J. Q. You, Xuedong Hu, S. Ashhab, Franco Nori
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Novel qubits with intrinsic noise protection constitute a promising route for improving the coherence of quantum information in superconducting circuits. However, many protected superconducting qubits exhibit relatively low transition frequencies, which could make their integration with conventional transmon circuits challenging. In this work, we propose and study a scheme for entangling a tunable transmon with a Cooper-pair parity-protected qubit, a paradigmatic example of a low-frequency protected qubit that stores quantum information in opposite Cooper-pair parity states on a superconductin…
We study a readout scheme of superconducting flux qubit state with a Cooper pair box as a transmon. The qubit states consist of the superpositions of two degenerate states where the charge and phase degrees of freedom are entangled. Owing to the robustness of transmon against external fluctuations, our readout scheme enables the quantum non-demolition and single-shot measurement of flux qubit states. The qubit state readout can be performed by using the non-linear Josephson amplifiers after a $π/2$-rotation driven by an ac-electric field.
Coupling Majorana fermion excitations to coherent external fields is an important stage towards their manipulation and detection. We analyse the charge and transmon regimes of a topological nano-wire embedded within a Cooper-Pair-Box, where the superconducting phase difference is coupled to the zero energy parity states that arise from Majorana quasi-particles. We show that at special gate bias points, the photon-qubit coupling can be switched off via quantum interference, and in other points it is exponentially dependent on the control parameter $E_J/E_C$. As well as a probe for topological-s…
Tomographic Signatures of Interacting Majorana and Andreev States in Superconductor-Semiconductor Transmon Qubits
本文研究超导体-半导体 transmons 量子比特中基态结库珀对电荷分布的层析重建，提出通过电荷态概率分布特征识别相互作用态与拓扑态，并利用信息论指标分类结中激发类型差异。
Anomalous magnetic flux via junction twist-angle in a triplet-superconducting transmon qubit
本研究提出由三重态超导体构成的带扭转角c轴结，可调控transmon量子比特的库珀对隧穿强度并产生零场反常磁通量，为量子比特优化设计提供了新的零场调控途径。
Short dephasing times pose one of the main challenges in realizing a quantum computer. Different approaches have been devised to cure this problem for superconducting qubits, a prime example being the operation of such devices at optimal working points, so-called "sweet spots." This latter approach led to significant improvement of $T_2$ times in Cooper pair box qubits [D. Vion et al., Science 296, 886 (2002)]. Here, we introduce a new type of superconducting qubit called the "transmon." Unlike the charge qubit, the transmon is designed to operate in a regime of significantly increased ratio o…
Experimental realization of a $\cos(2φ)$ transmon qubit
该研究通过实验实现了一个利用电荷-奇偶性对称性的$\cos(2φ)$量子比特，展示了低频量子比特的相干控制和单次读取能力，并发现电荷保护由100倍的岛电荷矩阵元抑制，但1/f磁通噪声限制了相干时间。
Generalized transmon Hamiltonian for Andreev spin qubits
研究基于转子哈密顿量的量子点嵌入超导体结中的相互作用问题，通过富尔曼带近似方法精确对角化并描述低能现象。
New Design of three-qubit system with three transmons and a single fixed-frequency resonator coupler
针对传统 transmon-谐振腔-transmon 结构的脆弱性，本文提出一种由单个固定频率谐振腔耦合器介导三个 transmon 量子比特的新型结构，其 CNOT 门平均保真度超过 0.98，为提升量子比特连接数并保持门性能提供了关键方法。
Experimental observation of dynamical blockade between transmon qubits via ZZ interaction engineering
通过电容工程实现强ZZ耦合，观察到transmon量子比特间的动态阻塞现象，为超导电路中相互作用主导物理的研究提供了新方法。
A First Demonstration of the SQUAT Detector Architecture: Direct Measurement of Resonator-Free Charge-Sensitive Transmons
本文首次演示了SQUAT探测器架构，通过直接测量无谐振器的电荷敏感超导谐振子，实现了同时检测电荷和准粒子信号的能力。
We present an experimental realization of the transmon qubit, an improved superconducting charge qubit derived from the Cooper pair box. We experimentally verify the predicted exponential suppression of sensitivity to 1/f charge noise [J. Koch et al., Phys. Rev. A 76, 042319 (2007)]. This removes the leading source of dephasing in charge qubits, resulting in homogenously broadened transitions with relaxation and dephasing times in the microsecond range. Our systematic characterization of the qubit spectrum, anharmonicity, and charge dispersion shows excellent agreement with theory, rendering t…
Probing Sub-eV Dark Photon, Scalar and Axion-like Particle Dark Matters with Transmon Qubits
利用超导量子比特探测子电子伏特暗光子、标量和轴子类暗物质的耦合限制
Coherence limitations of a Fourier-engineered $\cos(2φ)$ transmon qubit
本文研究了傅里叶工程化cos(2φ) transmon量子比特的相干限制，通过抑制有效量子比特势的奇次谐波，发现量子比特寿命受1/f磁通噪声限制，揭示了干涉保护方案的关键挑战。
Construction of new type of CNOT gate using cross-resonance pulse in the transmon-PPQ system
提出在可调transmon与奇偶保护量子比特组成的超导混合系统中，通过跨共振脉冲实现保真度超过0.998的新型CNOT门，为量子计算提供新平台。
A chiral photonic interface is a quantum system that has different probabilities for emitting photons to the left and right. An on-chip compatible chiral interface is attractive for both fundamental studies of light-matter interactions and applications to quantum information processing. We propose such a chiral interface based on superconducting circuits, which has wide bandwidth, rich tunability, and high tolerance to fabrication variations. The proposed interface consists of a core that uses Cooper-pair boxes (CPBs) to break time-reversal symmetry, and two superconducting transmons that conn…
We construct a minimal circuit, based on the top-transmon design, to rotate a qubit formed out of four Majorana zero-modes at the edge of a two-dimensional topological insulator. Unlike braiding operations, generic rotations have no topological protection, but they do allow for a full characterization of the coherence times of the Majorana qubit. The rotation is controlled by variation of the flux through a pair of split Josephson junctions in a Cooper pair box, without any need to adjust gate voltages. The Rabi oscillations of the Majorana qubit can be monitored via oscillations in the resona…
Supersymmetries in quantum mechanics offer a way to obtain degeneracies in the excitation spectrum which do not originate from selection rules. The mechanism behind the degeneracies is the same as the one that leads to the miraculous cancellations of divergences in supersymmetric field theories found in the high energy physics context. Even though of importance, there is up to now no realistic proposal of non-integrable systems that show level degeneracies due to a supersymmetric structure. Here, we propose an implementation of a quantum-mechanical supersymmetry in a Cooper-pair box shunted by…
Quasiparticle dynamics in a superconducting qubit irradiated by a localized infrared source
针对超导量子比特因准粒子导致的退相干问题，研究了transmon量子比特在不同参数聚焦红外辐射下的特性，验证了低能准粒子俘获动力学模型，为缓解高能辐射对超导电路的影响提供了技术途径。
The coherent tunnelling of Cooper pairs across Josephson junctions (JJs) generates a nonlinear inductance that is used extensively in quantum information processors based on superconducting circuits, from setting qubit transition frequencies and interqubit coupling strengths, to the gain of parametric amplifiers for quantum-limited readout. The inductance is either set by tailoring the metal-oxide dimensions of single JJs, or magnetically tuned by parallelizing multiple JJs in superconducting quantum interference devices (SQUIDs) with local current-biased flux lines. JJs based on superconducto…
The non-dissipative non-linearity of a Josephson junction converts macroscopic superconducting circuits into artificial atoms, enabling some of the best controlled quantum bits (qubits) today. Three fundamental types of superconducting qubits are known, each reflecting a distinct behavior of quantum fluctuations in a Cooper pair condensate: single charge tunneling (charge qubit), single flux tunneling (flux qubit), and phase oscillations (phase qubit). Yet, the dual nature of charge and flux suggests that circuit atoms must come in pairs. Here we introduce the missing one, named "blochnium". I…
Noise-induced quantum synchronization and maximally entangled mixed states in superconducting circuits
该研究在超导 transmon 量子比特链中实验观测到高斯白噪声诱导的全链量子同步，并证明两端形成最大纠缠混合态且对频率失谐稳定。
Error-corrected quantum computers can only work if errors are small and uncorrelated. Here I show how cosmic rays or stray background radiation affects superconducting qubits by modeling the phonon to electron/quasiparticle down-conversion physics. For present designs, the model predicts about 57\% of the radiation energy breaks Cooper pairs into quasiparticles, which then vigorously suppress the qubit energy relaxation time ($T_1 \sim$ 160 ns) over a large area (cm) and for a long time (ms). Such large and correlated decay kills error correction. Using this quantitative model, I show how this…
Quantum error correction will be an essential ingredient in realizing fault-tolerant quantum computing. However, most correction schemes rely on the assumption that errors are sufficiently uncorrelated in space and time. In superconducting qubits this assumption is drastically violated in the presence of ionizing radiation, which creates bursts of high energy phonons in the substrate. These phonons can break Cooper-pairs in the superconductor and, thus, create quasiparticles over large areas, consequently reducing qubit coherence across the quantum device in a correlated fashion. A potential m…
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