Abstract
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 faithfully extract information from it. Quantum error correction and simulation often make a more exacting demand: the ability to perform nondestructive measurements of specific correlations within that system. We realize such measurements by employing a protocol adapted from Nigg and 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 module of four highly coherent 3D transmon qubits, collectively coupled to a high- 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 backaction 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 nondemolition. We further show that both results are improved significantly by an additional error-heralding measurement. The analyses we present here form a useful basis for the future characterization and validation of quantum measurements, anticipating the demands of emerging quantum technologies.
- Received 2 June 2016
DOI:https://doi.org/10.1103/PhysRevX.6.031041
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Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Measurements play a special role in quantum mechanics and place fundamental limits on what can be known about a quantum system at any one time. Quantum measurement is most often discussed in the context of a two-state system, or qubit, such as the spin of an electron or the polarization of a photon. Developing a quantum computer, however, will require quantum error correction, which calls for subtle measurements of four, five, or even more qubits. Complicating matters further, researchers only want to know the state of qubits with certain properties: For example, is there an odd number of qubits in the state? Additionally, scientists wish to determine the quantum state after performing a measurement (i.e., the backaction). Here, we demonstrate an architecture and accompanying protocol uniquely suited to performing these measurements on ensembles of multiple qubits.
Our architecture builds on the success of superconducting three-dimensional transmon qubits, which exhibit excellent coherence times while still allowing high-fidelity control and measurement. We couple four of these qubits together via a high-Q superconducting resonator in such a way that they have negligible cross talk. We then use this resonator to mediate interactions between the qubits prior to directly measuring only one of them. Our algorithm is programmable to measure different properties of our three qubits. We quantitatively characterize each of these measurements, looking at both their accuracy as detectors and the fidelity of the quantum backaction. We distill this analysis into several metrics to benchmark the performance of our measurements.
We expect that our findings will be useful as quantum measurements become increasingly complex.