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arXiv:2602.23975

Coherent Control of Population and Quantum Coherence in Superconducting Circuits

Madan Mohan Mahana, Gunjan Yadav, Tarak Nath Dey

quant-phSubmitted Feb 27, 2026• Announced Mar 2, 2026• 0 citations• Revised Mar 3, 2026 (v2)
v1 · Feb 27, 2026v2 · Mar 3, 2026
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Abstract

Quantum mechanics, with its counterintuitive principles and probabilistic nature, has long been confined to the microscopic realm of atoms and photons. Yet, recent breakthroughs have pushed the boundaries of quantum behavior into the macroscopic world, where objects are visible to the naked eye and governed by classical physics. This review article traces the extraordinary progress toward achieving coherent control of population distributions among multiple quantum levels, as well as manipulation of absorption and refractive index, in such large-scale quantum systems, a feat once considered beyond reach.

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Quantum mechanics, with its counterintuitive principles and probabilistic nature, has long been confined to the microscopic realm of atoms and photons. Yet, recent breakthroughs have pushed the boundaries of quantum behavior into the macroscopic world, where objects are visible to the naked eye and governed by classical physics. This review article traces the extraordinary progress toward achieving coherent control of population distributions among multiple quantum levels, as well as manipulation of absorption and refractive index, in such large-scale quantum systems, a feat once considered beyond reach.
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It is widely believed that the microscopic, that is, atomic-scale or smaller, world is governed by quantum mechanics while the macroscopic world, where our direct and daily experiences reside, is governed by classical mechanics. But why? Why do we not observe quantum phenomena such as quantum tunneling and quantum superposition in the macroscopic world? Where is the boundary between the microscopic and macroscopic worlds when it comes to the governing physical principles? Macroscopic quantum phenomena were first examined theoretically by A. Leggett in the 1970s. This boldly audacious idea was experimentally demonstrated in a series of groundbreaking works on Josephson junctions by John Clarke, Michel Devoret, and John Martinis in 1985, that have been celebrated by the 2025 Nobel Prize in Physics. The trio’s work spurred the development of various qubit types based on superconducting circuits, ultimately leading to the current advancement of superconducting quantum computers.
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Superconducting qubits are Josephson junction-based circuits that exhibit macroscopic quantum behavior and can be manipulated as artificial atoms. Benefiting from the well-developed technology of microfabrication and microwave engineering, superconducting qubits have great advantages in design flexibility, controllability, and scalability. Over the past decade, there has been rapid progress in the field, which greatly improved our understanding of qubit decoherence and circuit optimization. The single-qubit coherence time has been steadily raised to the order of 10 to 100 μs, allowing for the demonstration of high-fidelity gate operations and measurement-based feedback control. Here we review recent progress in the coherence and readout of superconducting qubits.
Recent demonstrations of macroscopic quantum coherence in Josephson junction based electronic circuits have opened an entirely new dimension for research and applications in the established field of Josephson electronics. In this article we discuss basic Josephson circuits for qubit applications, methods of quantum description of these circuits, and circuit solutions for qubit couplings. Principles of manipulation and readout of superconducting qubits are reviewed and illustrated with recent experiments using various qubit types.
The quantum mechanical description of the world leads to predictions that defy our daily experi- ence when dealing with macroscopic objects. Schrodinger's famous cat illustrates most graphically the paradoxical situations that one encounters when extrapolating the quantum rules to systems possessing macroscopically distinguishable states. Leggett and Garg formulated this question qual- itatively by deriving a series of inequalities based upon the premises of macroscopic realism (1, 2) which have been recently tested within several microscopic systems (3-7) and micrometer-sized superconducting circuits (8). Here we present an experimental violation of Legget-Garg inequali- ties (LGI) with millimeter-sized crystals whose state can be thought of as a cat of cats. Through the reversible transfer of polarization state of single photons and the collective excitation states of crystals, we prepared superposition states of two macroscopic crystals separated by a distance in the mm range. The phase relation between the states in the superposition is governed by the dy- namics of a large number of individual atoms and can be controlled with a frequency detuned and polarization dependent atomic frequency comb (9-13). The quantum character of the considered global state is therefore governed by a macroscopic quantity, which allows for an additional level of complexity in the system. The observed violation of a LGI confirms the persistence of quantum coherence effects within macroscopic objects which can be seen with a naked eye.