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Quantum enhanced real-time sensing of protein-gold adsorption kinetics (Experimental work)
Quantum enhanced real-time sensing of protein-gold adsorption kinetics (Experimental work)
This project provides the most practical setup for improving the sensitivity of the time-dependent measurements involved in various biological processes at the molecular level.
Surface Plasmon Resonance (SPR) Sensor
The SPR sensor is based on the generation of surface plasmon polariton at metal-dielectric surface. The easiest way to understand surface plasmon polariton waves is by breaking down the term such that – Plasmon is the term used of electron oscillations in a metal, polariton comes from the EM field and it is referred to as a surface wave because of the coupling between the EM field and the electron oscillations confined at the metal dielectric interface. Here the prism, the gold film, and the dielectric medium form the apparatus useful in sensing the refractive index of the dielectric. As we plot the reflectivity of this sensor against the angle of incidence, we can see an absorption dip at the angle of resonance. If we keep the thickness of the metal and the frequency of the laser constant, even the slightest change in the refractive index of the sample can be detected using the shift in the resonance dip.
Two-Mode Bright Squeezed State (TMBSS)
The Two-Mode Bright Squeezed State (TMBSS) is an optical state with reduced photon number fluctuations below the Shot Noise Limit (SNL) set by the fluctuations in the coherent state of same intensity. The squeezing refers to the quantum noise reduction that is achieved via the subtraction of the intensity of the two simultaneously generated optical modes of light that share temporal as well as spatial correlations. The two beams namely the probe and the conjugate are also referred to as the entangled twin beams in the quantum optics community. Note that the observed quantum enhancement in this work arises from intensity-difference squeezing, which is different that the quadrature squeezing.
This work uses a χ(3) process called degenerate Four Wave Mixing (FWM) in which annihilation of two photons of same frequency, here pump frequency, leads to the creation of two new photons of slightly different frequencies, here probe and the conjugate frequencies. FWM takes place inside a 2.5 cm long cylindrical vapor cell containing Rb-85 atomic ensemble. The D1 line corresponding to the 5S1/2 → 5P1/2 transition including the hyperfine structure of the Rb85 atom constitutes a double-Λ energy level configuration which decides the frequency of the probe and the conjugate beams.
Improved sensitivity while measuring the binding parameters
It is easier to see the quantum advantage in the Signal-to-Noise Ratio (SNR) plot as a function of time. The quantum advantage here is defined as below -
Better SNR implies better sensitivity of measuring the parameter of study. We compare the SNR of two-mode bright squeezed state with that of the coherent state to show the quantum advantage. This quantum advantage translates into a 60% improvement in the estimation of protein-gold binding rate surpassing the limitations of the current shot noise limited techniques. Please check out the paper PRX Life 3 (2), 023010 for more details.
Quantum metrology of absorption and gain parameters (Theoretical work)
This project presents two measurement schemes: balanced photodetection and time-reversed metrology, both utilizing two-mode bright squeezed light. The quantum advantage of both the schemes is then compared to the theoretical limit given by the quantum Cramér-Rao bound.
The above figure shows schematics of the sensor with two Optical Parametric Amplifiers (OPA) which is the scheme for the time reversed metrology. The truncated version with only one OPA but with intensity difference detection constitutes the balanced photodetection method. Here we use the Heisenberg operator representation to describe the evolution of states in the sensor. |u> and |0> are the coherent and the vacuum states respectively. Inside the OPA the modes go under squeezing transformation representing the correlation between the two output modes. Alpha (α) represents the absorption coefficient of the sample. We find out that these setups can attain a more than sevenfold quantum advantage (QA) for α = 0.01 and threefold QA for α = 0.05. Similar results are found for the gain parameter. Please check out our paper PHYSICAL REVIEW A 109, 053715 (2024) for more details.
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