量子技术与卡弗里纳米科学研究所）；Netherlands Organization for Applied Scientific Research (TNO)（荷兰应用科学研究组织（TNO））；Quantum Engineering Centre for Doctoral Training, H. H. Wills Physics Laboratory and Department of Electrical and Electronic Engineering, University of Bristol（布里斯托大学量子工程博士培训中心、H.H.威尔斯物理实验室及电气与电子工程系）
Superconducting fluxonium qubits provide a promising alternative to transmons on the path toward large-scale superconductor-based quantum computing due to their better coherence and larger anharmonicity. A major challenge for multi-qubit fluxonium devices is the experimental demonstration of a scalable crosstalk-free multi-qubit architecture with high fidelity single-qubit and two-qubit gates, single-shot readout and state initialization. Here, we present a two-qubit fluxonium-based quantum processor with a tunable coupler element following our theoretical proposal [DOI: 10.1063/5.0064800]. We…
Quantum dot hybrid qubits exploit an extended charge-noise sweet spot that suppresses dephasing and has enabled the experimental achievement of high-fidelity single-qubit gates. However, current proposals for two-qubit gates require tuning the qubits away from their sweet spots. Here, we propose a two-hybrid-qubit coupling scheme, based on exchange interactions, that allows the qubits to remain at their sweet spots at all times. The interaction is controlled via the inter-qubit tunnel coupling. By simulating such gates in the presence of realistic quasistatic and $1\!/\!f$ charge noise, we sho…
The Mølmer-Sørensen gate is a state-of-the-art entangling gate in the ion trap quantum computing where the gate fidelity can exceed 99\%. Here we propose an analogous implementation in the setting of cavity QED. The cavity photon mode acts as the bosonic degree of freedom in the gate in contrast of that played by a phonon mode in ion traps. This is made possible by utilising cavity assisted Raman transitions interconnecting the logical qubit states embedded in a four-level energy structure, making the "anti-Jaynes-Cummings" (AJC) term available under the rotating-wave approximation. We identif…
We investigate an approach to universal quantum computation based on the modulation of longitudinal qubit-oscillator coupling. We show how to realize a controlled-phase gate by simultaneously modulating the longitudinal coupling of two qubits to a common oscillator mode. In contrast to the more familiar transversal qubit-oscillator coupling, the magnitude of the effective qubit-qubit interaction does not rely on a small perturbative parameter. As a result, this effective interaction strength can be made large, leading to short gate times and high gate fidelities. We moreover show how the gate …
Important tasks in cavity quantum electrodynamics include the generation and control of quantum states of spatially-separated particles distributed in different cavities. An interesting question in this context is how to prepare entanglement among particles located in different cavities, which are important for large-scale quantum information processing. We here consider a multi-cavity system where cavities are coupled to a superconducting (SC) qubit and each cavity hosts many SC qubits. We show that all intra-cavity SC qubits plus the coupler SC qubit can be prepared in an entangled Greenberg…
Current quantum technology is approaching the system sizes and fidelities required for quantum error correction. It is therefore important to determine exactly what is needed for proof-of-principle experiments, which will be the first major step towards fault-tolerant quantum computation. Here we propose a surface code based experiment that is the smallest, both in terms of code size and circuit depth, that would allow errors to be detected and corrected for both the $X$ and $Z$ basis of a qubit. This requires $17$ physical qubits initially prepared in a product state, on which $16$ two-qubit …
Polaritons are quasi-particles originating from the coupling of light with matter that demonstrated quantum phenomena at the many-particle mesoscopic level, such as BEC and superfluidity. A highly sought and long-time missing feature of polaritons is a genuine quantum manifestation of their dynamics at the single-particle level. Although they are conceptually perceived as entangled states and theoretical proposals abound for an explicit manifestation of their single-particle properties, so far their behaviour has remained fully accountable for by classical and mean-field theories. In this Arti…
We theoretically investigate the generation of heralded entanglement between two identical atoms via cavity-assisted photon scattering in two different configurations, namely either both atoms confined in the same cavity or trapped into locally separated ones. Our protocols are given by a very simple and elegant single-step process, whose key mechanism is a controlled-phase-flip gate implemented by impinging a single photon on single-sided cavities. In particular, when the atoms are localized in remote cavities, we introduce a single-step parallel quantum circuit instead of the serial process …
A key challenge for semiconductor quantum-dot charge qubits is the realization of long-range qubit coupling and performing high-fidelity gates based on it. Here, we describe a new type of charge qubit formed by an electron confined in a triple-quantum-dot system, enabling single and two-qubit gates working in the dipolar and quadrupolar detuning sweet spots. We further present the form for the long-range dipolar coupling between the charge qubit and the superconducting resonator. Based on the hybrid system composed of the qubits and the resonator, we present two types of entangling gates: the …
Arrays of qubits encoded in the ground-state manifold of neutral atoms trapped in optical (or magnetic) lattices appear to be a promising platform for the realization of a scalable quantum computer. Two-qubit conditional gates between nearest-neighbor qubits in the array can be implemented by exploiting the Rydberg blockade mechanism, as was shown by D. Jaksch et al. [Phys. Rev. Lett. 85, 2208 (2000)]. However, the energy shift due to dipole-dipole interactions causing the blockade falls off rapidly with the interatomic distance and protocols based on direct Rydberg blockade typically fail to …
Proposals for experimentally realizing quantum-autonomous gates
Joint Center for Quantum Information and Computer Science, NIST and University of Maryland（量子信息与计算机科学联合中心，美国国家标准与技术研究院和马里兰大学）；Department of Physics, University of Maryland（马里兰大学物理系）；Department of Physics, Harvard University（哈佛大学物理系）；Atominstitut, Technische Universität Wien（维也纳工业大学原子研究所）；Institute for Quantum Optics and Quantum Information, IQOQI Vienna, Austrian Academy of Sciences（奥地利科学院维也纳量子光学与量子信息研究所）；Joint Quantum Institute, University of Maryland（马里兰大学联合量子研究所）；Duke Quantum Center and Department of Physics, Duke University（杜克大学杜克量子中心及物理系）；National Quantum Laboratory (QLab), University of Maryland（马里兰大学国家量子实验室（QLab））；Department of Microtechnology and Nanoscience, Chalmers University of Technology（查尔姆斯理工大学微技术与纳米科学系）；Institute for Physical Science and Technology, University of Maryland（马里兰大学物理科学与技术研究所）
The ubiquitous presence of $1/f$ flux noise was a significant barrier to long-coherence in superconducting qubits until the development of qubits that could operate in static, flux noise insensitive configurations commonly referred to as `sweet-spots'. Several proposals for entangling gates in superconducting qubits tune the flux bias away from these spots, thus reintroducing the dephasing problem to varying degrees. Here we revisit one such proposal, where interactions are parametrically activated by rapidly modulating the flux bias of the qubits around these sweet-spots, and study the effect…
We propose an experiment to test the Weak Equivalence Principle (WEP) with a test mass consisting of two entangled atoms of different species. In the proposed experiment, a coherent measurement of the differential gravity acceleration between the two atomic species is considered, by entangling two atom interferometers operating on the two species. The entanglement between the two atoms is heralded at the initial beam splitter of the interferometers through the detection of a single photon emitted by either of the atoms, together with the impossibility of distinguishing which atom emitted the p…
在量子水平上对混合物理系统的精确控制对于许多基于量子的技术的实现是重要的。在量子信息处理（QIP）和量子网络领域，各种提议讨论了混合架构的可能性，其中将特定任务委托给最合适的子系统。例如，在量子网络中，将信息从具有良好存储器属性的子系统传送到在网络中的节点之间传输信息时更有效的另一个子系统可能是有利的。对于被捕获的离子，由不同种类形成的混合系统引入了额外的自由度，可以利用这些自由度来扩展和细化系统的控制。不同元素的离子以前曾在QIP实验中用于交感冷却，通过耗散产生纠缠，以及一个物种与另一个物种的量子非破坏（QND）测量。在这里，我们展示了不同元素的离子之间的纠缠量子门，它可以作为QIP，量子网络，精密光谱学，计量学和量子模拟的重要组成部分。通过激光束诱导的状态相关力产生的有效自旋-自旋相互作用，实现了$^9$ Be $^+$离子和$^{25}$ Mg $^+$离子之间的几何相位门。结合单量子比特门和同种纠缠门，这种混合元素纠缠门在这样的混合系统上为通用QIP提供了一套完整的门。使用一系列这样的门，我们演示了一个受控非（CNOT）门和交换门。我们进一步证明了这些门对热激发的鲁棒性，并在量子逻辑光谱（QLS）中显示出改进的检测。我们还观察到一个强烈的违反CHSH型贝尔不等式纠缠态组成的不同的离子物种。
The $i$swap gate is an entangling swapping gate where the qubits obtain a phase of $i$ if the state of the qubits is swapped. Here we present a simple implementation of the controlled-$i$swap gate. The gate can be implemented with several controls and works by applying a single flux pulse. The gate time is independent of the number of controls, and we find high fidelities for any number of controls. We discuss an implementation of the gates using superconducting circuits and present a realistic implementation proposal, where we have taken decoherence noise and fabrication errors on the superco…
We theoretically investigate the use of fast pulsed two-qubit gates for trapped ion quantum computing in a two-dimensional microtrap architecture. In one dimension, such fast gates are optimal when employed between nearest neighbours, and we examine the generalisation to a two-dimensional geometry. We demonstrate that fast pulsed gates are capable of implementing high-fidelity entangling operations between ions in neighbouring traps faster than the trapping period, with experimentally demonstrated laser repetition rates. Notably, we find that without increasing the gate duration, high-fidelity…
The utility of classical neural networks as universal approximators suggests that their quantum analogues could play an important role in quantum generalizations of machine-learning methods. Inspired by the proposal in [Torrontegui and García-Ripoll 2019 EPL 125 30004], we demonstrate a superconducting qubit implementation of an adiabatic controlled gate, which generalizes the action of a classical perceptron as the basic building block of a quantum neural network. We show full control over the steepness of the perceptron activation function, the input weight and the bias by tuning the adiabat…
We present a one-step scheme to construct the controlled-phase gate deterministically on remote transmon qutrits coupled to different resonators connected by a superconducting transmission line for an universal distributed quantum computing. Different from previous works on remote superconducting qubits, the present gate is implemented with coherent evolutions of the entire system in the all-resonance regime assisted by the dark photons to robust against the transmission line loss, which allows the possibility of the complex designation of a long-length transmission line to link lots of circui…
Generation of field mediated three qubit entangled state shared by Alice and Bob
提出了一种利用Alice控制的腔体中的两个原子（量子比特A1和A2）与Bob（B）控制的远程实验室中的一个原子纠缠产生共享三重纠缠态的方案.这种纠缠是通过囚禁原子与两个腔共享的双模压缩光场相互作用产生的。所提出的方案是文献[W.儿子，M。S. Kim，J. Lee，and D. Ahn，J.Mod.Opt.49，1739（2002）]，其中检查了使用二分连续变量状态纠缠两个远程量子比特的可能性。当整体负性检测三原子混合态的自由纠缠时，束缚纠缠则由态算符的纯态分解计算的负性检测。通过三原子混合态的选择性部分换位计算的部分负性检测了量子比特对A1B、A2B和A1A2的两两纠缠。发现三原子的纠缠是类W纠缠，没有产生类GHZ量子关联。
We present an efficient approach to optimising pulse sequences for implementing fast entangling two-qubit gates on trapped ion quantum information processors. We employ a two-phase procedure for optimising gate fidelity, which we demonstrate for multi-ion systems in linear Paul trap and microtrap architectures. The first phase involves a global optimisation over a computationally inexpensive cost function constructed under strong approximations of the gate dynamics. The second phase involves local optimisations that utilise a more precise ODE description of the gate dynamics, which captures th…
Time-Frequency Continuous-Variable Quantum Teleportation with Single Photons
Telecom Paris, Institut Polytechnique de Paris（巴黎电信学院，巴黎理工学院）；Centre for Nanosciences and Nanotechnology, CNRS, Université Paris-Saclay（纳米科学与工程中心，法国国家科学研究中心，巴黎萨克雷大学）
