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We autonomously stabilize arbitrary states of a qubit through parametric modulation of the coupling between a fixed frequency qubit and resonator. The coupling modulation is achieved with a tunable coupling design, in which the qubit and the resonator are connected in parallel to a superconducting quantum interference device. This allows for quasistatic tuning of the qubit-cavity coupling strength from 12 MHz to more than 300 MHz. Additionally, the coupling can be dynamically modulated, allowing for single-photon exchange in 6 ns. Qubit coherence times exceeding 20  μs are maintained over the majority of the range of tuning, limited primarily by the Purcell effect. The parametric stabilization technique realized using the tunable coupler involves engineering the qubit bath through a combination of photon nonconserving sideband interactions realized by flux modulation, and direct qubit Rabi driving. We demonstrate that the qubit can be stabilized to arbitrary states on the Bloch sphere with a worst-case fidelity exceeding 80%.

We study ultrafast excitonic population inversion resulting from the interaction of a semiconductor quantum dot (SQD) with localized surface plasmons. The plasmonic enhanced fields are generated when a metallic nanoparticle (MNP) is subject to a nonlinear chirped few-cycle pulse train. By numerically solving the time-dependent Bloch equations beyond the rotating-wave approximation, we show that the complete population inversion can be achieved for small interparticle distance and the dynamic in population inversion exhibits a steplike transition between absorption and amplifying. This phenomenon can be exploited as an all-optical ultrafast switching device. Moreover, the final state of population inversion is shown to be modified significantly with the interparticle distances, which is not only robust against the variation of probe pulse parameters but also suggests a straightforward method for measuring the interparticle distances via probing the final populations.

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Superconducting circuits extensively rely on the Josephson junction as a nonlinear electronic element for manipulating quantum information and mediating photon interactions. Despite continuing efforts in designing anharmonic Josephson circuits with improved coherence times, the best photon lifetimes have been demonstrated in microwave cavities. Nevertheless, architectures based on quantum memories need a qubit element for addressing these harmonic modules at the cost of introducing additional loss channels and limiting process fidelities. This work focuses on tailoring the oscillator Hilbert space to enable a direct Rabi drive on individual energy levels. For this purpose we implement a flux-tunable inductive coupling between two linear resonators using a superconducting quantum interference device. We dynamically activate a three-wave mixing process through parametric flux modulation in order to selectively address the lowest eigenstates as an isolated two-level system. Measuring the Wigner function confirms we can prepare arbitrary states confined in the single photon manifold, with measured coherence times limited by the oscillator intrinsic quality factor. This architectural shift in engineering oscillators with stimulated nonlinearity can be exploited for designing long-lived quantum modules and offers flexibility in studying non-equilibrium physics with photons in a field-programmable simulator.Quantized electromagnetic excitations in superconducting circuits have become a promising platform for processing quantum information 1 . A central piece to this hardware is the Josephson effect, which grants nonlinearity with negligible dissipation. Nonlinear mesoscopic oscillators have been attracting much interest in studying non-equilibrium physics 2 . Electrical circuits composed of Josephson junctions behave as quantum systems with discrete energy levels resembling artificial atoms 3 . Although the coherence properties of anharmonic Josephson circuits have been improving these past two decades, storing microwave photons in harmonic oscillators leads to significantly longer coherence times 4,5 . This promising development has launched efforts in building hardwareefficient architectures based on quantum memories 6,7 . Since it is impossible to selectively address individual transitions in a harmonic system, Josephson qubits are used for preparing and transferring quantum information with negative impact on coherence and gate fidelities. In this work we present a new paradigm in exploiting the Josephson effect to perform logical operations directly on the oscillator Hilbert space using a dynamically activated nonlinearity.The key concept behind this experiment is to engineer a three-wave interaction between the electromagnetic modes of two linear oscillators, which we refer to as logical and blockade oscillator (Fig. 1A). Parametrically modulating the interaction amplitude stimulates the conversion of two logical photons into one blockade photon and vice-versa. Provided the conversion rate is larger than the oscil...