The scalable application of quantum information science will stand on reproducible and controllable high-coherence quantum bits (qubits). Here, we revisit the design and fabrication of the superconducting flux qubit, achieving a planar device with broad-frequency tunability, strong anharmonicity, high reproducibility and relaxation times in excess of 40 μs at its flux-insensitive point. Qubit relaxation times T1 across 22 qubits are consistently matched with a single model involving resonator loss, ohmic charge noise and 1/f-flux noise, a noise source previously considered primarily in the context of dephasing. We furthermore demonstrate that qubit dephasing at the flux-insensitive point is dominated by residual thermal-photons in the readout resonator. The resulting photon shot noise is mitigated using a dynamical decoupling protocol, resulting in T2≈85 μs, approximately the 2T1 limit. In addition to realizing an improved flux qubit, our results uniquely identify photon shot noise as limiting T2 in contemporary qubits based on transverse qubit–resonator interaction.

As the field of superconducting quantum computing advances from the few-qubit stage to largerscale processors, qubit addressability and extensibility will necessitate the use of 3D integration and packaging. While 3D integration is well-developed for commercial electronics, relatively little work has been performed to determine its compatibility with high-coherence solid-state qubits. Of particular concern, qubit coherence times can be suppressed by the requisite processing steps and close proximity of another chip. In this work, we use a flip-chip process to bond a chip with superconducting flux qubits to another chip containing structures for qubit readout and control. We demonstrate that high qubit coherence (T1, T 2,echo > 20 µs) is maintained in a flip-chip geometry in the presence of galvanic, capacitive, and inductive coupling between the chips.

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Remarkable advancements in coherence and control fidelity have been achieved in recent years with cryogenic solid-state qubits. Nonetheless, thermalizing such devices to their milliKelvin environments has remained a long-standing fundamental and technical challenge. In this context, we present a systematic study of the first-excited-state population in a 3D transmon superconducting qubit mounted in a dilution refrigerator with a variable temperature. Using a modified version of the protocol developed by Geerlings et al., we observe the excited-state population to be consistent with a Maxwell-Boltzmann distribution, i.e., a qubit in thermal equilibrium with the refrigerator, over the temperature range 35-150 mK. Below 35 mK, the excited-state population saturates at approximately 0.1%. We verified this result using a flux qubit with ten times stronger coupling to its readout resonator. We conclude that these qubits have effective temperature T eff ¼ 35 mK. Assuming T eff is due solely to hot quasiparticles, the inferred qubit lifetime is 108 μs and in plausible agreement with the measured 80 μs. Superconducting qubits are increasingly promising candidates to serve as the logic elements of a quantum information processor. This assertion reflects, in part, several successes over the past decade addressing the fundamental operability of this qubit modality [1][2][3]. A partial list includes a 5-orders-of-magnitude increase in the coherence time T 2 [4], the active initialization of qubits in their ground state [1,5], the demonstration of low-noise parametric amplifiers [6][7][8][9][10][11][12] enabling high-fidelity readout [13][14][15][16], and the implementation of a universal set of high-fidelity gates [17]. In addition, prototypical quantum algorithms [18][19][20] and simulations [21,22] have been demonstrated with few-qubit systems, and the basic parity measurements underlying certain error detection protocols are now being realized with qubit stabilizers [23][24][25][26][27][28] and photonic memories [29].Concomitant with these advances is an enhanced ability to improve our understanding of the technical and fundamental limitations of single qubits. The 3D transmon [30] has played an important role in this regard, because its relatively clean electromagnetic environment, predominantly low-loss qubit-mode volume, and resulting long coherence times make it a sensitive test bed for probing these limitations.One such potential limitation is the degree to which a superconducting qubit is in equilibrium with its cryogenic environment. Consider a typical superconducting qubit with a level splitting E ge ¼ hf ge , with f ge ¼ 5 GHz, mounted in a dilution refrigerator at temperature T ¼ 15 mK, such that E ge ≫ k B T. Ideally, such a qubit in thermal equilibrium with the refrigerator will have a thermal population P jei ≈ 10 −5 % of its first excited state according to Maxwell-Boltzmann statistics. In practice, however, the empirical excited-state population reported for various superconducting qubits (featuring similar...