Enhanced absorption microscopy with correlated photon pairs

Ming Li, Chang-Ling Zou, Di Liu, Guo-Ping Guo, Guang-Can Guo, and Xi-Feng Ren

Phys. Rev. A 98, 012121 – Published 18 July 2018

Abstract

By harnessing the quantum states of light for illumination, precise phase and absorption estimations can be achieved with precision beyond the standard quantum limit. Despite their significance for precision measurements, quantum states are fragile in noisy environments which leads to difficulties in revealing the quantum advantage. In this paper, we propose a scheme to improve optical absorption estimation precision by using the correlated photon pairs from spontaneous parametric down-conversion as the illumination. This scheme performs better than classical illumination when the loss is below a critical value. Experimentally, the scheme is demonstrated by a scanning transmission-type microscope that uses correlated photon illumination. As a result, the signal-to-noise ratio of a two-photon image shows a 1.36-fold enhancement over that of the single-photon image for single-layer graphene with a 0.98 transmittance. This enhancement factor will be larger when using a multimode squeezed state as the illumination.

Received 26 October 2017

DOI:https://doi.org/10.1103/PhysRevA.98.012121

Physics Subject Headings (PhySH)

Quantum information processing Quantum metrology Quantum sensing

Quantum Information, Science & Technology

Authors & Affiliations

Ming Li, Chang-Ling Zou, Di Liu, Guo-Ping Guo, Guang-Can Guo, and Xi-Feng Ren^*

Key Laboratory of Quantum Information, University of Science and Technology of China, CAS, Hefei, Anhui 230026, China and Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China

^*renxf@ustc.edu.cn

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Vol. 98, Iss. 1 — July 2018

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Images

Figure 1
Theoretical calculations of the transmittance estimates for different illumination schemes. (a) Schematics of the single-pass (left) and the double-pass schemes (right). (b) Transmittance estimate precision for an ideal single-photon probe state. (c) Transmittance estimate precision for the probe state from SPDC. In this case, the input state has fluctuations of the photon number. Here, we set $β = 10^{- 2}$ . The estimation precision of the two-photon case exceeds that of the single-photon case when $t_{2}$ is greater than the critical value $t_{0}$ . Red line: single-pass scheme. Blue line: double-pass scheme. Pink line: coherent state. Black line: enhancement factor $Δ t_{SP} / Δ t_{DP}$ .
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Figure 2
Experimental setup. HWP: half-wave plate; Filter1: 650-nm long-pass filter; Filter2: 808-nm interference filter. PBS: polarization beam splitter. A 404-nm laser is focused on the periodic pooled potassium titanyl phosphate (KTP) crystal to generate twin photons with orthogonal polarizations ( $H, V$ ). We use a temperature controller to tune the wavelength of the photons. Single-pass scheme: After filtering and PBS, the photons are separated and coupled into different single-mode fibers. One photon is used as the trigger, and the other is used as the illumination. Double-pass scheme: After filtering, the two photons are coupled to one fiber, and then the two-photon source is used as the illumination and is focused on the sample. The coincidence measurement is performed by splitting the photons with the PBS. The difference between the two schemes is the position of the PBS. The single-pass scheme is realized by moving the PBS in the black dashed box upwards.
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Figure 3
Scanned images of graphene layers from the transmission microscope. (a) Single-pass scheme. (b) Double-pass scheme. Each pixel in these images corresponds to 100 nm. The whole area is $5 \times 5 μ m^{2}$ . The data used for calculating the enhancement are the counts in the three different areas. In the experiment, the photon fluxes are fixed to approximately 5000 counts. The SNRs and enhancements of the samples with different transmittances are calculated from the data in the red dashed rectangles. (c) and (d) The data are extracted from the images shown in this figure. The error bars are calculated from 25 datasets. (e) The relation between the maximal enhancement of the SNR and the estimated transmittance $t_{2}$ . The black line: Enhancement of the double-pass scheme compared to that of single-pass scheme. The orange points: experimental results for the graphene layers with different transmittances. From left to right, the transmittances are 0.66, 0.87, and 0.98.
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