The quantum Zeno effect (QZE) is the apparent freezing of a quantum system in one state under the influence of a continuous observation. It has been further generalized to the stabilizationof a manifold spanned by multiple quantum states. In that case, motion inside the manifold can subsist and can even be driven by the combination of a dissipative stabilization and an external force. A superconducting microwave cavity that exchanges pairs of photons with its environments constitutes an example of a system which displays a stabilized manifold spanned by Schr\“odinger cat states. For this driven-dissipative system, the quantum Zeno stabilization transforms a simple linear drive into photon number parity oscillations within the stable cat state manifold. Without this stabilization, the linear drive would trivially displace the oscillator state and push it outside of the manifold. However, the observation of this effect is experimentally challenging. On one hand, the adiabaticity condition requires the oscillations to be slow compared to the manifold stabilization rate. On the other hand, the oscillations have to be fast compared with the coherence timescales within the stabilized manifold. Here, we implement the stabilization of a manifold spanned by Schr\“odinger cat states at a rate that exceeds the main source of decoherence by two orders of magnitude, and we show Zeno-driven coherent oscillations within this manifold. While related driven manifold dynamics have been proposed and observed, the non-linear dissipation specific to our experiment adds a crucial element: any drift out of the cat state manifold is projected back into it. The coherent oscillations of parity observed in this work are analogous to the Rabi rotation of a qubit protected against phase-flips and are likely to become part of the toolbox in the construction of a fault-tolerant logical qubit.
We present a device demonstrating a lithographically patterned transmon integrated with a micromachined cavity resonator. Our two-cavity, one-qubit device is a multilayer microwaveintegrated quantum circuit (MMIQC), comprising a basic unit capable of performing circuit-QED (cQED) operations. We describe the qubit-cavity coupling mechanism of a specialized geometry using an electric field picture and a circuit model, and finally obtain specific system parameters using simulations. Fabrication of the MMIQC includes lithography, etching, and metallic bonding of silicon wafers. Superconducting wafer bonding is a critical capability that is demonstrated by a micromachined storage cavity lifetime 34.3 μs, corresponding to a quality factor of 2 million at single-photon energies. The transmon coherence times are T1=6.4 μs, and TEcho2=11.7 μs. We measure qubit-cavity dispersive coupling with rate χqμ/2π=−1.17 MHz, constituting a Jaynes-Cummings system with an interaction strength g/2π=49 MHz. With these parameters we are able to demonstrate cQED operations in the strong dispersive regime with ease. Finally, we highlight several improvements and anticipated extensions of the technology to complex MMIQCs.
There are two general requirements to harness the computational power of quantum mechanics: the ability to manipulate the evolution of an isolated system and the ability to faithfullyextract information from it. Quantum error correction and simulation often make a more exacting demand: the ability to perform non-destructive measurements of specific correlations within that system. We realize such measurements by employing a protocol adapted from [S. Nigg and S. M. Girvin, Phys. Rev. Lett. 110, 243604 (2013)], enabling real-time selection of arbitrary register-wide Pauli operators. Our implementation consists of a simple circuit quantum electrodynamics (cQED) module of four highly-coherent 3D transmon qubits, collectively coupled to a high-Q superconducting microwave cavity. As a demonstration, we enact all seven nontrivial subset-parity measurements on our three-qubit register. For each we fully characterize the realized measurement by analyzing the detector (observable operators) via quantum detector tomography and by analyzing the quantum back-action via conditioned process tomography. No single quantity completely encapsulates the performance of a measurement, and standard figures of merit have not yet emerged. Accordingly, we consider several new fidelity measures for both the detector and the complete measurement process. We measure all of these quantities and report high fidelities, indicating that we are measuring the desired quantities precisely and that the measurements are highly non-demolition. We further show that both results are improved significantly by an additional error-heralding measurement. The analyses presented here form a useful basis for the future characterization and validation of quantum measurements, anticipating the demands of emerging quantum technologies.