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Bulletin of the American Physical Society

Resonance fluorescence readout of a superconducting qubit

Nathanaël Cottet, Yen‐Hsiang Lin, Long B. Nguyen, Nicholas Grabon, Vladimir Manucharyan

Published Mar 16, 2017• 0 citations
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Non-demolition readout and high-fidelity unconditional reset of qubits are key requirements for practical quantum computation. For superconducting qubits, readout and reset are typically realized using resonators based on dispersive approximation. However, violations of the approximation often lead to detrimental effects on the qubit. In this work, we demonstrate non-demolition fluorescence readout and high-fidelity unconditional reset of a fluxonium qubit via dissipation engineering without employing a resonator. We design and implement a planar filter to protect the qubit from energy relaxation while enhancing the relaxation of the readout transition. By appropriately selecting the readout transition, we achieve fluorescence readout with enhanced quantum non-demolitionness. We also realize fast, high-fidelity and all-microwave unconditional reset of the fluxonium qubit. These results highlight the potential of superconducting-quantum computing architectures without relying on dispersive interaction between qubits and resonators.
We report high-fidelity state readout of a trapped ion qubit using a trap-integrated photon detector. We determine the hyperfine qubit state of a single ^{9}Be^{+} ion held in a surface-electrode rf ion trap by counting state-dependent ion fluorescence photons with a superconducting nanowire single-photon detector fabricated into the trap structure. The average readout fidelity is 0.9991(1), with a mean readout duration of 46 μs, and is limited by the polarization impurity of the readout laser beam and by off-resonant optical pumping. Because there are no intervening optical elements between the ion and the detector, we can use the ion fluorescence as a self-calibrated photon source to determine the detector quantum efficiency and its dependence on photon incidence angle and polarization.
This experimental thesis explores energetic and informational aspects of the fluorescence emitted by superconducting circuits. A quantum bit interacting with a resonant drive exchanges quanta of energy during absorption and emission cycles. Using low- noise amplification and linear detection of microwave light, we have developed a measurement setup to record the energy and complex amplitude of the fluorescence field and realized three experiments based on the record of fluorescence. First, we have realized and demonstrated a transfer of energy between two resonant drives. Owing to the properties of stimulated emission, the direction and magnitude of energy transfer can be controlled by the phase of a quantum superposition of qubit states. Second, we have used the information contained in fluorescence to implement a new readout scheme for superconducting circuits without the help of any ancillary quantum system. The circuit, directly coupled to the environment, encodes quantum information in a well-protected subspace and is read using the fluorescence of a strongly coupled transition. Finally, we have explored the interplay between information and energy in the quantum regime by realizing an autonomous Maxwell's demon. The demon is a cavity measuring the system, a superconducting qubit. Work is extracted in the form of a stimulated photon emitted by the qubit and is directly measured using our fluorescence measurement setup. It is linked with the variations of the system's internal energy. Using independent measurements, the transfer of information from the system to the demon is quantified. In particular, we demonstrated the quantum signatures of the demon when the system is initialized in a quantum superposition.