1. Quantum Metrology:
Improving the precision of measurement utilizing quantum mechanical resources is the central task of quantum metrology. Quantum metrology revolves around the measurement of phase shift that a quantum state goes through as precisely as possible. The importance of precise measurement is well manifested in applications like LIGO (Laser Interferometer Gravitational-Wave Observatory). Improving precision has been proven to be remarkably useful in microscopy and imaging of photosensitive samples. The most popular interferometric schemes used in metrology are Mach-Zehnder, Michelson, Fourier-transform, Sagnac, Fabry-Perot interferometers that rely on linear optical elements like beamsplitters. In addition to improving the performance of linear metrological schemes, my research interest includes the exploration of nonlinear interferometers like SU(1,1). I am also interested in incorporating modern techniques of machine learning, exotic quantum states, and novel detection strategies, etc into metrological applications. Modeling experimental effects like loss, decoherence, noise is also particularly important for the realistic implementation of all of these protocols.
2. Quantum Imaging:
Quantum imaging utilizes the quantum correlations or quantum properties of light to extract information about the object being imaged. Quantum phase imaging and quantum ghost imaging are the most common examples of quantum imaging. The main idea of quantum phase imaging is to utilize simultaneous phase measurement techniques to extract the phase-contrast image of the object. Most protocols perform this kind of imaging at a very low light level which is particularly beneficial for imaging photosensitive samples. Ghost imaging exploits the intensity correlation to reconstruct an image. Quantum lithography is another curious problem that is fundamentally similar to quantum imaging. It makes use of the non-classical properties of photons to perform lithography.
3. Classical/Quantum Optical Communication:
Optical communication, of course, a broad field which deals with all kind of communication protocols in which light photons are utilized as an information carrier. The most common scheme of optical communication is fiber-optics-based communication. Quantum optical communication protocols utilize the exotic quantum properties of light like entanglement, quantum fluctuation, correlation. The Quantum counterpart of the classical unit of information (bit) is called qubit which is defined as an unbiased superposition of quantum states representing 0 and 1. A d-dimensional "quantum bit" which uses a d-fold superposition is called a qudit. In addition to polarization, photon-number, and spin, it is very interesting to study spatial modes of light which can encode multiple bits of information in a single photon. This leads to the field of free-space optical communication utilizing spatial modes of light.
4. Dynamical Decoupling (NV-center diamond):
Protecting the coherence of the qubit in a noisy environment is vital to the sanctity of quantum information encoded in the state. Dynamical decoupling is an important technique that utilizes a sequence of flip pulses to effectively decouple the quantum state from the environment. Nitrogen vacancy center (NV center) is a type of point defect in the diamond lattice. It is a very good candidate for a solid-state quantum system. I am currently exploring this system from the spectator qubit approach. The nuclear spin state of the nitrogen is treated as a computational qubit that has a higher coherence time. Currently, I am exploring the possibilities of maintaining its coherence by applying a series of pulse sequences. For the experimental side, CPT (coherent population trapping) technique is under consideration to measure the environmental noise parameter in real-time.
5. Medical Imaging Physics:
Medical imaging is a very important application side of the overall physics of imaging. The most commonly used techniques are x-ray radiography, CT (computed tomography), MRI (magnetic resonance imaging), PET (positron emission tomography), SPECT (single-photon emission computed tomography), PCI (phase-contrast imaging), etc. All of these imaging techniques either use x-rays or gamma rays except for MRI. I have been mostly working on SPECT and PCI. Iterative image reconstruction to obtain the volumetric image is a very important aspect of both SPECT and PCI. My research is mainly focused on improving the sensitivity and/or resolution of these imaging modalities by exploring various detector designs. We have shown to improve the sensitivity of the cardiac SPECT system by 308% with our novel geometry of hemi-ellipsoidal CsI detectors. This is very significant to reduce the patient radiation dose. We have also designed a quadratically structured phase grating for PCI which enables us to obtain phase-contrast images of the sample without having to use multiple diffraction gratings and analyzer grating. We have filed a US/worldwide patent on our unique PCI system. "Phase-contrast x-ray interferometry", J Dey, N Bhusal, L Butler, JP Dowling, K Ham, V Singh. US Patent App. 16/044,111.