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Possible applications of a novel type of photon counting instrument for Intensity Interferometry observations Intensity Interferometry observations

Giampiero Naletto University of Padova

Workshop on Stellar Intensity Interferometry p y y Salt Lake City 29‐30 January 2009

Introduction

During the last years we realized in Padova two similar instruments, AquEYE and IquEYE, for astronomical applications . They are essentially extremely fast photon counters, with the capability of time tagging the collected photons with a 50 ps time accuracy and storing all the timing data in a mass memory. This type of instrument is really versatile because it allows to t i d d tl ith di t t t l if it bl l k

• perate independently with distant telescopes if a suitable clock

synchronization can be obtained. We are planning to further develop this type of instruments for We are planning to further develop this type of instruments for possible applications that can range from “quantum”

• bservations with future ELTs, as measurement of second and

, higher order correlation functions from remote light sources, to intensity interferometry with existing telescopes as VLT and Keck.

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The team

Many people are participating to the realization of this project: realization of this project:

• Univ. Padova: C. Barbieri, I. Capraro,
• G. Naletto, T. Occhipinti, E. Verroi, P.

, p , , Zoccarato, V. Da Deppo, C. Facchinetti, C. Germanà, E. Giro, M. Parrozzani, F. Tamburini, M. Zaccariotto, L. Zampieri INAF R A Di P l INAF Rome: A. Di Paola, INAF Cagliari: P. Bolli, C. Pernechele ll INAF Catania: S. Billotta, G. Bonanno, Collaborations: D. Dravins (Lund), A. C d (Lj blj )

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Cadez (Ljubljana)

Outline

• Some history: QuantEYE
• Some history: QuantEYE
• Description of AquEYE and of some of the obtained results
• Description of Iq EYE and of some of the obtained res lts
• Description of IquEYE and of some of the obtained results

(very preliminary)

• Results of the Joint Asiago Ljubljana Crab pulsar observation
• Results of the Joint Asiago‐Ljubljana Crab pulsar observation

(preliminary)

• Instrument present limitations and possible ways to overcome

Instrument present limitations and possible ways to overcome them

• Future applications

pp

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QuantEYE proposal

In Sept. 2005, we completed a study (QuantEYE, the ESO Quantum Eye) in the frame of the studies for the 100 m OWL telescope. The main goal of the study was to demonstrate the possibility to reach the ps time resolution needed to bring the ps time resolution needed to bring quantum optics concepts into the astronomical domain, with two main , scientific aims in mind: ‐Measure the entropy of the light through the statistics of the photon time of arrival (TOA) ‐ Demonstrate the feasibility of HBTII

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Why studying the photon time statistics ?

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Why Extremely Large Telescopes?

The above mentioned quantum correlations are fully developed

• n time scales of the order of the inverse optical bandwidth For
• n time scales of the order of the inverse optical bandwidth. For

instance, with the very narrow band pass Δλ = 0.1 nm in the visible, through a definite polarization state, typical time scales are 10 ps. However, the photon flux is very weak even from bright stars, so that only Extremely that only Extremely Large Telescopes (ELTs) can bring Quantum g Q Optical effects in the astronomical reaches.

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QuantEYE

QuantEYE was conceived for measuring second‐ and higher‐order correlation functions in the collected photon stream (up to 1 GHz) from OWL with the highest time resolution (better than 0 1 ns) time resolution (better than 0.1 ns).

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Key limitation: the detector

The most critical point, and driver for the possible optical designs

• f QuantEYE was the availability of very fast and accurate
• f QuantEYE, was the availability of very fast and accurate

photon counting detectors.

• Imaging PC detectors (ICCD, ICMOS, MCP) either do not allow

g g ( , , ) fast time tagging of the detected events, or have a rather low maximum total count rate

• Non‐imaging PC detectors (PMT, SPADs) either have a

relatively low QE, or have a small sensitive area SPADs are preferable: a 50 ps time resolution with count rates as high as 10 MHz can be obtained, with standard voltages and QE. f h l ld b bl f h However, even if the time resolution could be acceptable for this application, the total count rate was still two orders of magnitude smaller than what was necessary ! magnitude smaller than what was necessary !

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Solution: splitting the problems …

To suitably design the system and to overcome both the SPAD limitations and the difficulties of a reasonable optical design limitations and the difficulties of a reasonable optical design (coupling the 100 m pupil / 600 m focal length of OWL with a single 50 μm detector !), we decided to split the problems. In practice, we designed QuantEYE subdividing the system pupil into N × N sub‐pupils, each of them focused on a single SPAD (so giving a total of N2 distributed SPAD's). In such a way, a “sparse” SPAD array (SSPADA) coping with the i d hi h ld b b i d required very high count rate could be obtained. The SSPADA is sampling the telescope pupil, so a system of N2 parallel smaller telescopes is realized each one acting as a fast parallel smaller telescopes is realized, each one acting as a fast photometer.

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QuantEYE optical design

Schematic view of the telescope pupil subdivision

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Advantages of this optical design

Th l b l t t i t ti ti ll i d b f t N2

• The global count rate is statistically increased by a factor N2

with respect to the maximum count rate of a single SPAD. In the assumption of having N = 10 (100 SPAD's), the global count the assumption of having N 10 (100 SPAD s), the global count rate becomes 1 GHz (one photon every 100 ns on each SPAD)

• Simpler optical design

Simpler optical design

• Detector redundancy
• By suitable cross correlations of the detected signal a digital
• By suitable cross‐correlations of the detected signal, a digital

HBT intensity interferometer is realized among a large number

• f different sub‐apertures across the full OWL pupil

p p p

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Overall QuantEYE block diagram

The overall system: two heads controls storage time unit The overall system: two heads, controls, storage, time unit.

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AquEYE

While expecting the realization

• f the future E‐ELT we decided
• f the future E ELT, we decided

to apply the described concept to realize a much smaller version of the instrument, compatibly also with the f il bl f d few available funds. We named this instrument AquEYE the Asiago quantum AquEYE, the Asiago quantum eye: it has been applied to the AFOSC camera of the Asiago‐ Cima Ekar (Italy) 182 cm Telescope.

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AquEYE optomechanical design

A simple way of realizing this small prototype was to consider an

• ptical configuration in which the telescope pupil is divided in
• ptical configuration in which the telescope pupil is divided in

four parts only by means of a pyramidal mirror.

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AquEYE subsystems

AFOSC focus AFOSC focus Pyramid y Focusing lenses g Filters SPAD

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Selected detectors

As best compromise, the selected detectors are SPADs produced by Italian company MPD by Italian company MPD. Their main drawbacks are the small sensitive area (50 µm diameter) and a ≈70 ns dead time. diameter) and a 70 ns dead time.

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Advantages of multiple detectors

Differences between the photon times of arrival for 1 or 4 SPADs. (some MHz total rate) (some MHz total rate)

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AquEYE electronics schematics

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Time referencing and tagging

The CAEN TDC board samples the collected events at 40 GHz (25 ps time resolution), multiplying a reference frequency at 40 MHz. To maintain the desired 100 ps time accuracy over hours of

• bservation avoiding too expensive solutions like Hydrogen‐maser
• r Cesium clock a rubidium oscillator coupled to a Trimble Mini T
• r Cesium clock, a rubidium oscillator coupled to a Trimble Mini‐T

GPS disciplined OCXO (Oven Controlled X‐tal Oscillator) has been used as external reference frequency to the CAEN TDC board. used as external reference frequency to the CAEN TDC board. This clock is extremely accurate on short term, but has a drift for long periods. To remove this drift, the PPS signal from GPSDO g p g (GPS Disciplined oscillator, which is synchronized within 25 ns rms to UTC) is given in input to the CAEN board and time tagged h i h h h li fi l i f together with the events. Then a post‐process linear fit analysis of the collected PPS allows to estimate the rubidium drift, and to remove it remove it.

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Data handling

Obviously, all the data have to be stored and preliminary analyzed at their “production rate”. To store and analyze all the collected data a central storage unit with a capacity of 1 TB has been used. The arrival time of each photon is given as input t h to an asynchronous post processor which guarantees data guarantees data integrity for the following scientific investigation.

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The light curve of the Crab pulsar

Average Crab pulse profile from Asiago data (blue) and from 4 m g ( ) Kitt Peak telescope data (red; Fordham et all, ApJ. 581, 2002). The measured period in Asiago was P = 0.03362160125 s, to be compared with the P = 0.03362160253 s extrapolated from Jodrell Bank ephemerides

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Jodrell Bank ephemerides.

IquEYE

Thanks to the positive experience of AquEYE, it has been decided to realize q , IquEYE, a more complex instrument for applications to larger telescopes, as NTT and TNG.

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IquEYE optical layout

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IquEYE opto‐mechanics

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IquEYE electronics

CAEN b d Monitor CAEN board camera and motor controls CAEN board (redundant) GPS Rubidium clock Acquisition Control & data analysis & storage server server storage server

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IquEYE block diagram

EQuA ATFU Optics EQuA

Electronics for

Quantum Astronomy

ATFU

AquEYE Time and Frequency Unit

Optics

Telescope, Optical AquEYE and Detectors

Mass Storage

QuAS

Quantum Astronomy Software

Scientific DATA

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Equa schematics

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The Crab pulsar at NTT

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The PSR B0540‐69

IquEYE @ NTT (2009) HSP on HST (1993) CTIO 4 m (1985)

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At the other extreme: Eta Carinae

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The Asiago‐Ljubljana experiment

The Ljubljana telescope (80 cm diameter) is 230 km far from Asiago

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The Ljubljana telescope (80 cm diameter) is 230 km far from Asiago.

Joint observations of the Crab pulsar

On 10‐11 October 2008 we performed joint observations of the Crab pulsar. Both the observatories were equipped with a breadboard ACTS (Accurate and Certified Time System) clock unit provided by Th l Al i S Thi i i l i l Thales Alenia Space. This is an experimental setup to simulate the characteristics of timing of the future Galileo system. ACTS assures a time accuracy of 25 ns on UTC and certifies the ACTS assures a time accuracy of 25 ns on UTC, and certifies the

• time. These units were used to have a “common” clock, with

which we tried to synchronize the two observations. which we tried to synchronize the two observations.

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Obtained results

The obtained data have not been completely analyzed yet. The preliminary results (determination of the initial phase of the Crab pulsar period) show that the two measurements were about measurements were about 100 µs out of phase. This value is much larger than expected and suspected; investigations are going on to understand the reason p ; g g g

• f this discrepancy.

However the pulsar period obtained by this measurement was in h h h l b h l agreement within 1 ns with the value given by the value

• btained by means of Jodrell Bank ephemerides, demonstrating

a “perfect” internal clocking a perfect internal clocking.

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Instrument present limits

• Total count rate

The used SPADs have two outputs: an extreme timing accurate ‐ The used SPADs have two outputs: an extreme timing accurate (25 ps) NIM, which limits the linearity range of the detector to about 2 MHz; an about 10 times less accurate TTL, which gives ; , g up to 12 MHz count rate. To have the best timing, we used the NIM output, accepting a “low” count rate. ‐ The used CAEN board limits the total output count rate to 8 MHz

• Detector dead time

The used SPADs have an about 75 ns dead time, limiting the l h l ( f d) b l single channel maximum rate (if TTL output is used) but mainly inhibiting the capability of detecting very time‐close photons

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Possible instrument improvements (I)

• Total count rate

CAEN people assured that they will increase the board output ‐ CAEN people assured that they will increase the board output

• band. Anyway, we could simply use more boards in parallel

‐ It is rather difficult to improve the MPD SPAD time accuracy ‐ It is rather difficult to improve the MPD SPAD time accuracy

• performance. However, SPAD technology is fast improving:

several companies are now producing them, and SPAD arrays are becoming available. It is reasonable to suppose that in a few years it will be possible to have more performing SPADs

• Detector dead time

The use of multiple detectors statistically allows to greatly d h bl h h h h d b h reduce this problem. The higher the detector number, the higher the probability of detecting very time‐close photons, substantially reducing to zero the dead time substantially reducing to zero the dead time.

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Possible instrument improvements (II)

• The optical design can be improved
• The optical design can be improved.

In fact, the present design is a consequence of the limited availability of suitable detectors Presently the detector availability of suitable detectors. Presently, the detector limitations imposed a multi channel optical design, with all the related complexity. If SPAD arrays will be available in the future, a much simpler

• ptical design will be possible.
• The timing accuracy can be improved

In future it will be possible to use the better GNSS Galileo receiver with the aim to achieve a better synchronization to UTC.

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Future developments

We are planning to bring IquEYE also to TNG, which is very similar to NTT A hypothesis under investigation is to leave similar to NTT. A hypothesis under investigation is to leave IquEYE (upgraded) as a resident instrument for NTT. The next step will be to realize another version of this p instrument to be brought to one of the existing 8‐10 m telescopes (for example the Very Large Telescope at Cerro Paranal, Chile, or the Large Binocular Telescope in Tucson, or Keck on Mauna Kea). We have already applied to be funded for this experiment and contacts have already been taken with VLT this experiment, and contacts have already been taken with VLT. We are also considering the possibility of mounting a quantum detector in the central pixel of the Cherenkov light collector detector in the central pixel of the Cherenkov light collector MAGIC (Major Atmospheric Gamma Imaging Cherenkov) (Roque de los Muchachos, Canarias, Spain).

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HBTII possible application

This type of apparatus could be used with a network of telescopes allowing for example multi‐dimensional HBTII telescopes, allowing for example multi dimensional HBTII performed by means of post‐process data analysis.

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Possible performance of HBTII applications

Simulations have been performed to verify the possibility of realizing HBTII with this type of instrument realizing HBTII with this type of instrument. Test conditions:

• λ = 500 nm

λ 500 nm

• Δλ = 3 nm
• QE = 0.7
• Losses = 0.3
• Detector dead time = 70 ns
• Number of detectors = 4
• Number of detectors = 4

Two cases have been considered:

• 8 m telescopes 1 ns time accuracy 2 hours integration time
• 8 m telescopes, 1 ns time accuracy, 2 hours integration time
• 1.8 m telescopes, 20 ns time accuracy (Tempo2), 4 hours i.t.

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Possible performance of HBTII applications

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Comments on the plot

• Small telescopes are rather inefficient for HBTII applications,

but could be used with long exposure times but could be used with long exposure times

• To synchronize the observations, the photon TOA’s have to be

“homogenized” at the solar system baricentre by suitable s/w, as g y y / ,

• Tempo2. The time error associated with Tempo2 is 20 ns: this is

the error in time that has been considered for the present instrumentation applied to 1.8 m telescope. However it is not clear how it should be considered in these applications. Th SNR i i i l h f h

• The SNR ratio is proportional to the square root of the

integration time: very long observations can be done

• Flattening of the lines is due to saturation of the SPAD because
• Flattening of the lines is due to saturation of the SPAD because
• f high rate. If more SPADs can be used, the SNR can linearly

increase increase

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Another simulation

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Other possible applications

The realized instrument allows to perform measurements of

• ther very fast phenomena:
• ther very fast phenomena:
• Variabilities close to black holes
• Variabilities close to black holes
• Free electron lasers in magnetars
• Flare stars
• Flare stars
• Lunar occultations
• CV
• CV
• Exoplanetary transits
• …

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Conclusions

The characteristics of QuantEYE, AquEYE and IquEYE, the instruments studied, realized and tested have been reviewed. The proposed designs are very modular, and can be easily adapted to any “optical” telescope. The performed tests showed that this type of instrument performs very well as extremely fast photon counters / photometers. The instrument characteristics make it very suitable for HBTII applications also with the present design. It is reasonable to expect that in a few years much better performance can be expect that in a few years much better performance can be

• btained, mainly improving the time tagging accuracy.

The adopted philosophy of storing all the collected data allows the The adopted philosophy of storing all the collected data allows the possibility of using network of telescopes, also located in different sites.

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