2,239 research outputs found
Demonstration of Universal Parametric Entangling Gates on a Multi-Qubit Lattice
We show that parametric coupling techniques can be used to generate selective
entangling interactions for multi-qubit processors. By inducing coherent
population exchange between adjacent qubits under frequency modulation, we
implement a universal gateset for a linear array of four superconducting
qubits. An average process fidelity of is estimated for
three two-qubit gates via quantum process tomography. We establish the
suitability of these techniques for computation by preparing a four-qubit
maximally entangled state and comparing the estimated state fidelity against
the expected performance of the individual entangling gates. In addition, we
prepare an eight-qubit register in all possible bitstring permutations and
monitor the fidelity of a two-qubit gate across one pair of these qubits.
Across all such permutations, an average fidelity of
is observed. These results thus offer a path to a scalable architecture with
high selectivity and low crosstalk
Realizing a Deterministic Source of Multipartite-Entangled Photonic Qubits
Sources of entangled electromagnetic radiation are a cornerstone in quantum
information processing and offer unique opportunities for the study of quantum
many-body physics in a controlled experimental setting. While multi-mode
entangled states of radiation have been generated in various platforms, all
previous experiments are either probabilistic or restricted to generate
specific types of states with a moderate entanglement length. Here, we
demonstrate the fully deterministic generation of purely photonic entangled
states such as the cluster, GHZ, and W state by sequentially emitting microwave
photons from a controlled auxiliary system into a waveguide. We tomographically
reconstruct the entire quantum many-body state for up to photonic modes
and infer the quantum state for even larger from process tomography. We
estimate that localizable entanglement persists over a distance of
approximately ten photonic qubits, outperforming any previous deterministic
scheme
Coarse-graining in retrodictive quantum state tomography
Quantum state tomography often operates in the highly idealised scenario of
assuming perfect measurements. The errors implied by such an approach are
entwined with other imperfections relating to the information processing
protocol or application of interest. We consider the problem of retrodicting
the quantum state of a system, existing prior to the application of random but
known phase errors, allowing those errors to be separated and removed. The
continuously random nature of the errors implies that there is only one click
per measurement outcome -- a feature having a drastically adverse effect on
data-processing times. We provide a thorough analysis of coarse-graining under
various reconstruction algorithms, finding dramatic increases in speed for only
modest sacrifices in fidelity
Incomplete quantum process tomography and principle of maximal entropy
The main goal of this paper is to extend and apply the principle of maximum
entropy (MaxEnt) to incomplete quantum process estimation tasks. We will define
a so-called process entropy function being the von Neumann entropy of the state
associated with the quantum process via Choi-Jamiolkowski isomorphism. It will
be shown that an arbitrary process estimation experiment can be reformulated in
a unified framework and MaxEnt principle can be consistently exploited. We will
argue that the suggested choice for the process entropy satisfies natural list
of properties and it reduces to the state MaxEnt principle, if applied to
preparator devices.Comment: 8 pages, comments welcome, references adde
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