Photonic cat states stored in high-Q resonators show great promise for hardware efficient universal quantum computing. We propose an approach to efficiently prepare such cat states in a Kerr-nonlinear resonator by the use of a two-photon drive. Significantly, we show that this preparation is robust against single-photon loss. An outcome of this observation is that a twophoton drive can eliminate undesirable phase evolution induced by a Kerr nonlinearity. By exploiting the concept of transitionless quantum driving, we moreover demonstrate how non-adiabatic initialization of cat states is possible. Finally, we present a universal set of quantum logical gates that can be performed on the engineered eigenspace of such a two-photon driven resonator and discuss a possible realization using superconducting circuits. The robustness of the engineered subspace to higher-order circuit nonlinearities makes this implementation favorable for scalable quantum computation.

We observe measurement-induced qubit state mixing in a transmon qubit dispersively coupled to a planar readout cavity. Our results indicate that dephasing noise at the qubit-readout detuning frequency is up-converted by readout photons to cause spurious qubit state transitions, thus limiting the nondemolition character of the readout. Furthermore, we use the qubit transition rate as a tool to extract an equivalent flux noise spectral density at f ∼ 1 GHz and find agreement with values extrapolated from a 1/f α fit to the measured flux noise spectral density below 1 Hz.PACS numbers: 42.50. Lc, 42.50.Pq, 03.67.Lx, High-fidelity measurement is a crucial tool in quantum information science. For superconducting qubits [1,2], one widely used framework for performing quantum nondemolition (QND) [3] measurement is the circuit quantum electrodynamics (cQED) architecture [4,5]. In cQED, a qubit is coupled to a microwave-frequency resonant cavity through a Jaynes-Cummings-type interaction, in analogy to an atom in an optical Fabry-Perot cavity. In the dispersive limit, probing the qubit-statedependent resonant frequency of the cavity implements, to first order, a QND measurement of the qubit state.In the case of a linear readout cavity [6], only recently has single-shot sensitivity been demonstrated using a near-quantum-noise-limited superconducting parametric amplifier [7,8], enabling observation of individual qubit state transitions in real time [9]. Subsequent experiments [10,11] have reported single-shot fidelities of 94%-97%. Nonlinear circuit QED readout methods-using either the nonlinearity of the qubit [12][13][14] or a nonlinear cavity [15]-have shown single-shot fidelities of 86%-92%, but the former is not QND and the latter is too slow to allow continuous qubit monitoring.In this letter, we explore non-QND behavior in cQED readout with a linear cavity. We employ single shot readout [9] to directly quantify the rate of measurementinduced qubit transitions. We find that dephasing noise at the qubit-readout detuning frequency ∆ ro = ω q − ω ro combines with readout photons to induce qubit excitation and relaxation, making the measurement process no longer fully QND. The rate of qubit transitions due to such "dressed dephasing" depends linearly on the average cavity photon occupationn and the spectral density of dephasing noise at the detuning frequency S(±∆ ro ), consistent with recent calculations which keep higher or- * Electronic address: slichter@berkeley.edu; Present address:Time and Frequency Division, National Institute of Standards and Technology, Boulder CO 80305 der terms in the dispersive approximation [16]. Furthermore, the qubit transition rate provides a new probe of dephasing noise at |∆ ro |/2π ∼ 1 GHz, a frequency range not currently accessible by other techniques. We find that our extracted value of dephasing noise at GHz frequencies is consistent with the "universal" 1/f magnetic flux noise [17,18] typically observed in low frequency measurements, suggesting the persistence of this noise mechanis...

We present an experimental realization of resonance fluorescence in squeezed vacuum. We strongly couple microwave-frequency squeezed light to a superconducting artificial atom and detect the resulting fluorescence with high resolution enabled by a broadband traveling-wave parametric amplifier. We investigate the fluorescence spectra in the weak and strong driving regimes, observing up to 3.1 dB of reduction of the fluorescence linewidth below the ordinary vacuum level and a dramatic dependence of the Mollow triplet spectrum on the relative phase of the driving and squeezed vacuum fields. Our results are in excellent agreement with predictions for spectra produced by a two-level atom in squeezed vacuum [Phys. Rev. Lett. 58, 2539-2542], demonstrating that resonance fluorescence offers a resource-efficient means to characterize squeezing in cryogenic environments.

Single-mode Josephson junction-based parametric amplifiers are often modeled as perfect amplifiers and squeezers. We show that, in practice, the gain, quantum efficiency, and output field squeezing of these devices are limited by usually neglected higher-order corrections to the idealized model. To arrive at this result, we derive the leading corrections to the lumped-element Josephson parametric amplifier of three common pumping schemes: monochromatic current pump, bichromatic current pump, and monochromatic flux pump. We show that the leading correction for the last two schemes is a single Kerr-type quartic term, while the first scheme contains additional cubic terms. In all cases, we find that the corrections are detrimental to squeezing. In addition, we show that the Kerr correction leads to a strongly phase-dependent reduction of the quantum efficiency of a phase-sensitive measurement. Finally, we quantify the departure from ideal Gaussian character of the filtered output field from numerical calculation of third and fourth order cumulants. Our results show that, while a Gaussian output field is expected for an ideal Josephson parametric amplifier, higher-order corrections lead to non-Gaussian effects which increase with both gain and nonlinearity strength. This theoretical study is complemented by experimental characterization of the output field of a flux-driven Josephson parametric amplifier. In addition to a measurement of the squeezing level of the filtered output field, the Husimi Q-function of the output field is imaged by the use of a deconvolution technique and compared to numerical results. This work establishes nonlinear corrections to the standard degenerate parametric amplifier model as an important contribution to Josephson parametric amplifier's squeezing and noise performance.

No abstract