Research

1. Cavity QED:   A cavity consists of two highly reflecting mirrors that trap photons for a fraction of a second. By placing an atom inside the cavity (see figure), the interaction between the atom and the photon can be controlled to realize several interesting dynamics.  A cavity with a two-level atom provides the simplest setup for understanding the light-matter interaction. 

    For more on cavities, please see the review article "A review on quantum information processing in cavities".

 We have addressed the following aspects of light-matter interaction:

    (a) Photon blockade: If two cavities are nonlinearly coupled, transfer of photons from one cavity to another cavity can be blocked.  

    (b) Quantum control of photon transfer: The photon blockade phenomena may lead to universal control of photon transfer between any two cavities.

2. Cavity array: Two or more cavities can be coupled to form an array. An array of cavities becomes a conduit for photon transfer. Several cavities can be coupled to form a quantum network that has several applications for realizing quantum communication, quantum internet, etc. 

(a) Quantum state transfer: An unknown quantum state (state of photon) can be transferred between two cavities. The transfer probability depends on the form of coupling strengths between the cavities. We have investigated a "parabolic" form of coupling strength which provides a perfect transfer of photons between two cavities.  Another form of coupling strength, that satisfies a square-root law, has been investigated in this context. 

3. Entanglement:  It is one of the most puzzling features of quantum mechanics. Entanglement implies the inseparability of the states of two or more quantum systems. In the realm of quantum information processing, entangled states play an important role in numerous protocols, most notably in quantum teleportation, quantum dense coding, and quantum sensing.

(a) N00N state generation: This is an interesting entangled state between two cavities or field modes, in which there are equal probabilities of detecting all the photons in any one of the modes. 

(b) Entanglement measurement: We also propose an idea of measuring entanglement between two atoms by using cavities.

4. Quantum information processing:  Quantum information is encoded in qubits (quantum version of classical bits 0 and 1). These qubits are used for realizing various quantum information protocols and quantum computation using quantum gates. A few quantum information protocols are:

(i) Quantum state transfer (for communication)

(ii) Quantum teleportation

(iii) Quantum dense coding

In this context, we have worked on the following problems:

(a) Quantum state transfer in cavities:  Qubit state can be transferred in an array to form a quantum network. 

(b) Quantum dense coding: This is a protocol for transmitting two classical bits of information from a sender (Alice) to a remote receiver (Bob) by sending only one quantum bit (qubit). We have shown the possible realization of quantum dense coding in a cavity array. 

5. Quantum optomechanics:  An optomechanical system consists of a cavity whose one of the mirrors oscillates in its mean position (see figure).  

(a) Quantum gates: We proposed a scheme for realizing quantum gates in an optomechanical setup for quantum computation applications

6. Quantum interferometry:  Interferometers are the basis for demonstrating the interference of an EM field at a single photon level. Such a simple interferometer is the Mach-Zehnder interferometer. It consists of two beam splitters, a few mirrors, and a phase shifter. The first beam splitter divides the input field into two. A phase shifter in one of the arms/paths introduces a phase shift to the field passing through it, and the final beam splitter combines both beams. 

Interferometers are used for gravitational wave detection, quantum sensing, quantum communication, quantum security, etc. 

We have worked on the following applications:

(a) Quantum sensing: Detecting small phase change using an interferometer is an important issue in quantum mechanical applications. The precision with which the phase can be detected is bounded by the Heisenberg limit.  This limit can be overcome in nonlinear interferometers.  

(b) Enhancing phase sensitivity by photon filtration from a coherent state. 

7. Quantum thermodynamics:  It is another interesting field where we study various quantum systems that interact with reservoirs (large systems with non-zero temperatures).  Due to the interaction, the system exchanges heat energy with the reservoir, resulting in the decoherence of the system.    

(a) Heat engine: We show that a four-mode nonlinear interferometer can act like a heat engine that concentrates energy in one of the output modes and coherently produces work.  

(b) Work production in a Mach-Zehnder interferometer: A simple Mach-Zehnder interferometer can be used for generating a non-passive state (having non-zero work capacity) out of thermal fields (zero work capacity). 

(c) Thermal diode: We show that a thermal diode can be realized in a coupled cavities which are connected to two thermal reservoirs. An atom inside one of the cavities can be used to control the heat transfer between the reservoirs.

7. Fundamentals of quantum optics:  

(a) Nonclassical state generation: Nonclassical states are important for realizing quantum information protocols and computation. We have addressed problems related to the generation of a special class of state, the so-called "number state filtered coherent state".

(b) Quantifying coherence

(c) Photon statistics in parametric down-conversion process