Comparing and combining measurement-based and driven-dissipative entanglement stabilization

We demonstrate and contrast two approaches to the stabilization of qubit entanglement by feedback. Our demonstration is built on a feedback platform consisting of two superconducting qubits coupled to a cavity which are measured by a nearly-quantum-limited measurement chain and controlled by high-speed classical logic circuits. This platform is used to stabilize entanglement by two nominally distinct schemes: a "passive" reservoir engineering method and an "active" correction based on conditional parity measurements. In view of the instrumental roles that these two feedback paradigms play in quantum error-correction and quantum control, we directly compare them on the same experimental setup. Further, we show that a second layer of feedback can be added to each of these schemes, which heralds the presence of a high-fidelity entangled state in realtime. This "nested" feedback brings about a marked entanglement fidelity improvement without sacrificing success probability.Comment: 40 pages, 12 figure

Confining the state of light to a quantum manifold by engineered two-photon loss

Physical systems usually exhibit quantum behavior, such as superpositions and entanglement, only when they are sufficiently decoupled from a lossy environment. Paradoxically, a specially engineered interaction with the environment can become a resource for the generation and protection of quantum states. This notion can be generalized to the confinement of a system into a manifold of quantum states, consisting of all coherent superpositions of multiple stable steady states. We have experimentally confined the state of a harmonic oscillator to the quantum manifold spanned by two coherent states of opposite phases. In particular, we have observed a Schrodinger cat state spontaneously squeeze out of vacuum, before decaying into a classical mixture. This was accomplished by designing a superconducting microwave resonator whose coupling to a cold bath is dominated by photon pair exchange. This experiment opens new avenues in the fields of nonlinear quantum optics and quantum information, where systems with multi-dimensional steady state manifolds can be used as error corrected logical qubits

Demonstrating a superconducting dual-rail cavity qubit with erasure-detected logical measurements

A critical challenge in developing scalable error-corrected quantum systems is the accumulation of errors while performing operations and measurements. One promising approach is to design a system where errors can be detected and converted into erasures. A recent proposal aims to do this using a dual-rail encoding with superconducting cavities. In this work, we implement such a dual-rail cavity qubit and use it to demonstrate a projective logical measurement with erasure detection. We measure logical state preparation and measurement errors at the $0.01\%$-level and detect over $99\%$ of cavity decay events as erasures. We use the precision of this new measurement protocol to distinguish different types of errors in this system, finding that while decay errors occur with probability $\sim 0.2\%$ per microsecond, phase errors occur 6 times less frequently and bit flips occur at least 170 times less frequently. These findings represent the first confirmation of the expected error hierarchy necessary to concatenate dual-rail erasure qubits into a highly efficient erasure code

Flying Qubit Operations in Superconducting Circuits

The quantum non-demolition (QND) measurement process begins by entangling the system to be measured, a qubit for example, with an ancillary degree of freedom, usually a system with an infinite-dimensional Hilbert space. The ancilla is amplified to convert the quantum signal into a measurable classical signal. The continuous classical signal is recorded by a measurement apparatus; a discrete measurement outcome is recovered by thresholding the integrated signal record. Measurements play a central role in technologies based on quantum theory, like quantum computation and communication. They form the basis for a wide range of operations, ranging from state initialization to quantum error correction. Quantum measurements used for quantum computation must satisfy three essential requirements of being high fidelity, quantum non-demolition and efficient. Satisfying these criteria necessitates control over all the parts of the quantum measurement process, especially generating the ancilla, entangling it with the qubit and amplifying it to complete the measurement. For superconducting quantum circuits, a promising platform for realizing quantum computation, a natural choice for the ancillae are modes of microwave-frequency electromagnetic radiation. In the paradigm of circuit quantum electrodynamics (cQED) with three-dimensional circuits, the most commonly used ancillae are coherent states, since they are easy to generate, process and amplify. Using these flying coherent states, we present results for achieving QND measurements of transmon qubits with fidelities of F> 0.99 and efficiencies of η = 0.56 ± 0.01. By also treating the measurement as a more general quantum operation, we use the ancillae as carriers of quantum information to generate remote entanglement between two transmon qubits in separate cavities. By using microwave single photons as the flying qubits, it is possible to generate remote entanglement that is robust to loss since the generation of entanglement is uniquely linked to a particular measurement outcome. We demonstrate, in a single experiment, the ability to efficiently generate and detect single microwave photons and use them to generate robust remote entanglement between two transmon qubits. This operation forms a crucial primitive in modular architectures for quantum computation. The results of this thesis extend the experimental toolbox at the disposal to superconducting circuits. Building on these results, we outline proposals for remote entanglement distillation as well as strategies to further improve the performance of the various tools