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  • 1. Asplund, R.
    et al.
    Björk, Gunnar
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Bourennane, M.
    An expectation value expansion of Hermitian operators in a discrete Hilbert space2001In: Journal of Optics B-Quantum and Semiclassical Optics, ISSN 1464-4266, E-ISSN 1741-3575, Vol. 3, no 3, p. 163-170Article in journal (Refereed)
    Abstract [en]

    We discuss a real-valued expansion of any Hermitian operator defined in a Hilbert space of finite dimension N, where N is a prime number, or an integer power of a prime. The expansion has a direct interpretation in terms of the operator expectation values for a set of complementary bases. The expansion can be said to be the complement of the discrete Wigner function. We expect the expansion to be of use in quantum information applications since qubits typically are represented by a discrete, and finite-dimensional, physical system of dimension N = 2(p), where p is the number of qubits involved. As a particular example we use the expansion to prove that an intermediate measurement basis (a Breidbart basis) cannot be found if the Hilbert space dimension is three or four.

  • 2.
    Björk, Gunnar
    et al.
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Porta Mana, Piero Giovanni Luca
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    A size criterion for macroscopic superposition states2004In: Journal of Optics B-Quantum and Semiclassical Optics, ISSN 1464-4266, E-ISSN 1741-3575, Vol. 6, no 11, p. 429-436Article in journal (Refereed)
    Abstract [en]

    An operational measure to quantify the sizes of some 'macroscopic quantum superpositions', realized in recent experiments, is proposed. The measure is based on the fact that a superposition presents greater sensitivity in interferometric applications than its superposed constituent states. This enhanced sensitivity, or 'interference utility', may then be used as a size criterion among superpositions.

  • 3.
    Björk, Gunnar
    et al.
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Söderholm, Jonas
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Sanchez-Soto, L. L.
    Two-photon imaging and quantum holography2004In: Journal of Optics B-Quantum and Semiclassical Optics, ISSN 1464-4266, E-ISSN 1741-3575, Vol. 6, no 6, p. S478-S482Article in journal (Refereed)
    Abstract [en]

    We discuss when the use of entangled photon pairs in an imaging system can be simulated with a classically correlated source. In particular, we consider two recently proposed schemes with 'bucket detection' of one of the photons. We argue that these schemes give identical results for entangled states as for appropriately prepared classically correlated states.

  • 4.
    Heydari, Hoshang
    et al.
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Björk, Gunnar
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Mutual first-order coherence of phase-locked lasers2004In: Journal of Optics B-Quantum and Semiclassical Optics, ISSN 1464-4266, E-ISSN 1741-3575, Vol. 6, no 1, p. 46-53Article in journal (Refereed)
    Abstract [en]

    We argue that (first-order) coherence is a relative, and not an absolute, property. It is shown how feedforward or feedback can be employed to make two (or more) lasers relatively coherent. We also show that after the relative coherence is established, the two lasers will,stay relatively coherent for some time even if the feedforward or feedback loop has been turned off, enabling, e.g., demonstration of unconditional quantum teleportation using lasers.

  • 5. Inoue, S.
    et al.
    Björk, Gunnar
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Experimental demonstration of exposure-free imaging and contrast amplification2000In: Journal of Optics B-Quantum and Semiclassical Optics, ISSN 1464-4266, E-ISSN 1741-3575, Vol. 2, no 3, p. 338-345Article in journal (Refereed)
    Abstract [en]

    Interference between waves can yield quite counterintuitive results. Some time ago, Elizur and Vaidman (1993 Found. Phys. 23 987) proposed a way to detect the presence or absence of a perfectly absorbing object by probing it with light, without any photons being absorbed. Here we present experimental results where a photographic film's shape is imaged on an identical piece of film without exposing the first film. This is possible by using resonant imaging, i.e. by holding the film to be imaged inside an optical resonator. Using the same technique, it is also possible to image nearly transparent objects. The resonant imaging will 'amplify' small transmission variations rendering the nearly invisible clearly visible.

  • 6. Loo, F. Y.
    et al.
    Brusch, A.
    Sauge, Sebastien
    Allegrini, M.
    Arimondo, E.
    Andersen, N.
    Thomsen, J. W.
    Investigations of a two-level atom in a magneto-optical trap using magnesium2004In: Journal of Optics B-Quantum and Semiclassical Optics, ISSN 1464-4266, E-ISSN 1741-3575, Vol. 6, no 1, p. 81-85Article in journal (Refereed)
    Abstract [en]

    Magnesium atoms are cooled in a magneto-optical trap (MOT) using the 3s(2 1)S(0) --> 3s3p P-1(1) resonance transition. Magnesium is a simple atom which offers a unique possibility for comparison to the Doppler theory of laser cooling. We measure trap parameters as a function of laser-intensity, -detuning and magnetic field gradient. We find the main features well accounted for by Doppler theory, but temperature measurements gave significant higher values than predicted by the Doppler theory. We also observe radiation pressure effects within the MOT that limit the maximum achievable density. Comparing our results to predictions by models in the literature shows a good agreement. Recently we have improved our set-up and now trap about 150 x 10(6) atoms.

  • 7.
    Shen, Jianqi
    KTH, School of Information and Communication Technology (ICT), Centres, Zhejiang-KTH Joint Research Center of Photonics, JORCEP.
    A scheme of measurement of quantum-vacuum geometric phases in a noncoplanar fibre system2004In: Journal of Optics B-Quantum and Semiclassical Optics, ISSN 1464-4266, E-ISSN 1741-3575, Vol. 6, no 7, p. L13-L17Article in journal (Refereed)
    Abstract [en]

    We study the quantum-vacuum geometric phases resulting from the vacuum fluctuation of photon fields in a Tomita-Chiao-Wu noncoplanar curved fibre system, and suggest a scheme to test for the potential existence of such a vacuum effect. Since the signs of the quantum-vacuum geometric phases of left- and right-handed (LRH) circularly polarized light are opposite, the sum of the geometric phases at the vacuum level is necessarily zero in the fibre experiments performed previously by other authors. By using the present approach where a fibre made of gyroelectric media is employed, the quantum-vacuum geometric phases of LRH light cannot be exactly cancelled, and it may therefore be possible to test this experimentally.

  • 8. Trifonov, A.
    et al.
    Tsegaye, T.
    Björk, Gunnar
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Soderholm, J.
    Goobar, E.
    Atature, N.
    Sergienko, A. V.
    Experimental demonstration of the relative phase operator2000In: Journal of Optics B-Quantum and Semiclassical Optics, ISSN 1464-4266, E-ISSN 1741-3575, Vol. 2, no 2, p. 105-112Article in journal (Refereed)
    Abstract [en]

    We have experimentally demonstrated a realization of the two-mode relative phase operator introduced by Luis and Sanchez-Soto (Luis A and Sanchez-Soto L L 1993 Phys. Rev. A 48 4702). The relative phase distribution function was measured for a weakly excited relative phase eigenstate and weakly excited two-mode coherent states. The experiment is also (using the eigenstates) a demonstration of Heisenberg-limited interferometry.

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