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.
|Advisor:||Devoret, Michel H.|
|School Location:||United States -- Connecticut|
|Source:||DAI-B 79/05(E), Dissertation Abstracts International|
|Subjects:||Quantum physics, Physics|
|Keywords:||Quantum Computation, Quantum Information, Remote Entanglement, Superconducting Circuits, Superconducting Qubits, Ultra Low-Noise Measurement|
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