Measurement of the Entanglement of Two Superconducting Qubits via State Tomography Matthias Steffen, et al. Science 313 , 1423 (2006); DOI: 10.1126/science.1130886

Department of Physics, University of California, Santa Barbara, Santa Barbara, California, United States
Science (Impact Factor: 33.61). 10/2006; 313(5792):1423-5. DOI: 10.1126/science.1130886
Source: PubMed


Demonstration of quantum entanglement, a key resource in quantum computation arising from a nonclassical correlation of states,
requires complete measurement of all states in varying bases. By using simultaneous measurement and state tomography, we demonstrated
entanglement between two solid-state qubits. Single qubit operations and capacitive coupling between two super-conducting
phase qubits were used to generate a Bell-type state. Full two-qubit tomography yielded a density matrix showing an entangled
state with fidelity up to 87%. Our results demonstrate a high degree of unitary control of the system, indicating that larger
implementations are within reach.

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    • "The Wigner function reconstruction has been also proposed to be of use to prove the quantum nature of the superposition of very massive particles [14] and even macroscopic opto-mechanical systems [15]. Furthermore, to prove entanglement in superconducting qubits, quantum state tomography has been applied [16]. In more technical terms the Wigner function is a quasi-probability distribution of states in phase space. "
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    ABSTRACT: We report on the theoretical investigation of Wigner distribution function (WDF) reconstruction of the motional quantum state of large molecules in de Broglie interference. De Broglie interference of fullerenes and as the like already proves the wavelike behaviour of these heavy particles, while we aim to extract more quantitative information about the superposition quantum state in motion. We simulate the reconstruction of the WDF numerically based on an analytic probability distribution and investigate its properties by variation of parameters, which are relevant for the experiment. Even though the WDF described in the near-field experiment cannot be reconstructed completely, we observe negativity even in the partially reconstructed WDF. We further consider incoherent factors to simulate the experimental situation such as a finite number of slits, collimation, and particle-slit van der Waals interaction. From this we find experimental conditions to reconstruct the WDF from Talbot interference fringes in molecule Talbot-Lau interferometry.
    New Journal of Physics 02/2012; 14(4). DOI:10.1088/1367-2630/14/4/045001 · 3.56 Impact Factor
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    • "In subsequent research, the state of each qubit was read out independently in a coupled two qubit system, and quantum entanglement was clearly shown by the quantum state tomography technique.34) By this observation, the quantum state is projected into the |0〉 and |1〉 states, losing a large amount of information. "
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    ABSTRACT: Intensive research on the construction of superconducting quantum computers has produced numerous important achievements. The quantum bit (qubit), based on the Josephson junction, is at the heart of this research. This macroscopic system has the ability to control quantum coherence. This article reviews the current state of quantum computing as well as its history, and discusses its future. Although progress has been rapid, the field remains beset with unsolved issues, and there are still many new research opportunities open to physicists and engineers.
    Proceedings of the Japan Academy Ser B Physical and Biological Sciences 04/2010; 86(4):275-292. DOI:10.2183/pjab.86.275 · 2.65 Impact Factor
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    • "This setup is promising for the study of circuit cavity Quantum Electrodynamics (circuit QED) [3] [4] [48]. With superconducting circuits one can now realize simple algorithms [43], quantum nondemolition measurements [49] and qudits [50], generate entangled states [51], test Bell's inequality [50] and the Leggett-Garg inequality [52]. "
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    ABSTRACT: Remarkable progress towards realizing quantum computation has been achieved using natural and artificial atoms as qubits. This article presents a brief overview of the current status of different types of qubits. On the one hand, natural atoms (such as neutral atoms and ions) have long coherence times, and could be stored in large arrays, providing ideal "quantum memories". On the other hand, artificial atoms (such as superconducting circuits or semiconductor quantum dots) have the advantage of custom-designed features and could be used as "quantum processing units". Natural and artificial atoms can be coupled with each other and can also be interfaced with photons for long-distance communications. Hybrid devices made of natural/artificial atoms and photons may provide the next-generation design for quantum computers.
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