We report a superconducting artificial atom with an observed quantum coherence time of T * 2 =95µs and energy relaxation time T1=70µs. The system consists of a single Josephson junction transmon qubit embedded in an otherwise empty copper waveguide cavity whose lowest eigenmode is dispersively coupled to the qubit transition. We attribute the factor of four increase in the coherence quality factor relative to previous reports to device modifications aimed at reducing qubit dephasing from residual cavity photons. This simple device holds great promise as a robust and easily produced artificial quantum system whose intrinsic coherence properties are sufficient to allow tests of quantum error correction. PACS numbers: 03.67.Ac, 42.50.Pq, 85.25.-j Superconducting quantum circuits are a leading candidate technology for large scale quantum computing. They have been used to show a violation of a Bell-type inequality [1]; implement a simple two-qubit gate favorable for scaling [2]; generate three-qubit entanglement [3]; perform a routine relevant to error correction [4];and very recently to demonstrate a universal set of quantum gates with fidelities greater than 95% [5]. Most of these devices employ small angle-evaporated Josephson junctions as their critical non-linear circuit components. Devices designs appear to be consistent with the basic requirements for quantum error correction (QEC) and fault tolerance [6]. However, the construction and operation of much larger systems capable of meaningful tests of such procedures will require individual qubits and junctions with a very high degree of coherence. Current estimates for threshold error rates -and the cumulative nature of errors originating from control, measurement, and decoherence -make likely the need for quantum lifetimes at least 10 3 times longer than gate and measurement times [7], corresponding to 20 to 200µs for typical systems.To this end, improvements in qubit lifetimes have continued for the past decade, spurred largely by clever methods of decoupling noise and loss mechanisms from the qubit transition and thus realizing Hamiltonians more closely resembling their idealized versions. Recently, Paik, et al. made a breakthrough advance [8] by embedding a transmon qubit [9, 10] in a superconducting waveguide cavity. Dubbed three-dimensional circuit QED (3D cQED), this system produced significantly enhanced qubit lifetimes of T 1 =25-60µs and T * 2 =10-20µs, corresponding to quality factors for dissipation and decoherence of Q 1 ≈1.8×10 6 and Q 2 ≈7×10 5 , respectively.These results lead to two important questions. First, are similar coherence properties observable using other fabrication processes, facilities, and measurement setups? Second, what is the origin of the dephasing process suppressing T * 2 well below the no-pure-dephasing limit of 2T 1 ? Is it intrinsic to the junctions or to this qubit ar-chitecture? The weight and urgency of these questions are increased by implications on scaling potential: if the results are reproducible and decoherence tim...

We demonstrate an all-microwave two-qubit gate on superconducting qubits which are fixed in frequency at optimal bias points. The gate requires no additional subcircuitry and is tunable via the amplitude of microwave irradiation on one qubit at the transition frequency of the other. We use the gate to generate entangled states with a maximal extracted concurrence of 0.88, and quantum process tomography reveals a gate fidelity of 81%.

We use quantum process tomography to characterize a full universal set of all-microwave gates on two superconducting single-frequency single-junction transmon qubits. All extracted gate fidelities, including those for Clifford group generators, single-qubit π/4 and π/8 rotations, and a two-qubit controlled-not, exceed 95% (98%), without (with) subtracting state preparation and measurement errors. Furthermore, we introduce a process map representation in the Pauli basis which is visually efficient and informative. This high-fidelity gate set serves as a critical building block towards scalable architectures of superconducting qubits for error correction schemes and pushes up on the known limits of quantum gate characterization.

We characterize a superconducting qubit before and after embedding it along with its package in an absorptive medium. We observe a drastic improvement in the effective qubit temperature and over a tenfold improvement in the relaxation time up to 5.7 µs. Our results suggest the presence of external radiation inside the cryogenic apparatus can be a limiting factor for both qubit initialization and coherence. We infer from simple calculations that relaxation is not limited by thermal photons in the sample prior to embedding, but by dissipation arising from quasiparticle generation.Energy loss in superconducting qubits remains a major object of study on the road towards scalable qubit architectures. The primary origins of relaxation in superconducting qubits can be intrinsic to the material, encompassing dielectric, 1 and resistive (quasiparticle) losses, 2,3 or external through radiative 4,5 and electromagnetic (EM) environmental losses. 6,7 The current understanding of these loss mechanisms is still incomplete, as it is difficult to experimentally separate the different contributions to qubit relaxation. It is equally critical to consider the bath to which the qubits relax, as the equilibrium temperature of such a bath determines the degree of purity for qubit initialization. 8,9 Thermal heating of qubits is well known in experiments, 10 although not often pointed out, and its elimination is necessary for future quantum computing applications.In this Letter, we present an experiment where extrinsic loss mechanisms on a superconducting qubit are removed, resulting in both a lower effective qubit temperature and a significant increase in qubit coherence. We characterize the same superconducting qubit in two separate experimental configurations: in a printed-circuit board (PCB) package mounted within a standard cryogenic environment with no additional shielding, and then in nominally the same setup with the only change of embedding the entire PCB and qubit device in absorptive material. We find, surprisingly, that the embedding of highly sensitive quantum circuits in an absorptive, lossy medium can be useful for shielding and attenuating losses caused by radiation and EM environmental factors within the cryogenic setup. 11,12 Our experimental procedure isolates the qubit loss mechanisms to be only those which are material related or on-chip/on-package, and linked to a bath that is thermalized to the base temperature.The experimental device tested is a capacitively shunted flux qubit 13 (CSFQ) in the circuit quantum electrodynamics architecture. The qubit is capacitively coupled to a coplanar waveguide λ/2 resonator (ω cav /2π = 10.3 GHz), with a 6 µm center strip and 3 µm spacing to ground, via a C qr ∼ 5 fF interdigitated capacitor. A vacuum Rabi experiment (not shown) gives a qubitresonator coupling strength g/π ∼ 200 MHz. The resonator is also capacitively coupled to a microwave feed line (C c ∼ 2 fF), with a linewidth κ/2π = 470 kHz, cor-Figure 1. (a) Optical micrographs of the qubit device. (b) Zoom in of qubit l...

We measure the coherence of a new superconducting qubit, the low-impedance flux qubit, finding T * 2 ∼ T1 ∼ 1.5µs. It is a three-junction flux qubit, but the ratio of junction critical currents is chosen to make the qubit's potential have a single well form. The low impedance of its large shunting capacitance protects it from decoherence. This qubit has a moderate anharmonicity, whose sign is reversed compared with all other popular qubit designs. The qubit is capacitively coupled to a high-Q resonator in a λ/2 configuration, which permits the qubit's state to be read out dispersively. PACS numbers:While there have been many successful superconducting qubit types, their large diversity suggests that the optimal qubit will be a hybrid combining favorable features of all: the tunability of the flux qubit [1][2][3], the simplicity, robustness and low impedance of the phase qubit [4][5][6] and the high coherence and compatibility with high-Q superconducting resonators of the transmon [7,8]. We have built such a hybrid, related to a suggested design of You et al. [9]. Our capacitively shunted flux qubit begins as a traditional three-junction loop[1], but is made to have low impedance by virtue of a large capacitive shunt (C s = 100fF) of the small junction. This new superconducting qubit is as coherent as the best currently reported; we measure T * 2 ∼ T 1 ∼ 1.5µs. Since the key to this qubit is the large shunting capacitance C s and therefore its low effective impedance L J /C s , we will call it the low-impedance flux qubit ( Z flux qubit). As Fig. 1(a) shows, the shunt capacitor is realized using a simple, reliable single-level interdigitated structure. We choose the ratio of the small and large junction critical currents I 0 to be around α = 0.3. For this α the qubit potential has only one minimum (see Eq. (3) below), and the qubit shows only a weak dependence of the qubit frequency ω 01 on applied flux Φ. As for the original flux qubit, a "sweet spot" exists at which the qubit is to first order insensitive to Φ, giving rise to long dephasing times, but even away from this degeneracy point our frequency sensitivity is about a factor of 30 smaller than in the traditional flux qubit. Our flux sensitivity is comparable to that of the phase qubit (∂ω 01 /∂Φ ∼ 30 GHz/Φ 0 ) which permits tunability without completely destroying phase coherence, despite the presence of significant flux noise amplitude on the order of S Φ = 1 − 2µΦ 0 / √ Hz. Modeling indicates that our qubit at the sweet spot still has appreciable anharmonicity, with |ω 12 − ω 01 |/2π in the neighborhood of several 100 MHz (or about 2 − 10% of the qubit resonance frequency, depending on α), but interestingly, with ω 12 > ω 01 , the opposite of any important qubit except the flux qubit. Such anharmonicity leads to a situation where all of the lowest energy levels for a two-qubit system would be those of the computational manifold |0 and |1 , which will facilitate coupled qubit experiments. The reduced impedance of this qubit has several advantages. Qubits...