Quantum imaging of faint objects

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1 Italian Quantum Information Science Conference 008, Camerino Quantum imaging of faint objects Theory : Lucia Caspani, Enrico Brambilla, Luigi Lugiato, Alessandra Gatti Experiment: Ottavia Jederkiewicz, Paolo Di Trapani IFM-CR-CISM, Università dell Insubria, Como, Italy

2 QUATUM IMAGIG This field exploits the quantum nature of light and the natural parallelism of optical signals to devise novel technique for optical imaging and for parallel information processing at the quantum level. Gatti, Brambilla, Lugiato, Quantum Imaging, Progress in Optics, Vol. 5, p.5, 008

3 MEU Ghost Imaging (not necessarily quantum) Detection of a weak object in the quantum regime

4 Ghost imaging by means of two-photon quantum entanglement Belinsky and Klyshko, Sov. Phys JETP 78, 59 (994) Pittman, Shih, Strekalov and Sergienko, PRA 5, R349 (995) Ribeiro, Padua, Machado da Silva, Barbosa, PRA. 49, 476, (994) Strekalov, Sergienko, Klyshko and Shih, PRL 74, 3600 (995) Abouraddy, Saleh, Sergienko, Teich, Phys.Rev.Lett. 87, 360 (00) GHOST IMAGE EXP GHOST DIFFRACTIO EXP THEORY Photon-pair created by PDC in the ultralow gain regime Pump χ () OBJECT h (x,x ) TEST ARM h (x,x ) x POIT-LIKE DETECTOR, FIXED POSITIO OR BUCKET DETECTOR Coincidence counts as a function of x REFERECE ARM ARRAY OF DETECTORS The imaging information is extracted from the coincidence counts as a function of the position of the reference photon which never passed through the object x

5 DEBATE : is entanglement of the two beams necessary for ghost imaging or not?

6 An old favourite of the 70-ties: the speckle pattern generated by impinging a laser beam on a ground glass TO CCD LASER ROTATIG GROUD GLASS BS Splitting symmetrically: twin speckle patterns Gatti, Brambilla, Bache, Lugiato, PRL 93, (004), Phys. Rev. A 70, 0380 (004)

7 Experimental evidence of high resolution ghost image and ghost diffraction with classically correlated beams from a pseudo thermal source Ferri, Magatti,Gatti, Bache, Brambilla, Lugiato, Phys. Rev. Lett. 94, 8360 (005) He-e LASER near-field plane 400 mm OBJECT F F CCD D=3mm GROUD GLASS TURBID MEDIUM BS p F' q coherence time ~ 0. s speckles ~5 µm + = q p Feff Feff focal of the two lens system

8 Further debate: can two-photon correlation of thermal light be considered as correlation of intensity fluctuations? (Scarcelli, Berardi, Shih, PRL 96, 06360, 006) See - Gatti, Bondani, Lugiato, Paris and Fabre, PRL 98, 3930 (007) - Erkmen, Shapiro, PRA 77, (008)

9 Detection of a weak absorption (e.g. a spectroscopic signal): typically a differential measurement is used Weak absorption α SOURCE (not shot-noise limited) 50/50 BS reference ' = ' The signal is retrieved from _' = α The source beam may have some excess noise, but the signal-to-noise ratio SR ' δ' SR SQL = α for small α standard quantum limit, coherent source The differential scheme suppresses the excess noise in the incoming beam, but is affected by the shot-noise in -

10 Detection of a weak absorption A classical differential scheme suppresses the excess noise in the incoming beam, but is limited by the partition noise (=shot-noise) in - χ () signal Weak absorption α ' = ' idler OPO twin beams reference The shot-noise can be beaten by replacing the classical copies with quantum copies: twin-beams with sub-shot noise intensity correlation signal-to-noise ratio is improved with respect to the standard quantum limit SR twin SR beams Signal-idler degree of correlation σ = SQL σ δ + for small α = splitted beams < sub - shot noise twin - beams

11 Improvement of the SR when detecting a weak spectroscopic signal with single-mode twin beams generated by an OPO: - Souto Ribeiro, Schwob, Maitre, Fabre, Opt. Lett., 893 (997) - Jiangrui Gao et al., Opt.Lett. 3, 870 (998) Question: can we use the same technique to detect the spatial distribution of an absorption coefficient α(x), i.e. the image of a weak object? - o if the twin beams are single mode. - Yes, if we use multi-mode twin beams, as those generated by singlepass parametric down-conversion (PDC) we need pixel-to-pixel sub-shot-noise correlation between the signal and idler cross-section, i.e. quantum correlation in the spatial domain.

12 Quantum Spatial correlation in the far-field of PDC Brambilla, Gatti, Bache, Lugiato, Phys Rev A 69, 0380 (004) q p =0, ω p pump q, ω p + ω -q, ω p -ω signal idler Microscopic process: momentum conservation creates correlation in the directions of propagation of twin photons Detection of Quantum Spatial Correlation in high-gain PDC Experiment performed at Como Lab. (Jedrkiewicz, Jiang, Di Trapani) signal - idler emission cones ps pump nm mm waist χ () x λf coh w p coherence length z 0 l c 4 mm type II BBO crystal Far-field nm in a single shot

13 Signal and idler distributions on the high efficiency CCD Zoomed nm evident spatial correlation between the two images - Degree of correlation: noise in the difference - of photocounts from symmetric signal-idler pixels σ = δ + Zoomed nm - CCD noise variance is subtracted δ = δ measured δ background

14 Experimental evidence of sub-shot noise spatial correlation σ σ = δ + Shot-noise 0 00 Jedrkiewicz, Jiang, Brambilla, Gatti, Bache, Lugiato, Di Trapani, PRL 93, 4360 (004) + Twin beam effect over several phase conjugate signal/idler modes. Can be used to enhance the sensitivity of imaging. Problem: rapid deterioration of signal-idler correlation with increasing gain. Quantum correlation only at relatively low photon number (<0 ph/pixel): the CCD noise δ background 90 ph/pixel would be detrimental for high-sensitivity measurements.

15 Proposed solution: decrease the excess noise (Brambilla, Caspani, Jedrkiewicz, Lugiato, Gatti, Phys. Rev. A 69, , 008) Regime of low-gain - long pulse duration. The degeneracy factor M τ pump τcoh increases with pulse duration signal and idler statistics become Poissonian-like umerics: degree of correlation σ as a function of the gain (photon number), X shift fixed = 4 µm, M~5 τ pump =5ps τ pump =50ps τ pump =50 ps σ = δ +,0 0,8 0,6 0,4 shot-noise τ pump =000ps M~000 slopes M τ τ coh pump 0, 0, < + > Correlation is more robust for long pump pulses, low excess noise sub-shot noise correlation at higher photon number

16 umerical simulation of the detection of a weak object α(x) weak object α(x) CCD α=0,04 Gaussian pump pulse w pump =500 µm τ pump up to 8ns signal χ () r r r ( x) = ( x) '( x) idler reference f lens FAR FIELD ' Relevant quantities evaluated from the simulations: σ SR R = σ SR σ SR SQL degree of spatial correlation (without object) signal - to - noise ratio improvement of SR with respect to SQL

17 Signal-to-noise ratio as a function of photon number (pump pulse duration) SR τ pump (ps) σ=0.04, R=4 σ=0.5, R=.7 σ=0.4, R=.95 η= η=0.8 η=0.9 twin-beams coherent source (SQL) < > object PDC SQL τ pump η = 0.9 = 500 ps = 000 ph./pixel α obj = 0.04

18 Brida, Genovese, Meda, Ruo Berchera, IRIM Torino F ()() yx=± +± σ ()() yx=± +± - Large photon number per pixel - o need for substraction of background

19 COCLUSIOS The quantum nature of light can really improve the SR in the detection of faint objects There is a good progress towards the experimental realization of this concept