Steffen, M. et al. Measurement of the entanglement of two superconducting qubits via state tomography. Science 313, 1423-1425

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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    • "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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    • "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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