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Quantum Spectroscopy for Ultra-Sensitive Detection

Idea Proposed

A quantum-enhanced technique for detecting mid-to-far infrared electric fields absorbed and emitted by molecules. The method improves the sensitivity of time-domain spectroscopy (TDS) by using two-mode squeezed states (a quantum optical technique) to reduce noise beyond classical limits.

Image

Schematic of the experimental setup. A 1030-nm, 250- fs-duration, 100-khz- repetition- rate laser is split into two branches. the bottom branch (in red in the fig-ure) pumps a GaP crystal, leading to the generation of a single-cycle thz pulse (yellow). the top branch (p-polarized, also in red) seeds a parametric amplifier pumped bya synchronized laser (s-polarized, green in the figure) at 515 nm, generating a two-mode squeezed vacuum (p-polarized), consisting of photon number correlated signal(purple) and idler (cyan) pulses. the idler polarization is rotated to s-polarization using a half-wave plate (hWP). the signal pulse is used for the electro-optical (eO) detec-tion of the thz electric field while the idler is delayed to not interact with the thz pulse and serves as a reference. the eO modulation is analyzed by a polarimetric arrange-ment comprising an hWP, a Wollaston polarizer, and a low-noise, high–quantum-efficiency balanced detector. the temporal resolution is achieved by delaying the thzwith respect to the signal and idler pulses using a linear translation stage. note that the idler polarization impinging on the eO crystal is orthogonal (s-polarized) to thatof the signal, and both are rotated by 45° with an hWP before interacting with the linearly polarized thz field in the eO crystal, and then are rotated back to almost thesame initial condition before reaching the Wollaston prism.



How Does It Work?

  1. THz Radiation Generation:

    • A laser pulse is used to generate terahertz (THz) radiation in a gallium phosphide (GaP) crystal.
    • This THz pulse is crucial for probing materials and molecules.

  2. Electro-Optical Sampling (EOS):

    • The THz electric field interacts with a probe pulse in a nonlinear crystal.
    • This modifies the phase of the probe pulse, which is detected using a balanced detector.

  3. Quantum Enhancement Using Two-Mode Squeezed States:

    • Traditional detection is limited by shot noise.
    • Quantum-entangled photons (twin beams) are used to reduce this noise.
    • This results in a twofold noise reduction and improves sensitivity.

  4. Spectroscopy:

    • The method allows high-precision measurements of spectral data.
    • It improves both amplitude and phase measurements of the THz field.

How Can We Use It?

This technique has potential applications in:

  • Security screening
  • Quality control
  • Medical diagnostics
  • Chemical composition analysis
  • Environmental monitoring (air, water, and food safety)

Sources & citation

Dionysis Adamou et al. ,Quantum-enhanced time-domain spectroscopy.Sci. Adv.11,eadt2187(2025).DOI:10.1126/sciadv.adt2187

https://phys.org/news/2025-02-quantum-technique-spectroscopy-sensitivity-revealing.html