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Quantum image distillation

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Science Advances  18 Oct 2019:
Vol. 5, no. 10, eaax0307
DOI: 10.1126/sciadv.aax0307
  • Fig. 1 Experimental apparatus.

    Light emitted by a diode laser (λp = 405 nm) illuminates β-barium borate (BBO) crystal with a thickness of 0.5 mm to produce spatially entangled pairs of photons by type I SPDC. Long-pass filters (LPF) positioned after the crystal remove pump photons. Lenses f1 = 35 mm and f2 = 75 mm image the crystal surface onto an object O1 (dead cat). Simultaneously, an object O2 (alive cat) is illuminated by a spatially filtered light-emitting diode (LED). Images of both objects are superimposed onto an EMCCD camera using a single-lens imaging configuration (f3 = 50 mm) and an unbalanced beam splitter (BS; 92% transmission). Band-pass filters (BPF) at 810 ± 5 nm and a polarizer (P) in front of the camera select near-degenerate photons. The single and double red arrows indicate respectively classical and photon-pair illuminations.

  • Fig. 2 Separation of mixed quantum-classical images.

    The direct-intensity image (A) acquired by accumulating photons on the camera sensor shows a superposition of both objects O1 (quantum) and O2 (classical), representing a dead cat and an alive cat, respectively. Intensity correlation function Γ(r, r) (B) measured with the camera shows the image of O1. An image of O2 (C) is obtained by subtracting the reconstructed image of O1 from the mixed image. The residual image of O1 observed in the background is due to single photons created by absorption of one photon of a pair propagating through the dead cat mask. A similar experiment is performed using positive (O1) and negative (O2) resolution charts, as shown by its corresponding (D) direct-intensity image, (E) Γ(r, r), and (F) reconstructed classical image. Both experiments are performed by acquiring N ∼ 107 frames using an exposure time of τ = 6 ms. a.u, arbitrary units.

  • Fig. 3 Characterization of residual single-photon intensity.

    Direct-intensity image (A) acquired with the LED turned off shows object O3 (the number “3”). The image is deliberately slightly defocused by positioning it out of the focal plane of the imaging system. Direct-intensity image (B) acquired with the SPDC turned off shows the ground-truth image of O4 (the number “6”). Direct-intensity image (C) acquired with both sources on shows a superimposition of both objects. The intensity correlation function Γ(r, r) (D) reveals the number “3”; image subtraction between this and the mixed image reveals the classical image (E) number “6.” In this case, the residual intensity created by absorption of one photon of a pair is concentrated near the edge of the number “3.” The residual single-photon intensity (F) is isolated by subtracting the reconstructed classical (E) from its ground truth (B). Experiments are performed by acquiring N = 6 × 106 frames using an exposure time of τ = 6 ms.

  • Fig. 4 SNR in quantum-distilled images.

    (A) SNRs are represented as a function of average intensity ratio between classical and quantum light Icl/Iqu (black crosses) together with a theoretical model (blue dashed line). In this experiment, both sources homogeneously illuminate the camera sensor (B) and SNRs are measured by dividing the peak intensity by the SD of the noise in the minus-coordinate projections of Γ. (B) (C), and (D) show minus-coordinate projections acquired for intensity ratios of 0, +∞, and 11, respectively. All experiments are performed by acquiring N = 251,600 images with an exposure time of τ = 6 ms. With these settings, intensity of the quantum source averaged over camera pixels is equal to Iqu = 939 gl. Inset, normalized quantum image of a dead cat reconstructed with an average classical/quantum intensity ratio of 5.5. Scale bar, 400 μm.

  • Fig. 5 Conditional projections.

    Direct-intensity image (A) measured under simultaneous illumination of classical and quantum light. Conditional image Γ(rA) (B) shows an intense peak centered around position A. Conditional images Γ(rB) (C) are null and flat.

Supplementary Materials

  • Supplementary Materials

    This PDF file includes:

    • Section S1. Theory
    • Section S2. Measurement of Γ (r, r)
    • Section S3. Projections of Γ

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