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Abstract: Very recently we have seen the first proof of principle demonstrations of entanglement-based quantum networks. However, analogous to quantum computers, near-term quantum networks will feature noisy devices communicating at modest rates. Here, I will present some recent ideas for performing communication and computation applications with near-term quantum networks. Finally, I will discuss the potential of quantum networks for performing quantum cryptographic applications beyond quantum key distribution. 

Abstract: We will present the results of two experimental projects, one on repurposing a conventional EMCCD for photon counting requirements, and the other regarding an on-the-fly measurement of Schmidt number during quantum optical experiments. We will then bring out the connection of these results with our ongoing quantum communication research. Finally, I will conclude with discussing our technological proposal on free-space long-distance quantum communication.

No abstract needed.

Abstract: Establishing secure communication in the quantum world is quite demanding and challenging task. The quantum era is governed by Quantum Physics, which offers a framework to test fundamental aspects of quantum mechanics such as coherence, entanglement, and non-classical properties of light-matter. More recently, it has led to the development of quantum technologies which aim to harness quantum principles for promising applications in computing, communication, sensing and precision metrology. Quantum key distribution (QKD) is a method of quantum cryptography, which has become a new generation security solution and does not rely on the computation assumptions of problems presumed difficult.

QKD can provide a secure means for information exchange even in the presence of quantum computers. Optical fiber offers a reliable quantum channel for QKD well tested for existing telecommunication, which can offer not only point to point connectivity but also a means for long distance information exchange. This talk will begin with realizable sources of single and entangled photons useful for quantum secure communication, and would further highlight some implementations of fiber based QKD made by our group at IIT Delhi in lab scale and in real field environment. Notably, the first Indian long distance intercity fiber QKD ranging more than 100kms, and first global long distance baseline error optimized DPS QKD over 380km fiber would be discussed. Some aspects of entanglement based QKD will also be outlined, focussing on the need of coexistence of classical and quantum signal on the same fibre. The prospects of photonic quantum technologies would also be highlighted emphasizing on field deployable devices for QKD and hybrid quantum networks for future quantum internet.

Abstract: A general quantum information task can be carried out in a classical, quantum, or hybrid register. While most computing tasks are efficiently and routinely carried out by classical computers, for certain tasks we are seeing increasing competition from the NISQ era quantum processors. In a hybrid method, one divides a given task into two parts, classical efficient and quantum efficient. In our lab, nuclear spin ensembles coherently controlled by radiowaves via nuclear magnetic resonance constitute the quantum registers. As examples of the above three cases, I will describe efficient characterization of quantum evolutions via a recommender system, recommender system expedited quantum control optimization, physics-informed neural network for quantum control of NMR registers, maximal work extraction unitarily from an unknown quantum state, and certain ongoing works.

References:

1. Efficient Characterization of Quantum Evolutions via a Recommender System, Priya Batra, Anukriti Singh, T. S. Mahesh, Quantum 5, 598 (2021).

2. Recommender System Expedited Quantum Control Optimization, Priya Batra, M. Harshanth Ram, and T. S. Mahesh, Physics Open 14, 100127 (2023).

3. Physics-informed neural network for quantum control of NMR registers, Priya Batra and T. S. Mahesh, arXiv:2407.00444 [quant-ph].

4. Maximal work extraction unitarily from an unknown quantum state: Ergotropy estimation via feedback experiments, Jitendra Joshi and T. S Mahesh, arXiv:2409.04087 [quant-ph].

For invitees, QUICST faculty and QUICST students.

Abstract: In this talk, I will first explore several strange behaviors of quantum communication, highlighting how noise can exhibit context-dependent nature. Specifically, I will show how the conventional notion of noise in quantum communication may behave as 'anti-noise' depending on the auxiliary resources. I will then present some communication tasks that are impossible with noisy, entanglement-breaking channels in a definite (classical) causal order but can be achieved with the same noisy channels when the order of these channels is coherently controlled by a quantum bit, allowing for indefinite (quantum) causal order. Finally, I will address practical aspects of quantum state estimation within noisy communication networks. I will show how measurement statistics from entanglement distillation protocols can be used to efficiently estimate the network states, reducing resource consumption and eliminating the need for separate estimation protocols in communication networks.

Abstract: Hybrid quantum devices, combining diamond-based color defects like nitrogen-vacancy (NV) centers with other quantum systems, offer promising avenues for enhancing quantum memory capabilities. Coupling NV centers with optical cavities can facilitate efficient quantum state manipulation and readout. By controlling the interaction between the NV centers and the cavity photons, it's possible to store and retrieve quantum information with high fidelity. Such hybrid systems hold great potential for realizing scalable and high-performance quantum memories. In this talk I will give a brief overview of our activities on developing hybrid quantum devices. 

Abstract: Atomic and solid-state spin ensembles are promising platforms for implementing quantum technologies, but the unavoidable presence of noise imposes the needs for error correction. Typical quantum error correction (QEC) requires addressing specific qubits, but this is practically challenging in most realistic architectures. In this work, we propose QEC schemes for unresolvable spin ensembles. By using degenerate superpositions of excited states, which are fundamentally mixed, we find codes that can protect against both individual and collective errors, including dephasing, decay, and pumping. We show how information recovery can be achieved with only collective measurement and control, and illustrate its applications in extending memory lifetime.

Ref: arXiv:2408.11628v1

Abstract: The task of discriminating between two known quantum channels is a well-known binary-hypothesis-testing task. We present a variational quantum algorithm with a parameterized state preparation and two-outcome positive operator-valued measure which defines the acceptance criteria for the hypothesis test. Both the state preparation and measurement are simultaneously optimized using the success probability of single-shot discrimination as an objective function which can be calculated using localized measurements. Under constrained signal-mode photon-number quantum illumination we match the performance of known optimal two-mode probes by simulating a bosonic circuit. Our results show that variational algorithms can prepare optimal states for binary hypothesis testing with resource constraints. 

Joint work with M.Subramanian, PRA 2024