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Collective Charge of Bosons (CCB)

Jincheng Zhang

Published Aug 17, 2026• 0 citations
Publisher

Abstract

The concept of electric charge is traditionally associated with elementary fermionic particles such as electrons. In contrast, bosonic quasiparticles like photons, magnons, and phonons are usually regarded as electrically neutral. Here we propose that a macroscopic occupation of a bosonic mode can give rise to an emergent collective charge, denoted as Qm_collective. We formulate a theoretical framework based on the many-body wavefunction of a Bose–Einstein condensate and show that the collective charge can be expressed as the expectation value of the total charge operator over the condensate state. When the number of bosons in the condensate becomes large, the collective charge tends to a finite value even though each individual boson remains neutral. We discuss possible experimental platforms for observing this effect, including photon condensation in microcavities, polariton condensates in semiconductor microstructures, and acoustic phonon condensates in nanomechanical systems. The emergence of a collective charge in bosonic systems opens a new avenue for investigating quasiparticle interactions, non-linear optical phenomena, and the design of novel quantum devices with controllable charge properties.

DOI
10.5281/zenodo.21973148

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The concept of electric charge is traditionally associated with elementary fermionic particles such as electrons. In contrast, bosonic quasiparticles like photons, magnons, and phonons are usually regarded as electrically neutral. Here we propose that a macroscopic occupation of a bosonic mode can give rise to an emergent collective charge, denoted as Qm_collective. We formulate a theoretical framework based on the many-body wavefunction of a Bose–Einstein condensate and show that the collective charge can be expressed as the expectation value of the total charge operator over the condensate state. When the number of bosons in the condensate becomes large, the collective charge tends to a finite value even though each individual boson remains neutral. We discuss possible experimental platforms for observing this effect, including photon condensation in microcavities, polariton condensates in semiconductor microstructures, and acoustic phonon condensates in nanomechanical systems. The emergence of a collective charge in bosonic systems opens a new avenue for investigating quasiparticle interactions, non-linear optical phenomena, and the design of novel quantum devices with controllable charge properties.
The notion that a single bosonic particle may carry no net electric charge while a macroscopic ensemble of such particles can exhibit a collective charge has emerged as a provocative concept in contemporary condensed matter physics. This paper develops a comprehensive theoretical framework for the Collective Charge of Bosons (CCB) phenomenon, presenting a quantitative model of how bosonic condensation can generate a non‑zero effective charge density even when individual constituents are neutral. We formulate the collective charge operator Q_m_collective, derive its expectation value in various bosonic states, and analyze the stability conditions for the emergence of CCB in systems such as photon Bose–Einstein condensates and phononic lattices. The proposed mechanism predicts new quasiparticle excitations that couple to electromagnetic fields despite being composed of electrically neutral constituents, opening avenues for novel optoelectronic devices and quantum information platforms. The paper concludes with a discussion of experimental signatures, potential realization in existing platforms, and the broader implications for the classification of quasiparticles in many‑body systems.
Abstract The original Bose–Einstein condensation (BEC) was theoretically established in the 1920s, for the case of an ideal gas of boson particles (a consequence of the symmetry of the wave function). In the 1990s, it was realized in systems of atomic alkali gases. At the beginning of the 21st century, investigators considered its emergence in solid‐state matter, and a BEC‐like phenomenon was observed in the case of bosonic quasiparticles (elementary excitations in solids). Both types of BEC can occur in a condensation in conditions of thermodynamic equilibrium. A third type of what has been dubbed a BEC in non‐equilibrium conditions can occur in systems of bosonic quasiparticles (phonons, magnons, excitons); this phenomenon (which can be considered as cases of systems with complex behavior) follows after a certain threshold of intensity of an external pumping source is achieved. In this review, after brief comments on the two types of BEC in equilibrium, a description is presented on how a BEC‐like phenomenon arises in systems of quasiparticles that are driven away from thermodynamic equilibrium, and several examples are described. This description is given within a unique theoretical framework, and a comparison with experimental results is performed; in addition, the possible technological applications are outlined.
Abstract The seminal phenomenon of the Bose-Einstein condensation has been observed on a large variety of physical platforms, e.g., superfluid helium, fermions with attractive interactions, quasiparticles in solids, and ultracold atomic gases. Recently, the Bose–Einstein condensation of non-interacting photons in an optical microcavity has been confirmed experimentally. Here, we report the experimental observation of Bose-Einstein like condensation and collective Bogoliubov excitations in a low dissipative microwave resonator with embedded superconducting quantum network composed of multiple flux qubits. Under strong coherent pumping the resonator mode becomes macroscopically populated, giving rise to hybridized photon–qubit collective modes. The Bogoliubov-like collective excitations appearing in a response to a nearby-frequency probe tone, accompanied by pronounced bistability and hysteresis in the measured transmission coefficient, provide the first experimental evidence of a Bose–Einstein–like condensate of microwave photons on a superconducting circuit platform. A theoretical framework based on Gross-Pitaevskii formalism and taking into account the ac Stark effect inducing the effective interaction between photons, quantitatively reproduces the experimental observations and identifies the regime of appearance of microwave Bogoliubov excitations. These results establish a proof of principle for exploiting driven collective states in superconducting circuits as a platform for quantum-limited microwave detection and engineered many-body photonic systems for quantum information processing.
Bose-Einstein condensation has in the last two decades been observed in cold atomic gases and in solid-state physics quasiparticles, exciton-polaritons and magnons, respectively. The perhaps most widely known example of a bosonic gas, photons in blackbody radiation, however exhibits no Bose-Einstein condensation, because the particle number is not conserved and at low temperatures the photons disappear in the system’s walls instead of massively occupying the cavity ground mode. This is not the case in a small optical cavity, with a low-frequency cutoff imprinting a spectrum of photon energies restricted to values well above the thermal energy. The here reported experiments are based on a microscopic optical cavity filled with dye solution at room temperature. Recent experiments of our group observing Bose-Einstein condensation of photons in such a setup are described. Moreover, we discuss some possible applications of photon condensates to realize quantum manybody states in periodic photonic lattices and photonic Josephson devices.
We explore the formation and collective modes of Bose-Einstein condensate of Dirac magnons (Dirac BEC).While we focus on two-dimensional Dirac magnons, an employed approach is general and could be used to describe Bose-Einstein condensates with linear quasiparticle spectrum in various systems.By using a phenomenological multicomponent model of pumped boson population together with bosons residing at Dirac nodes, the formation and time evolution of condensates of Dirac bosons is investigated.The condensate coherence and its multicomponent nature are manifested in the Rabi oscillations whose period is determined by the gap in the spin-wave spectrum.A Dirac nature of the condensates could be also probed by the spectrum of collective modes.It is shown that the Haldane gap provides an efficient means to tune between the gapped and gapless collective modes as well as controls their stability.
The paradigm of Bose-Einstein condensation has been associated with zero momentum to which a macroscopic fraction of bosons condense. Here we propose a new quantum state where bosonic alkali-metal atoms condense at nonzero momenta, defying the paradigm. This becomes possible when the atoms are confined in the ppp-orbital Bloch band of an optical lattice rather than the usual sss-orbital band. The new condensate simultaneously forms an order of transversely staggered orbital currents, reminiscent of orbital antiferromagnetism or ddd-density wave in correlated electronic systems but different in fundamental ways. We discuss several approaches of preparing atoms to the ppp-orbital band and propose an ``energy blocking'' mechanism by Feshbach resonance to protect them from decaying to the lowest sss-orbital band. Such a model system seems very unique and novel to atomic gases. It suggests a new concept of quantum collective phenomena of no prior example from solid state materials.