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.

The spectral density S(Φ)(f) = A(2)/(f/1 Hz)(α) of magnetic flux noise in ten dc superconducting quantum interference devices (SQUIDs) with systematically varied geometries shows that α increases as the temperature is lowered; in so doing, each spectrum pivots about a nearly constant frequency. The mean-square flux noise, inferred by integrating the power spectra, grows rapidly with temperature and at a given temperature is approximately independent of the outer dimension of a given SQUID. These results are incompatible with a model based on the random reversal of independent, surface spins.

Dynamical error suppression techniques are commonly used to improve coherence in quantum systems. They reduce dephasing errors by applying control pulses designed to reverse erroneous coherent evolution driven by environmental noise. However, such methods cannot correct for irreversible processes such as energy relaxation. In this work, we investigate a complementary, stochastic approach to reducing errors: instead of deterministically reversing the unwanted qubit evolution, we use control pulses to shape the noise environment dynamically. In the context of superconducting qubits, we implement a pumping sequence to reduce the number of unpaired electrons (quasiparticles) in close proximity to the device. We report a 70% reduction in the quasiparticle density, resulting in a threefold enhancement in qubit relaxation times, and a comparable reduction in coherence variability.Since Hahn's invention of the spin-echo in 1950 [1], coherent control techniques have been crucial tools for reducing errors, improving control fidelity, performing noise spectroscopy and generally extending coherence in both natural and artificial spin systems. All of these methods are similar: they correct for dephasing errors by reversing unintended phase accumulations due to a noisy environment through the application of a sequence of control pulses, thereby improving the dephasing time T 2 . However, such coherent control techniques cannot correct for irreversible processes that reduce the relaxation time T 1 , where energy is lost to the environment. Improving T 1 requires either reducing the coupling between the spin system and its noisy environment, reducing the noise in the environment itself [2], or implementing full quantum error correction.We demonstrate a pumping sequence that dynamically reduces the noise in the environment and improves T 1 of a superconducting qubit through an irreversible pumping process. The sequence contains the same type of control pulses common to all dynamical-decoupling sequences, but instead of coherently and deterministically controlling the qubit time evolution, the sequence is designed to shape the noise stochastically via inelastic energy exchange with the environment. Similar methods have been used to extend T 2 of spin qubits by dynamic nuclear polarization [3], and irreversible control techniques are commonly used to prepare systems into well-defined quantum states through optical pumping [4,5] and sideband cooling [6], but outside of quantum error correction, to our knowledge no dynamic enhancement of T 1 has been previously reported.We implement the pumping sequence in a superconducting flux qubit, with the aim of reducing the population of unpaired electrons or quasiparticles in close vicinity to the device. As a superconducting circuit is cooled well below its critical temperature, the quasiparticle density via BCS theory is expected to be exponentially suppressed, but a number of experimental groups have reported higher-than-expected values in a wide variety of systems [7][8][9][10]. Although...

For many types of superconducting qubits, magnetic flux noise is a source of pure dephasing. Measurements on a representative dc superconducting quantum interference device (SQUID) over a range of temperatures show that S (f ) = A 2 /(f/1 Hz) α , where S is the flux noise spectral density, A is of the order of 1 μ 0 Hz −1/2 , 0.61 α 0.95, and 0 is the flux quantum. For a qubit with an energy level splitting linearly coupled to the applied flux, calculations of the dependence of the pure dephasing time τ φ of Ramsey and echo pulse sequences on α for fixed A show that τ φ decreases rapidly as α is reduced. We find that τ φ is relatively insensitive to the noise bandwidth, f 1 f f 2 , for all α provided the ultraviolet cutoff frequency f 2 > 1/τ φ . We calculate the ratio τ φ,E /τ φ,R of the echo (E) and Ramsey (R) sequences and the dependence of the decay function on α and f 2 . We investigate the case in which S (f 0 ) is fixed at the "pivot frequency" f 0 = 1 Hz while α is varied and find that the choice of f 0 can greatly influence the sensitivity of τ φ,E and τ φ,R to the value of α. Finally, we present calculated values of τ φ in a qubit corresponding to the values of A and α measured in our SQUID.