abstractWe show that parametric coupling techniques can be used to generate selective entangling interactions for multi-qubit processors. By inducing coherent population exchange between adjacent qubits under frequency modulation, we implement a universal gate set for a linear array of four superconducting qubits. An average process fidelity of ℱ = 93% is estimated for three two-qubit gates via quantum process tomography. We establish the suitability of these techniques for computation by preparing a four-qubit maximally entangled state and comparing the estimated state fidelity with the expected performance of the individual entangling gates. In addition, we prepare an eight-qubit register in all possible bitstring permutations and monitor the fidelity of a two-qubit gate across one pair of these qubits. Across all these permutations, an average fidelity of ℱ = 91.6 ± 2.6% is observed. These results thus offer a path to a scalable architecture with high selectivity and low cross-talk.
journaltitleScience Advances
titleDemonstration of universal parametric entangling gates on a multi-qubit lattice

IOP Publishing

journaltitleNew Journal of Physics
titleMicrowave-activated conditional-phase gate for superconducting qubits

American Physical Society

journaltitlePhysical Review Applied
titleSuppression of Qubit Crosstalk in a Tunable Coupling Superconducting Circuit

American Physical Society

journaltitlePhysical Review Applied
titleBenchmarking Coherent Errors in Controlled-Phase Gates due to Spectator Qubits

American Physical Society

journaltitlePhysical Review A
titleHigh-fidelity controlled-${\ensuremath{\sigma}}^{Z}$ gate for resonator-based superconducting quantum computers

journaltitlenpj Quantum Information
titleRealisation of high-fidelity nonadiabatic CZ gates with superconducting qubits

American Physical Society

journaltitlePhysical Review X
titleRealization of High-Fidelity CZ and ZZ-Free iSWAP Gates with a Tunable Coupler

journaltitleScience China Information Sciences
titleSuperconducting quantum computing: a review

Nature Publishing Group

journaltitlenpj Quantum Information
titleScalable quantum computer with superconducting circuits in the ultrastrong coupling regime

IOP Publishing

journaltitleQuantum Science and Technology
titleDemonstration of quantum volume 64 on a superconducting quantum computing system

Nature Publishing Group

journaltitleNature Physics
titleOn-chip quantum simulation with superconducting circuits

journaltitleJournal of Low Temperature Physics
titleRecent progress in quantum simulation using superconducting circuits

Nature Publishing Group

journaltitleNature Communications
titleDigital quantum simulation of fermionic models with a superconducting circuit

journaltitlePhysical Review Letters
titleDissipation-based quantum sensing of magnons with a superconducting qubit

American Association for the Advancement of Science

titleEntanglement-based single-shot detection of a single magnon with a superconducting qubit

Nature Publishing Group

journaltitleNature Communications
titleExponentially-enhanced quantum sensing with non-Hermitian lattice dynamics

journaltitlePhysical Review B
titleCircuit-QED-based scalable architectures for quantum information processing with superconducting qubits

IOP Publishing

journaltitleReports on Progress in Physics
titleQuantum information processing with superconducting circuits: a review

Nature Publishing Group

journaltitleNature Physics
titleQuantum information processing and quantum optics with circuit quantum electrodynamics

abstractShort 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 T2 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 of Josephson energy and charging energy EJ∕EC. The transmon benefits from the fact that its charge dispersion decreases exponentially with EJ∕EC, while its loss in anharmonicity is described by a weak power law. As a result, we predict a drastic reduction in sensitivity to charge noise relative to the Cooper pair box and an increase in the qubit-photon coupling, while maintaining sufficient anharmonicity for selective qubit control. Our detailed analysis of the full system shows that this gain is not compromised by increased noise in other known channels.
journaltitlePhysical Review A
titleCharge-insensitive qubit design derived from the Cooper pair box

American Physical Society

journaltitlePhysical Review B
titleLow-decoherence flux qubit

abstractWe report quantum coherence measurements of a superconducting qubit whose design is a hybrid of several existing types. Excellent coherence times are found: T2*∼T1∼1.5 μs. The topology of the qubit is that of a traditional three-junction flux qubit, but it has a large shunting capacitance, and the ratio of the junction critical currents is chosen so that the qubit potential has a single-well form. The qubit has a sizable nonlinearity, but its sign is reversed compared with most other popular qubit designs. The qubit is read out dispersively using a high-Q resonator in a λ/2 configuration.
journaltitlePhysical Review Letters
titleHigh-Coherence Hybrid Superconducting Qubit
PS Snapshot:/home/jaseung/pCloudDrive/Zotero Files/storage/TTN5PUWX/e100502.html:text/html
flux qubit,CSFQ

Nature Publishing Group

titleStrong coupling of a single photon to a superconducting qubit using circuit quantum electrodynamics

journaltitleReviews of Modern Physics
titleHybrid quantum circuits: Superconducting circuits interacting with other quantum systems

Nature Publishing Group

journaltitleNature Physics
titleHybrid quantum systems with circuit quantum electrodynamics

Wiley Online Library

journaltitleInternational Journal of Circuit Theory and Applications
titleIntroduction to quantum electromagnetic circuits

journaltitlePhysical Review A
shorttitleCavity quantum electrodynamics for superconducting electrical circuits
titleCavity quantum electrodynamics for superconducting electrical circuits: An architecture for quantum computation
cQED,charge qubit,qubit,resonator

journaltitlePhysical Review Letters
titleCoherent Josephson qubit suitable for scalable quantum integrated circuits

journaltitlePhysical Review Applied
titleScalable quantum circuit and control for a superconducting surface code

journaltitlePhysical Review A
titleScalable nonadiabatic holonomic quantum computation on a superconducting qubit lattice

American Physical Society

journaltitlePhysical Review Applied
titleTunable Superconducting Qubits with Flux-Independent Coherence

American Physical Society

journaltitlePhysical Review A
titleAnalytical modeling of parametrically modulated transmon qubits

journaltitlePhysical Review B
titleDecoherence rates in complex Josephson qubit circuits

Springer, Berlin, Heidelberg

titleSpin-orbit Coupling Effects in Two-Dimensional Electron and Hole Systems

abstractThe Schrieffer–Wolff (SW) method is a version of degenerate perturbation theory in which the low-energy effective Hamiltonian Heff is obtained from the exact Hamiltonian by a unitary transformation decoupling the low-energy and high-energy subspaces. We give a self-contained summary of the SW method with a focus on rigorous results. We begin with an exact definition of the SW transformation in terms of the so-called direct rotation between linear subspaces. From this we obtain elementary proofs of several important properties of Heff such as the linked cluster theorem. We then study the perturbative version of the SW transformation obtained from a Taylor series representation of the direct rotation. Our perturbative approach provides a systematic diagram technique for computing high-order corrections to Heff. We then specialize the SW method to quantum spin lattices with short-range interactions. We establish unitary equivalence between effective low-energy Hamiltonians obtained using two different versions of the SW method studied in the literature. Finally, we derive an upper bound on the precision up to which the ground state energy of the nth-order effective Hamiltonian approximates the exact ground state energy.
journaltitleAnnals of Physics
titleSchrieffer–Wolff transformation for quantum many-body systems
Quantum many-body system,Perturbative expansion,Canonical transformation

American Physical Society

journaltitlePhysical Review A
titleEffective Hamiltonian models of the cross-resonance gate

journaltitlePhysical Review B
titleSuperconducting qubits beyond the dispersive regime
circuit Hamiltonian

abstractWhen a two-level system (TLS) is coupled to an electromagnetic resonator, its transition frequency changes in response to the quantum vacuum fluctuations of the electromagnetic field, a phenomenon known as the Lamb shift. Remarkably, by replacing the TLS by a harmonic oscillator, normal-mode splitting leads to a quantitatively similar shift, without taking quantum fluctuations into account. In a weakly anharmonic system, lying in between the harmonic oscillator and a TLS, the origins of such shifts can be unclear. An example of this is the dispersive shift of a transmon qubit in circuit quantum electrodynamics (QED). Although often referred to as a Lamb shift, the dispersive shift observed in spectroscopy in circuit QED could contain a significant contribution from normal-mode splitting that is not driven by quantum fluctuations, raising the question: how much of this shift is quantum in origin? Here we treat normal-mode splitting separately from shifts induced by quantum vacuum fluctuations in the Hamiltonian of a weakly anharmonic system, providing a framework for understanding the extent to which observed frequency shifts can be attributed to quantum fluctuations.
journaltitlePhysical Review A
shorttitleNature of the Lamb shift in weakly anharmonic atoms
titleNature of the Lamb shift in weakly anharmonic atoms: From normal-mode splitting to quantum fluctuations

journaltitleIEEE Transactions on Microwave Theory and Techniques
titleSimple Impedance Response Formulas for the Dispersive Interaction Rates in the Effective Hamiltonians of Low Anharmonicity Superconducting Qubits

American Physical Society

journaltitlePhysical Review A
titleEfficient $Z$ gates for quantum computing

American Physical Society

journaltitlePhysical Review Applied
titleUniversal Gate for Fixed-Frequency Qubits via a Tunable Bus

American Physical Society

journaltitlePhysical Review Applied
titleRapid High-Fidelity Single-Shot Dispersive Readout of Superconducting Qubits

American Physical Society

journaltitlePhysical Review Letters
titleHigh-Fidelity Controlled-Z Gate with Maximal Intermediate Leakage Operating at the Speed Limit in a Superconducting Quantum Processor

American Physical Society

journaltitlePhysical Review Letters
titleDemonstrating a Continuous Set of Two-Qubit Gates for Near-Term Quantum Algorithms

American Physical Society

journaltitlePhysical Review Research
titleBenchmarking the noise sensitivity of different parametric two-qubit gates in a single superconducting quantum computing platform

journaltitlePhysical Review Letters
titleThree-Qubit Randomized Benchmarking
randomized benchmarking,RB

American Physical Society

journaltitlePhysical Review Letters
titleSuppression of Unwanted $ZZ$ Interactions in a Hybrid Two-Qubit System

American Physical Society

journaltitlePhysical Review Applied
titleZZ Freedom in Two-Qubit Gates

titleRealization of three-qubit quantum error correction with superconducting circuits

American Physical Society

journaltitlePhysical Review Letters
titleHigh-Contrast $ZZ$ Interaction Using Superconducting Qubits with Opposite-Sign Anharmonicity

American Physical Society

journaltitlePhysical Review Applied
titleTunable Coupler for Realizing a Controlled-Phase Gate with Dynamically Decoupled Regime in a Superconducting Circuit

American Physical Society

journaltitlePhysical Review A
titleFirst-principles analysis of cross-resonance gate operation

American Physical Society

journaltitlePhysical Review Letters
titleDemonstration of a High-Fidelity cnot Gate for Fixed-Frequency Transmons with Engineered $ZZ$ Suppression

American Physical Society

journaltitlePhysical Review Applied
titleDemonstration of an All-Microwave Controlled-Phase Gate between Far-Detuned Qubits

American Physical Society

journaltitlePhysical Review Letters
titleImplementation of Conditional Phase Gates Based on Tunable $ZZ$ Interactions

American Physical Society

journaltitlePhysical Review Letters
titleHigh-Fidelity, High-Scalability Two-Qubit Gate Scheme for Superconducting Qubits

American Physical Society

journaltitlePhysical Review A
titleWhat is measured when a qubit measurement is performed on a multiqubit chip

Forschungszentrum Jülich

booktitleLecture Notes of the 44th IFF Spring School
titleControl of Superconducting Qubits

abstractWe present a method for detecting electromagnetic (EM) modes that couple to a superconducting qubit in a circuit quantum electrodynamics architecture. Based on measurement-induced dephasing, this technique allows the measurement of modes that have a high quality factor (Q) and may be difficult to detect through standard transmission and reflection measurements at the device ports. In this scheme, the qubit itself acts as a sensitive phase meter, revealing modes that couple to it through measurements of its coherence time. Such modes are indistinguishable from EM modes that do not couple to the qubit using a vector network analyzer. Moreover, this technique provides useful characterization parameters including the quality factor and the coupling strength of the unwanted resonances. We demonstrate the method for detecting both high-Q coupling resonators in planar devices as well as spurious modes produced by a 3D cavity.
journaltitleApplied Physics Letters
titleCharacterization of hidden modes in networks of superconducting qubits
bus cavity spectroscopy

abstractA method for obtaining NMR spectra of organic liquids free of J-couplings is described. The coupling between a group of equivalent protons and an adjacent 13C nets as a local decoupling field for the protons. A Pure chemical shift spectrum of ethanol is presented and possible applications to strongly coupled systems are discussed.
journaltitleChemical Physics Letters
titleBilinear rotation decoupling of homonuclear scalar interactions

abstractRobust quantum computation requires encoding delicate quantum information into degrees of freedom that are hard for the environment to change. Quantum encodings have been demonstrated in many physical systems by observing and correcting storage errors, but applications require not just storing information; we must accurately compute even with faulty operations. The theory of fault-tolerant quantum computing illuminates a way forward by providing a foundation and collection of techniques for limiting the spread of errors. Here we implement one of the smallest quantum codes in a five-qubit superconducting transmon device and demonstrate fault-tolerant state preparation. We characterize the resulting code words through quantum process tomography and study the free evolution of the logical observables. Our results are consistent with fault-tolerant state preparation in a protected qubit subspace.
journaltitlePhysical Review Letters
titleExperimental Demonstration of Fault-Tolerant State Preparation with Superconducting Qubits

journaltitlePhysical Review A
titleProcedure for systematically tuning up cross-talk in the cross-resonance gate

IOP Publishing

journaltitleJournal of Physics A: Mathematical and General
titleBlock diagonalisation of Hermitian matrices

American Physical Society

journaltitlePhysical Review A
titleProcess verification of two-qubit quantum gates by randomized benchmarking

American Physical Society

journaltitlePRX Quantum
titleReducing Unitary and Spectator Errors in Cross Resonance with Optimized Rotary Echoes

American Physical Society

journaltitlePhysical Review Letters
titleUniversal Quantum Gate Set Approaching Fault-Tolerant Thresholds with Superconducting Qubits

American Physical Society

journaltitlePhysical Review A
titleQuantum-process tomography: Resource analysis of different strategies

abstractA linear map Φ from Mn to Mm is completely positive iff it admits an expression Φ(A)=ΣiV∗iAVi where Vi are n×m matrices.
journaltitleLinear Algebra and its Applications
titleCompletely positive linear maps on complex matrices

abstractWe present a derivation and numerous applications of a compact explicit formula for the average fidelity of a quantum operation on a finite-dimensional quantum system. The formula can be applied to averages over particularly relevant subspaces; it is easily generalized to multi-component systems, and as a special result, we show that when the same completely positive trace-preserving map is applied to a large number of qubits with one-bit fidelity F close to unity, the average fidelity of the operation on the full K-bit register scales as F3K/2.
journaltitlePhysics Letters A
titleFidelity of quantum operations

American Physical Society

journaltitlePhysical Review Letters
titleScalable and Robust Randomized Benchmarking of Quantum Processes

journaltitlePhysical Review Applied
titleDirect Dispersive Monitoring of Charge Parity in Offset-Charge-Sensitive Transmons
parity measurement

American Physical Society

journaltitlePhysical Review Letters
titleControllable Coupling between Flux Qubits

American Physical Society

journaltitlePhysical Review Letters
titleQubit Architecture with High Coherence and Fast Tunable Coupling

American Physical Society

journaltitlePhysical Review Letters
titleUniversal Stabilization of a Parametrically Coupled Qubit

journaltitlePhysical Review Letters
titleHigh-fidelity, high-scalability two-qubit gate scheme for superconducting qubits

American Physical Society

journaltitlePhysical Review Letters
titleImpact of Spectators on a Two-Qubit Gate in a Tunable Coupling Superconducting Circuit

American Physical Society

journaltitlePhysical Review Letters
titleTunable Coupling Architecture for Fixed-Frequency Transmon Superconducting Qubits

American Physical Society (APS)

journaltitlePhysical Review Applied
titleParasitic-Free Gate: An Error-Protected Cross-Resonance Switch in Weakly Tunable Architectures

abstractBy exploiting two key aspects of quantum mechanics —the superposition and entanglement of physical states —quantum computers may eventually outperform their classical equivalents. A team based at Yale has achieved an important step towards that goal —the demonstration of the first solid-state quantum processor, which was used to execute two quantum algorithms. Quantum processors based on a few quantum bits have been demonstrated before using nuclear magnetic resonance, cold ion traps and optical systems, all of which bear little resemblance to conventional computers. This new processor is based on superconducting quantum circuits fabricated using conventional nanofabrication technology. There is still a long way to go before quantum computers can challenge the classical type. The processor is very basic, containing just two quantum bits, and operates at a fraction of a degree above absolute zero. But the chip contains all the essential features of a miniature working quantum computer and may prove scalable to more quantum bits and more complex algorithms.
titleDemonstration of two-qubit algorithms with a superconducting quantum processor

Reading, MA

titleModern quantum mechanics; rev. ed.

IOP Publishing

journaltitleNew Journal of Physics
titleToward tripartite hybrid entanglement in quantum dot molecules

titleLattice Hamiltonians and Stray Interactions Within Quantum Processors

Baltimore, The Johns Hopkins University Press

titleMatrix Computations

journaltitlePhysical Review Letters
titleDecoherence of Flux Qubits due to $1/f$ Flux Noise

abstractScalable quantum information processing requires controllable high-coherence qubits. Here, the authors present superconducting flux qubits with broad frequency tunability, strong anharmonicity and high reproducibility, identifying photon shot noise as the main source of dephasing for further improvements.
journaltitleNature Communications
titleThe flux qubit revisited to enhance coherence and reproducibility

American Physical Society

journaltitlePhysical Review B
titleDecoherence in a superconducting quantum bit circuit