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arXiv:1205.2587 · Physical Review Letters

Hyperentanglement-Enabled Direct Characterization of Quantum Dynamics

Trent Graham, Julio T. Barreiro, Masoud Mohseni, Paul G. Kwiat

Published Feb 5, 2013• 34 citations
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Abstract

We use hyperentangled photons to experimentally implement an entanglement-assisted quantum process tomography technique known as direct characterization of quantum dynamics. Specifically, hyperentanglement-assisted Bell-state analysis enabled us to characterize a variety of single-qubit quantum processes using far fewer experimental configurations than are required by standard quantum process tomography. Furthermore, we demonstrate how known errors in Bell-state measurement may be compensated for in the data analysis. Using these techniques, we have obtained single-qubit process fidelities over 98% but with one-third the number of experimental configurations required for standard quantum process tomography. Extensions of these techniques to multiqubit quantum processes are discussed.

DOI
10.1103/physrevlett.110.060404

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We use hyperentangled photons to experimentally implement an entanglement-assisted quantum process tomography technique known as direct characterization of quantum dynamics. Specifically, hyperentanglement-assisted Bell-state analysis enabled us to characterize a variety of single-qubit quantum processes using far fewer experimental configurations than are required by standard quantum process tomography. Furthermore, we demonstrate how known errors in Bell-state measurement may be compensated for in the data analysis. Using these techniques, we have obtained single-qubit process fidelities over 98% but with one-third the number of experimental configurations required for standard quantum process tomography. Extensions of these techniques to multiqubit quantum processes are discussed.
Direct characterization of quantum dynamics (DCQD) introduced by Mohseni and Lidar [Phys. Rev. Lett. 97, 170501 (2006)] offers an effective way for estimation of quantum dynamical systems. Employing single-photon two-qubit states, we experimentally implement DCQD of one-qubit dynamics. The requirement of two-body interactions is thus eliminated such that the algorithm can be realized more conveniently. Both DCQD and standard quantum process tomography (SQPT) measurements are performed for several processes. The data from both methods show good agreement. Although the average process fidelities of the data obtained via DCQD are a little lower than that of SQPT, the number of experimental configurations is quadratically reduced evidencing the quadratic advantage of DCQD scheme. Partial information of the quantum dynamics can also be directly extracted via DCQD with fewer measurements.
We experimentally implement direct characterization of quantum dynamics (DCQD) algorithm of a single qubit proposed in M. Mohseni and D. A. Lidar [Phys. Rev. Lett. 97, 170501 (2006)], which does not require quantum state tomography. The number of experimental configurations is reduced by a factor of 2 compared with that of standard quantum process tomography, which can be increased four times with an ideal Bell state analyzer. We combine the DCQD algorithm with maximum-likelihood estimation to give the physical characterization of quantum dynamics. It can also be applied to obtaining partial information about quantum dynamics.
We report on the implementation of a quantum process tomography technique known as direct characterization of quantum dynamics applied on coherent and incoherent single-qubit processes in a system of trapped (40)Ca(+) ions. Using quantum correlations with an ancilla qubit, direct characterization of quantum dynamics reduces substantially the number of experimental configurations required for a full quantum process tomography and all diagonal elements of the process matrix can be estimated with a single setting. With this technique, the system's relaxation times T(1) and T(2) were measured with a single experimental configuration. We further show the first, complete characterization of single-qubit processes using a single generalized measurement realized through multibody correlations with three ancilla qubits.
The characterization of quantum dynamics is a fundamental and central task in quantum mechanics. This task is typically addressed by quantum process tomography (QPT). Here we present an alternative "direct characterization of quantum dynamics" (DCQD) algorithm. In contrast to all known QPT methods, this algorithm relies on error-detection techniques and does not require any quantum state tomography. We illustrate that, by construction, the DCQD algorithm can be applied to the task of obtaining partial information about quantum dynamics. Furthermore, we argue that the DCQD algorithm is experimentally implementable in a variety of prominent quantum-information processing systems, and show how it can be realized in photonic systems with present day technology.