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Astronomical applications of quantum optics forextremely large telescopes

To cite this Article: , 'Astronomical applications of quantum optics for extremely largetelescopes', Journal of Modern Optics, 54:2, 191 - 197xxxx:journal To link to this article: DOI: 10.1080/09500340600742270URL: http://dx.doi.org/10.1080/09500340600742270

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© Taylor and Francis 2007

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Journal of Modern OpticsVol. 54, Nos. 2–3, 20 January–15 February 2007, 191–197

Astronomical applications of quantum opticsfor extremely large telescopes

C. BARBIERI*y, D. DRAVINSz, T. OCCHIPINTIy, F. TAMBURINIy,G. NALETTOy, V. DA DEPPOy, S. FORNASIERy, M. D’ONOFRIOy,

R. A. E. FOSBURYx, R. NILSSONz and H. UTHASz

yDepartment of Astronomy, University of Padova,Vicolo dell’ Osservatorio 2, Padova 35122, Italy

zLund Observatory, Box 43, Lund SE-22100, SwedenxSpace Telescope – European Coordinating Facility & European

Southern Observatory, Karl-Schwarzschild-2, Garching bei MunchenDE-85748, Germany

(Received 6 March 2006; in final form 7 April 2006)

A programme has been started to investigate photon properties that are notcurrently exploited in astronomical instruments, namely second- and higher-ordercoherence functions encoded in their arrival time, and the orbital angularmomentum. This paper expounds the first results achieved in the study of a novelastronomical photometer capable of pushing time tagging towards the picosecondregion. This conceptual device has been developed as a possible focal planeinstrument for the future OverWhelmingly Large Telescope (OWL) of theEuropean Southern Observatory. This instrument has been named QuantEYE,that is, the Quantum Eye of OWL.

1. Introduction

The frontiers of astronomy have expanded through observational breakthroughs.In the past, one major thrust in expanding parameter envelopes was the addition ofnew wavelength regions, and today almost all spectral regions are accessible toground and space telescopes. However, current astronomical instrumentationexploits the property of spatial coherence (imaging) or temporal coherence(spectroscopy) of the incoming photon stream. Beyond this first-order coherence,and encoded in the arrival times of the individual photons, lies detailed informationabout the emission mechanisms, such as stimulated emission (laser) or scattering.We recall the classic paper by Glauber [1] concerning second- and higher-ordercorrelation functions.

To explore this new realm of light, the time resolution and time taggingcapabilities of astronomical instruments must be pushed well beyond currentcapabilities. Numerous discoveries have been made with time resolution on thescale of milliseconds and slower: optical pulsars; lunar and planetary-ring

*Corresponding author. Email: cesare.barbieri@unipd.it

Journal of Modern Optics

ISSN 0950–0340 print/ISSN 1362–3044 online � 2007 Taylor & Francis

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DOI: 10.1080/09500340600742270

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occultations; rotation of cometary nuclei; pulsations from X-ray pulsars; cataclysmicvariable stars; pulsating white dwarfs; flickering high-luminosity stars; X-raybinaries; gamma-ray burst afterglows; and many others. A limit for such opticalstudies has been that CCD-like detectors do not readily allow frame rates faster than1–10ms, while photon-counting detectors either had low efficiency or photon-counting rates limited to no more than a few hundreds of kilohertz (see for instancethe pioneering work MANIA [2]).

Modern technology now permits us to go well beyond these achievements.Therefore, we have carried out a conceptual study for QuantEYE, the fastest timeresolution instrument available to astronomers. QuantEYE is in theory capable ofgoing where astronomy has not gone before, that is, into the domains of microsecondand nanosecond variability and beyond, with gigahertz photon count rates tomatch, all the way to the realm of quantum optics. QuantEYE will thus be capableof detecting and measuring photon-stream statistics, e.g. power spectra orautocorrelation functions. Such functions increase with the square of the intensity,implying an enormously increased sensitivity at future Extremely Large Telescopes(ELTs), over the existing 10m class telescopes.

These capabilities will enable detailed studies of millisecond pulsars; variabilityclose to black holes; surface convection on white dwarfs; non-radial oscillationspectra in neutron stars; fine structure on neutron-star surfaces; photon-gas bubblesin accretion flows; and possible free-electron lasers in the magnetic fields aroundmagnetars. Nanosecond-resolution photon-correlation spectroscopy will enablespectral resolutions exceeding R¼ 100 million (as probably required to resolvelaser-line emission around sources such as Eta Carinae). QuantEYE will have thepower to examine quantum statistics of photon arrival times (e.g. photon bunchingin time). It can also be adapted to more conventional high-speed astrophysicalproblems, such as stellar occultations by Kuiper Belt Objects, using small telescopes.A more complete description of the astrophysical problems that can be tackled byQuantEYE can be found elsewhere [3–5].

On the technical side, QuantEYE was designed [6] taking into account thecharacteristics which were foreseen in 2005 for the 100 m OverWhelmingly Large(OWL) telescope of the European Southern Observatory (ESO). Although the finalOWL design will differ from these adopted characteristics, our concept maintains itsfull scientific appeal, and can easily be adapted to different ELTs. Furthermore,given two distant ELTs, QuantEYE would permit a modern realization of theHanbury Brown–Twiss intensity interferometer, as shown in [3].

We wish to underline also that other important, and still unexplored,information could be encrypted in the photons arriving from a star.Electromagnetic radiation carries momentum: its linear momentum is associatedwith radiation pressure, while the orbital angular momentum (OAM) is usuallyassociated with the presence of a non-null component along the direction ofpropagation of the polarization vector, and is described by Laguerre–Gaussianmodes. Single photons can also carry OAM and a possible number of astrophysicalapplications emerge. A turbulent screen of interstellar medium presenting densityspikes, through and around which the light emitted from a maser beam has to pass,is thought to induce OAM on the photons with a mechanism similar to that

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produced by a spiral phase plate or a holographic phase plate. Also the radiationemitted by luminous pulsars and quasars may acquire the same properties intraversing discontinuous media in the immediate surroundings of these respectivesources or even interacting with the geometry of a compact rotating body such as aKerr Black Hole [7]. As an application of OAM to astronomy, we realized theprototype of a coronagraphic mask by imposing OAM on the light from a star, todetect the presence of close faint companions in double systems. Our device imposesLaguerre–Gaussian modes to the stellar light by means of a suitable holographicmask which acts as a coronagraphic mask [8]. This application does not require alarge telescope, and we plan to report our results in a future paper. In the same lineof experiment with the POAM, we recall the papers by Foo et al. and bySwartzlander [9, 10].

2. Instrument characteristics

The optical design of the proposed instrument has been driven by two main factors.The first is obviously the OWL telescope characteristics and performance: OWL hasa 100m aperture with an f/6 focal ratio; it is fully corrected for geometric aberrationsand is limited by seeing (in the absence of an adaptive optics system). On the basis ofthe present experience with large telescopes in ‘normal’ seeing conditions, we haveassumed that OWL will be able to concentrate a large fraction of the light comingfrom a point-like source at infinity within a 1 arc sec image over a fairly satisfactorypercentage of the observing time. Just to have an idea of the spot size at the telescopefocus, we can observe that owing to the 600m telescope focal length, 1 arc sec field ofview gives a 3mm diameter spot diameter at the OWL focus.

The second factor driving the QuantEYE design is the limited selection ofpresently available very fast photon counting detectors. At present, the best selectionis the single photon avalanche photodiode (SPAD), but unfortunately availableSPAD ‘arrays’ consist at most of active cells greatly separated by dead zones,because of a cross-talking problem among contiguous cells when high speeddetection is required. Since this type of format is useless for our application,we decided to use a sort of ‘distributed’ detector array that is a sparse array ofsingle SPADs. After this choice, the instrument optical design followed as aconsequence.

In brief, the optical design of QuantEYE is based on a preliminary collimation ofthe beam after the OWL focus, and on a following subdivision of the system pupilinto N�N sub-pupils, each of them focused on a single SPAD (so giving a total ofN2 SPADs). In this way, since the detector ‘distributed array’ is essentially samplingthe telescope pupil, a system of N2 parallel smaller telescopes is realized, each oneacting as a fast photometer (i.e. with no imaging capability).

In more detail, QuantEYE consists of an inverse Cassegrain telescope with600mm focal length and 100mm diameter ( f/6) that collimates the light beamafter the focus of the OWL telescope (figure 1). After collimation, the radiationbeam has an annular shape of about 100mm maximum radius. Here, the possibilityis foreseen of inserting filters and/or polarizers stacked in a suitable location on

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the side of the telescope (figure 2). The filtered beam is then collected by a 10� 10lenslet array sampling the instrument pupil. Each lenslet is an ad-hoc compoundsystem of two doublets with a 10� 10mm2 section and 10mm focal length ( f/1). Thedoublet conceptual design adopts only spherical surfaces for ease of fabrication.The 1/60 demagnification of this system reduces the 3mm diameter (1 arc sec) focusof OWL to 50 mm, corresponding to the baseline SPAD active area. To direct the lensfocus onto the SPADs, an optical fibre link has been assumed. Also different opticalcouplings, entirely avoiding optical fibres, not shown here for simplicity, have beenstudied and shown feasible.

Figure 2. 3D view of the focal reducer/collimator. Light entering the baffle cone is collimatedby the telescope and is collected by the lenslet array. Each lenslet is coupled to a singleSPAD via optical fibres. The cylinder on the side of the telescope is the box accommodatingthe stack of filters and polarizers. (The colour version of this figure is included in the onlineversion of the journal.)

Figure 1. Optical concept of QuantEYE. (a) Collimator–lenslet system magnifies 1/60 times(collimator focal length¼ 600mm, lens focal length¼ 10mm), giving a nominal spot sizeof 50 mm for a 1 arc sec source. For the sake of simplicity, only one lenslet of the whole 10� 10array is shown. (The colour version of this figure is included in the online version of thejournal.)

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To gain an idea of the optical performance of this instrument, figure 3 shows a plot

of the extended source encircled energy. It is possible to see that about 90% of the

1 arc sec source energy falls within the 50 mm diameter of the fibre core. In this

simulation, the extended source is assumed to have a uniform intensity: since in the

real case the illumination distribution is of Gaussian type, we expect a still higher

percentage of energy flux to enter the fibre.A second moving detector head has also been foreseen to observe a comparison

star, which can span the whole 3 arcmin scientific field of OWL. The instrument

working spectral range is 400–900 nm. In this spectral region, SPADs exist with a

quantum efficiency better than 40%, with very good timing accuracy (better than

100 ps), and with very low dark count rate (of the order of 100 c/s): putting together

the 100 SPADs, a photon counting rate up to 1GHz can be obtained.In order to preserve this great amount of data, QuantEYE has a central storage

unit with a minimum capacity of 50 terabytes connected to an a posteriori analysis

system with a high bandwidth transport channel. The arrival time of each photon is

given as input to an asynchronous post-processor, which guarantees data integrity

for the following scientific investigation. This a posteriori processing unit is a

cluster of CPUs. Specific parallel algorithms will work together, optimizing the

computations of the high-order correlation functions between the time tagged

photons. Furthermore, QuantEYE performs real-time correlation functions among

different detectors (online correlation unit). The number of detectors to be analysed

in real time depends on the computational power of reprogrammable logic

circuits and processors, which is determined by the technological progress of

CPUs, FPGAs (field programmable gate arrays) and ASICs (application-specific

integrated circuits).

Figure 3. Extended source encircled energy plot: percentage of energy falling within a circleof given radius for a uniformly illuminated 1 arcsec extended circular source. The verticaldashed line indicates the radius of the fibre core.

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Moreover, to assure a precise time reference, the SPADs are coupled to ahydrogen maser clock, allowing continuous photon counting with a time taggingprecision better than 100 ps for several hours (figure 4).

The proposed solution of sampling the telescope pupil and saving the photoninformation in 10� 10 parallel channels greatly reduces the limiting dead timeproblem of the detectors: in such a way a fixed-area very high speed photometer witha tremendous dynamic range (from the 5th to the 30th stellar magnitude, a factor1010 in brightness) can be obtained. In addition, it is likely that in the next few yearsthe development of new detectors and new optical clocks can push the timeresolution into the picosecond region for several hours of continuous operation.

A final consideration is relative to the analysis of the huge, multidimensionaldatabase generated by QuantEYE. The necessary computational power is very large,and new algorithms have to be developed: to this end, we hope that in the futurequantum computing and quantum algorithms might be available, greatly improvingthe computational capabilities.

3. Conclusions

The QuantEYE instrument proposed as a focal plane instrument for OWL has beenbriefly described together with its anticipated capabilities. This instrument isdedicated to the very high speed observation of many active phenomena with aphoton counting capability up to 1GHz, that is a programme to investigate photonproperties that are not currently exploited in astronomical instruments. The opticaldesign applied to this instrument, in which the incoming signal is sampled

Figure 4. The full QuantEYE scheme. (The colour version of this figure is included in theonline version of the journal.)

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by 100 parallel channels, allows one to overcome the present technological limitationof detector speed. However, we are confident that in the near future, with thepossibility of using SPAD arrays with contiguous sensitive cells, it will be reasonableto consider simpler optical designs allowing not only fast photometry, but alsoimaging of the observed objects.

Acknowledgements

The QuantEYE project has been partly supported by ESO, by the Swedish ResearchCouncil, the Royal Physiographical Society in Lund, and the University of Padova.We acknowledge valuable discussions with Hans-Gunter Ludwig, Bo Nilsson,and Torbjorn Wiesel in Lund, and Antonio Bianchini, Sandro Centro, SergioOrtolani and Luca Zampieri in Padova. We are greatly indebted to severalpersons who provided advice and material: Dr M. Perryman (ESTEC), Dr G. Bianco(ASI Matera), De. I. Prochazka (Czech Technical University in Prague), Prof.S. Cova (Politecnico Milano), Dr G. Biscotti (Becker&Hickle).

References

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[3] D. Dravins, C. Barbieri, V. Da Deppo, D. Faria, S. Fornasier, R.A.E. Fosbury,L. Lindegren, G. Naletto, R. Nilsson, T. Occhipinti, F. Tamburini, H. Uthas,L. Zampieri, QuantEYE quantum optics instrumentation for astronomy, OWLInstrument Concept Study, ESO document OWL-CSR-ESO-00000-0162 (2005).

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Quantum Coronagraphic Mask. The Photon Orbital Angular Momentum and itsApplication to Astronomy, Memorie Societa Astronomica Italiano, 75 282 (2005).

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