Manipulating phase of digitized photons: An application to radio astronomy

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  • Vistas in Astronomy, Vol. 37, pp. 555-558, 1993 Printed in Great Britain. All rights rese~ed.

    0083-6656/93 $24.00 @ 1993 Prxgamon Press Ltd

    MANIPULATING PHASE OF DIGITIZED PHOTONS: AN APPLICATION TO RADIO ASTRONOMY

    Tsuneaki Daishido, Kuniyuki Asuma, Kazuhiko Nishibori, Junichi Nakajima, Eiichiro Otobe, Naoki Watanabe, Yoshitaka Aramaki,

    Hiromi Kobayashi, Tomohiro Saito, Naoki Tanaka and Tomoyuki Hoshikawa

    Astrophysics Group, Department of Science, School of Education, Waseda University, 1-6-1 Nishiwaseda, Shinjuku-ku, Tokyo 169, Japan

    . . .noises.. .became frozen on the air.

    But just now, the rigours of winter being over ...,

    these noises are meltig, and you can hear them.

    Gargantua and Pantagruel

    (Francois Rabelais /trans. by J .M.Cohen)

    INTRODUCTION

    We dist ingush detect ion and sampling in measurement. Sampling is a

    kind of propagat ing stage for photons. We manipulate phase of sampled

    photons for the observat ion in radio astronomy.

    A photon behaves as a part ic le in emission and absorption, while it

    behaves as a wave in propagation. Phase information is lost in detect ion

    ( power measurement process). But it is not lost in sampling( ampl itude

    measurement process). These features are true even in VLBI( Very Long

    Basel ine Interferometer; a kind of recording and playback interferome-

    ter) or in the digital interferometer of Waseda University. A photon am-

    pl i tude could be recorded on the two separate devices s imultaneously in

    a way that phase information is preserved, with the format of analogue or digital. A recorded photon is a "frozen photon". However, one cannot say which device does a photon pickup. A "frozen photon" could be re-

    garded to be under "propagation". Palybacking the two devices melts a photon, and it is possible to be interfered itself. Result ing fringe

    pattern is the same as the one obtained by usual interferometer.

  • 556 Z D~shidoet~.

    PHOTON NUMBER UNCERTAINTY IN THE DIGITAL INTERFEROMETER

    In the simple two element interferometer, it could not be determin- ed that a photon arrives which antenna A nor B. It is valid even in the digital interferometer of Waseda University or the recording interferom- eter as VLBI. It is understood as follows.

    Since there is an uncertainty relation of A NA ~ = 1/2 between photon number N and phase ~ , precise measurement in phase ~ requires large uncertaity in photon number N.

    Y Y

    digitising or I I digitising or recording A .recordin S B

    Which antenna does

    a photon p ickup ?

    I digital or recorded dataJ A

    digital or recorded data B

    f r inge pat tern

    AMPLIFIRES AND AMPLITUDE RECORDINGS OF BOSON AND FERMION

    Classical waves which we observe by the radio interferometers are discribed quantum mechanicaly as coherent states. The coherent state is a superposition of photon number states as

    [ a > = 5 , i n >(e . . . . /n ! ) 1"~

    which gives the probability of Poisson distribution when one measures

  • Digitized Photons 557

    the number of photons. This means that the simultaneous amplitude re-

    cording on separate two devices is not prohibited even for the very weak

    rad io waves as one photon per phase space .

    Thus , a " f rozen photon" on the separate two magnet ic tape cou ld be

    in ter fe red . Th is i s on ly poss ib le fo r boson . For fe rmion , on the cont ra - ry , ampl i f ie rs nor ampl i tude recod ings does not ex is t accord ing to Pau l i

    ' s p r inc ip le , though the beaut i fu l in ter fe rometr i c exper iment by H .Rauch

    shows the ex is tence o f phase sh i f t fo r a neut ron .

    Interferometer

    Amplifier

    Amplitude Recording

    Boson Fermion

    possible possible

    possible impossible

    possible impossible

    DIGITAL PHASE MANIPULATION IN WASEDA ARRAY TELESCOPE

    Phase manipulation is done by multiplying the complex number in the

    Digital Lens( Complex amplitude equalizer + 2D FFT processor ), and it

    has capabilty of real time digital imaging . The radio patrol camera project was started at Waseda University in

    1979 using the 8 element radio interferometer, and the 1 dimentinal dig- ital processor was developed in 1984-85. The following FFT digital im- age was obtained in 1985 by MkI Digital Lens.

    Gain Pattern by the Osbervation of the Sun

    Observat ion by MkI D ig i ta l In ter fe rometer (1985)

  • 558 Z D~shidoetaL

    Two dimentinal Radio Patrol Camera Project has been started in 1989 with the over all array size of 20m x 20m. Each element is a 2.4 m an-

    tenna. The process speed of the Digital Lens is 150 -200 GOPS( Giga Op-

    erat ion Per Second), which is I00 times faster than super computer. The

    present radio interferometer of 8x8=64 element watchs 64 direct ions si-

    multaneously with the sensit iv i ty of 20m radio telescope at 10.6 GHz.

    Sky survey to search radio novae will start in 1993.

    Note: D ig i ta l Phase Cont ro le r D ig i ta l complex mul t ip l ie r i s a phase cont ro le dev ise .

    exp( i L + i2) = exp( i ~)exp( i2) = (a~ + ib,)(a2 + ibm)

    . . . . . >

    A/D

    A/D

    x X

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