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The dominant source of decoherence in contemporary frequency-tunable superconducting qubits is 1/f flux noise. To understand its origin and find ways to minimize its impact, we systematically study flux noise amplitudes in more than 50 flux qubits with varied superconducting quantum interference device (SQUID) geometry parameters and compare our results to a microscopic model of magnetic spin defects located at the interfaces surrounding the SQUID loops. Our data are in agreement with an extension of the previously proposed model, based on numerical simulations of the current distribution in the investigated SQUIDs. Our results and detailed model provide a guide for minimizing the flux noise susceptibility in future circuits.

We perform full time resolved tomographic measurements of the polarization state of pairs of photons emitted during the radiative cascade of the confined biexciton in a semiconductor quantum dot. The biexciton was deterministically initiated using a π-area pulse into the biexciton two-photon absorption resonance. Our measurements demonstrate that the polarization states of the emitted photon pair are maximally entangled. We show that the measured degree of entanglement depends solely on the temporal resolution by which the time difference between the emissions of the photon pair is determined. A route for fabricating an on demand source of maximally polarization entangled photon pairs is thereby provided.The ability to generate entangled photons on-demand is crucial for many future applications in quantum information processing. Devices based on the biexcitonexciton radiative cascade in single semiconductor quantum dot are considered to be one of the best candidates for these applications [1][2][3]. The ability to deterministically excite the biexciton using its two-photon absorption resonance [4,5] makes this avenue even more promising. A remaining challenge, however, is the excitonic fine structure, which splits the two exciton eigenstates thus providing spectral "which-path" information on the radiative cascade and preventing the pairs of emitted photons from being polarization entangled [2]. Various strategies were tried in an attempt to reduce the influence of the fine structure splitting. Spectral [2] and temporal filtering [2,6], which introduce non desired, nondeterministic post selection. Enhancement of the radiative rate using the Purcell effect [3], thereby reducing, but not limiting the effect of exciton precession. Attempts to reduce the fine-structure splitting using heat treatment [7] or growth along the [111] crystalographic direction [8] were reported as well as applications of external stress [9], electric [10] and magnetic fields tuning [11,12]. These efforts, usually result in unwanted loss of emission quantum efficiency [10], and increase in the exciton spin decoherence [6].We present here a novel study of a single semiconductor quantum dot, optically depleted [13] and then resonantly excited on-demand by a π-area pulse to the biexciton two photon absorption resonance [5]. The resulting pairs of biexciton and exciton photons are detected by two superconducting detectors synchronized to the exciting laser pulse. By performing synchronized time resolved polarization tomography of the two emitted photons, we unambigously show that the photons remain maximally polarization entangled during the whole radiative decay, and that the measured degree of entanglement does not depend on the QD source, but rather depends on the temporal resolution by which the time difference between the two photon emissions can be determined. Since during the radiative decay the exciton does not lose coherence, there is no need to eliminate the excitonic fine structure splitting. A relatively simple arrangement [14,15] ...

Noise that exhibits significant temporal and spatial correlations across multiple qubits can be especially harmful to both fault-tolerant quantum computation and quantum-enhanced metrology. However, a complete spectral characterization of the noise environment of even a two-qubit system has not been reported thus far. We propose and experimentally demonstrate a protocol for two-qubit dephasing noise spectroscopy based on continuous-control modulation. By combining ideas from spin-locking relaxometry with a statistically motivated robust estimation approach, our protocol allows for the simultaneous reconstruction of all the single-qubit and two-qubit cross-correlation spectra, including access to their distinctive nonclassical features. Only single-qubit control manipulations and state-tomography measurements are employed, with no need for entangled-state preparation or readout of two-qubit observables. While our experimental demonstration uses two superconducting qubits coupled to a shared, colored engineered noise source, our methodology is portable to a variety of dephasing-dominated qubit architectures. By pushing quantum noise spectroscopy beyond the single-qubit setting, our work heralds the characterization of spatiotemporal correlations in both engineered and naturally occurring noise environments.