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  • 1.
    Almlöf, Jonas
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Quantum Electronics and Quantum Optics, QEO.
    Björk, Gunnar G. E.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Quantum Electronics and Quantum Optics, QEO.
    Fidelity as a figure of merit in quantum error correction2013In: Quantum information & computation, ISSN 1533-7146, Vol. 13, no 1-2, p. 0009-0020Article in journal (Refereed)
    Abstract [en]

    We discuss the fidelity as a figure of merit in quantum error correction schemes. We show that when identifiable but uncorrectable errors occur as a result of the action of the channel, a common strategy that improves the fidelity actually decreases the transmitted mutual information. The conclusion is that while the fidelity is simple to calculate and therefore often used, it is perhaps not always a recommendable figure of merit for quantum error correction. The reason is that while it roughly speaking encourages optimisation of the "mean probability of success", it gives no incentive for a protocol to indicate exactly where the errors lurk. For small error probabilities, the latter information is more important for the integrity of the information than optimising the mean probability of success.

  • 2. Chen, Yuntian
    et al.
    Björk, Gunnar
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Generation and detection of photonic qutrit states by linear optics2008In: Quantum information & computation, ISSN 1533-7146, Vol. 8, no 5, p. 386-398Article in journal (Refereed)
    Abstract [en]

    We address the problem of generation and detection of the four mutually unbiased biphoton polarization-qutrit bases by linear optics. First, the generation of the bases is studied. Our numeric results show that the linear optics method can be used to generate the 4 mutually unbiased basis qutrit states probabilistically with high fidelity. Second, we investigate whether or not linear polarization-optics components are sufficient to realize the simultaneous detection of the qutrit states forming a complete basis. Analytical results show that every state in two of the bases, namely only half of the 4 mutually unbiased bases qutrit states can be identified.

  • 3. Heydari, Hoshang
    et al.
    Björk, Gunnar
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Concurrence and the Schwarz inequality2006In: Quantum information & computation, ISSN 1533-7146, Vol. 6, no 05-apr, p. 455-460Article in journal (Refereed)
    Abstract [en]

    We show that generalized concurrence is closely related to, and can be derived from, the Schwarz inequality. This connection places concurrence in a geometrical and functional-analytical setting.

  • 4.
    Heydari, Hoshang
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Björk, Gunnar
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Entanglement tensor for a general pure multipartite quantum state2005In: Quantum information & computation, ISSN 1533-7146, Vol. 5, no 2, p. 146-155Article in journal (Refereed)
    Abstract [en]

    We propose an entanglement tensor to quantitatively compute the entanglement of a general pure multipartite quantum state. We compare the ensuing tensor with the concurrence for bipartite state and apply the tensor measure to some interesting examples of entangled three-qubit and four-qubit states. It is shown that in defining the degree of entanglement of a multi-partite state, one needs to make assumptions about the willingness of the parties to cooperate. For such states our tensor becomes a measure of generalized entanglement of assistance. We also discuss the degree of entanglement and the concurrence of assistance of two generic multi-qubit states.

  • 5. Ramos, R. V.
    et al.
    Karlsson, Anders
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Numerical analysis of entanglement properties of density matrices in C-2 circle times C-2 systems2002In: Quantum information & computation, ISSN 1533-7146, Vol. 2, no 3, p. 222-239Article in journal (Refereed)
    Abstract [en]

    Quantum entanglement is an enigmatic and powerful property that has attracted much attention due to its usefulness in new ways of communications, like quantum teleportation and quantum key distribution. Much effort has been done to quantify entanglement. Indeed, there exist some well-established separability criterion and analytical formulas for the entanglement of bipartite systems. In both, the crucial element is the partial transpose of the density matrix. In this paper, we show numerically that one can also have information about the entanglement of bipartite state, in C(2)circle timesC(2), without looking at the partial transpose. We furthermore study properties of disentanglement operation, as well as properties of the relative entropy.

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