# /research_overview

### Multipartite entanglement measures

In the last two decades, correlation measures originating from the entanglement-separability paradigm have attracted a lot of attention, and substantial theoretical as well as experimental advancements in detecting and computing such measures have been achieved in a bipartite scenario. However, quantifying entanglement in multipartite domain still remains a challenging task. Our research aims to address the problem of characterizing, detecting, and quantifying quantum entanglement in multiparty domain, and developing strategies for reducing the computational resource required in calculation of there quantum correlation measures.

In the quest for computable multiparty entanglement measure, in one of our recent Physical Review A paper, we have proposed a localizable multipartite measure of entanglement based on the geometric measures of entanglement and local projective measurements. In another paper, we have also investigated the patterns in distributions of localizable entanglement, calculated using the entanglement of formation as the entanglement measure, over a pair of qubits for random multi-qubit pure states under noise, and pointed out an uniformity in the behaviours of the distributions.

**Uniform Decoherence Effect on Localizable Entanglement in Random Multi-qubit Pure States**, by Banerjee, AKP, and Sen(De), Phys. Rev. A**101**, 042339 (2020)**Multipartite Entanglement Accumulation in Quantum States: Localizable Generalized Geometric Measure**, by Sadhukhan, Singha Roy, AKP, Rakshit, Sen(De), and Sen, Phys. Rev. A**95**, 022301 (2017)

### Characterizing topological quantum error correcting codes

An immense effort is being given all over the world towards implementing large-scale fault-tolerant quantum computers, having the potential to solve problems that are intractable by existing classical computers, such as simulating large quantum systems, efficient decryption of codes, etc. Towards this aim, topological quantum error correcting codes, such as the surface codes and the color codes, have emerged as the most promising candidates. These topological quantum codes are interacting quantum spin systems constituted of spin-1/2 particles, which represents the physical qubits, arranged on lattices of specific geometry, and the ground states of these systems are used as resource states in quantum error correction. Generally the figures of merit of these quantum states to be used in a quantum protocol are quantum correlations, such as entanglement, which can be used as resource in that quantum protocol. However, it is known that in order to perform successful error correction using these states in a laboratory setup, taking into account errors on multiple physical qubits, one needs to deal with large systems in the presence of noise. This makes the characterisation of these systems using entanglement measures difficult. Our research aims to quantify and compute bipartite as well as multipartite entanglement in subsystems of a large-scale quantum many-body system like the topological quantum codes in the presence of noise.

In two recent papers published in the New Journal of Physics, we have proposed a methodology to compute a lower bound of localizable entanglement over a set of qubits in noisy surface and color codes. Our methodology approaches the problem along two different yet connected avenues. One of the two avenues exploits entanglement witness operators designed specifically for the topological quantum codes, while the other one employs graph states in order to obtain a computable lower bound. The methodology scales with the system size as N^{3}. Watch the video if you are interested to know more about this work.

**Scalable characterization of localizable entanglement in noisy topological quantum codes**, by Amaro, Müller, and AKP, New J. Phys.**22**, 053038 (2020)**Estimating localizable entanglement from witnesses**, by Amaro, Müller, and AKP, New J. Phys.**20**, 063017 (2018)

### Small quantum thermal machines

Quantum thermodynamics aims to design effective quantum thermal machines in order to understand thermodynamic principles at the quantum mechanical level, and to explore whether these machines can provide quantum advantages over their classical counterparts. It has grown to become a thriving multidisciplinary field of research, including statistical physics, quantum information theory, the theory of open quantum systems, and quantum many-body physics. Among the quantum thermal machines, small quantum absorption refrigerators, constituted off a small number of qubits and/or qutrits are the focus of our research. We study the different models and properties of these machines from a quantum information perspective, using the formalisms of open quantum systems. We also investigate possible construction of such machines using low-dimensional quantum spin models.

Recently, we have demonstrated that a three-qubit quantum refrigerator can perform in a regime of the relevant parameter space where a transient cooling of one of the qubits may take place with little, or no steady-state cooling. The work has been published in Europhysics Letters.

**Necessarily transient quantum refrigerator**, by Das, Misra, AKP, Sen(De), and Sen, Europhys. Lett.**125**, 20007 (2019)

### Quantum many-body systems

Quantum many-body systems are considered as ideal candidate systems to carry out quantum information processing tasks. They are crucial for the realizations of quantum protocols like quantum state transfer via a spin-chain, and measurement-based quantum computation. This highlights the necessity for a better understanding of the novel phases and phenomena occurring in quantum many-body systems using a language consistent with quantum information theory. This is achieved by investigating the quantum correlations, such as entanglement, that are of importance for quantum information processing tasks to be performed in these quantum many-body systems. Our research deals with the static as well as dynamical trends of entanglement measures in quantum many-body systems leading to its efficient characterization.

There exist zero-temperature states in quantum many-body systems that are fully factorized, thereby possessing vanishing entanglement, and hence being of no use as resource in quantum information processing tasks. In our recent works, we have demonstrated how entanglement can be generated via thermal as well as non-thermal noise in these states occurring in an alternating-field XY model, thereby using noise in our advantage. We have also shown a "freezing" behaviour of two-spin entanglement in this model, where entanglement remains constant over time for a finite time interval, and connected this phenomena with the well-known Lieb-Robinson bound in quantum many-body Physics. These results have been published in Physical Review A.

**Scale-invariant freezing of entanglement**, by Chanda, Das, Sadhukhan, AKP, Sen(De), and Sen, Phys. Rev. A**97**, 062324 (2018)**Emergence of entanglement with temperature and time in factorization-surface states**, by Chanda, Das, Sadhukhan, AKP, Sen(De), and Sen, Phys. Rev. A**97**, 012316 (2018)