We present measurements of superconducting flux qubits embedded in a three dimensional copper cavity. The qubits are fabricated on a sapphire substrate and are measured by coupling them inductively to an on-chip superconducting resonator located in the middle of the cavity. At their flux-insensitive point, all measured qubits reach an intrinsic energy relaxation time in the 6-20 microseconds range and a pure dephasing time comprised between 3 and 10 microseconds. This significant improvement over previous works opens the way to the coherent coupling of a flux-qubit to individual spins.
We present measurements of superconducting flux qubits embedded in a three dimensional copper cavity. The qubits are fabricated on a sapphire substrate and are measured by coupling them inductively to an on-chip superconducting resonator located in the middle of the cavity. At their flux-insensitive point, all measured qubits reach an intrinsic energy relaxation time in the 6-20 microseconds range and a pure dephasing time comprised between 3 and 10 microseconds. This significant improvement over previous works opens the way to the coherent coupling of a flux-qubit to individual spins.
We present measurements of superconducting flux qubits embedded in a three dimensional copper cavity. The qubits are fabricated on a sapphire substrate and are measured by coupling them inductively to an on-chip superconducting resonator located in the middle of the cavity. At their flux-insensitive point, all measured qubits reach an intrinsic energy relaxation time in the 6-20 μs range and a pure dephasing time comprised between 3 and 10 μs. This significant improvement over previous works opens the way to the coherent coupling of a flux qubit to individual spins.
We report experiments on superconducting flux qubits in a circuit quantum electrodynamics (cQED) setup. Two qubits, independently biased and controlled, are coupled to a coplanar waveguide resonator. Dispersive qubit state readout reaches a maximum contrast of 72%. We measure energy relaxation times at the symmetry point of 5 and $10\ensuremath{\mu}\mathrm{s}$, corresponding to 7 and $20\ensuremath{\mu}\mathrm{s}$ when relaxation through the resonator due to Purcell effect is subtracted out, and levels of flux noise of 2.6 and $2.7\phantom{\rule{0.222222em}{0ex}}\ensuremath{\mu}{\mathrm{\ensuremath{\Phi}}}_{0}/\sqrt{\text{Hz}}$ at 1 Hz for the two qubits. We discuss the origin of decoherence in the measured devices. The strong coupling between the qubits and the cavity leads to a large, cavity-mediated, qubit-qubit coupling. This coupling, which is characterized spectroscopically, reaches 38 MHz. These results demonstrate the potential of cQED as a platform for fundamental investigations of decoherence and quantum dynamics of flux qubits.
In this work, we study a series of tunable flux qubits inductively coupled to a coplanar waveguide resonator fabricated on a sapphire substrate. Each qubit includes an asymmetric superconducting quantum interference device, which is controlled by the application of an external magnetic field and acts as a tunable Josephson junction. The tunability of the qubits is typically $\ifmmode\pm\else\textpm\fi{}3.5\phantom{\rule{0.2em}{0ex}}\mathrm{GHz}$ around their central gap frequency. The measured relaxation times are limited by dielectric losses in the substrate and can attain ${T}_{1}\ensuremath{\sim}8\phantom{\rule{0.2em}{0ex}}\text{\ensuremath{\mu}}\mathrm{s}$. The echo dephasing times are limited by flux noise even at optimal points and reach ${T}_{2E}\ensuremath{\sim}4\phantom{\rule{0.2em}{0ex}}\text{\ensuremath{\mu}}\mathrm{s}$, almost an order of magnitude longer than state of the art.
We report the experimental realization of a 3D capacitively shunt superconducting flux qubit with long coherence times. At the optimal flux bias point, the qubit demonstrates energy relaxation times in the range of 60–90 μs and a Hahn-echo coherence time of about 80 μs, which can be further improved by dynamical decoupling. Qubit energy relaxation can be attributed to quasiparticle tunneling and unwanted two-level-system defects, while qubit dephasing is caused by flux noise away from the optimal point. Our results show that 3D c-shunt flux qubits demonstrate improved performance over other types of flux qubits, which is advantageous for applications such as quantum magnetometry and spin sensing.
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 40μs 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≈85μs, 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.
Advancements in the field of cavity quantum electrodynamics (c-QED) have deepened understanding of quantum phenomena, enabled reliable Josephson junction-based devices, and facilitated their integration with other quantum technologies to explore new physics and applications. Motivated by these developments, this thesis aims to construct a testbed for studying quantum thermodynamics using a superconducting flux qubit. The flux qubit is particularly suited for thermodynamic experiments due to its strong anharmonicity and its galvanic coupling architectures that facilitate efficient energy transport. In this thesis, the on-chip components include harmonic oscillators realized as superconducting resonators, a strongly nonlinear superconducting flux qubit, two copper thermal reservoirs modelled as Johnson–Nyquist noise sources and NIS thermometers, integrated on a highly resistive silicon platform that acts as a phonon bath. These components enable the study and optimization of heat transport between the metallic reservoirs, mediated by the quantum circuit, with the flux qubit as the central, flux-tunable working substance. Heat transport through the qubit occurs via sequential excitation and relaxation cycles between its ground and excited states, where excitation extracts heat equal to the qubit’s energy gap from the hot reservoir, while relaxation releases the same energy into the cold reservoir. Enhancing the thermal device’s performance critically depends on the coupling strength between on-chip components, particularly between the qubit and resonator. This thesis commences with the investigation and demonstration of a robust qubit–resonator coupling design that achieves the ultrastrong regime experimentally. Building on this result, the developed architecture is extended to connect the qubit with two identical resonators, symmetrically positioned on the two sides. These resonators are, in turn, coupled to metallic reservoirs, enabling the first ever demonstration of flux qubit-controlled heat transport between the two reservoirs with excellent device performance. Added geometric asymmetry in the coupler, combined with the flux qubit’s strong anharmonicity, enables the successful demonstration--presented in this thesis--of a non-reciprocal device such as the Microwave Quantum Diode, which, together with the reservoirs, could potentially form an efficient thermal diode. Finally, the thesis concludes with a brief outlook on future directions and ongoing experiments that build on the flux qubit–based testbed developed here for exploring quantum thermodynamic phenomena and device applications.