- Fundamentals
Quantum Battery
A quantum battery is a quantum system that stores energy using quantum mechanical effects such as entanglement and coherence, potentially allowing faster charging and higher energy density than classical batteries.
A quantum battery is a finite-dimensional quantum system whose internal energy can be controllably increased and later extracted to do work. The concept sits at the intersection of quantum thermodynamics, quantum information, and open systems physics. Unlike chemical or electrochemical batteries, a quantum battery stores energy directly in the quantum degrees of freedom of matter, including the population of excited states, coherent superpositions, and entangled configurations across many subsystems, and releases it on demand through coherent driving.
Why quantum mechanics might help
The central question in quantum battery research is whether quantum resources (coherence, entanglement, and many-body correlations) can outperform classical energy storage in any operationally meaningful sense. Two figures of merit have emerged as primary targets: charging power (energy stored per unit time) and ergotropy (the maximum work extractable by unitary operations on the battery state).
For charging power, a key theoretical result is that collective charging of N two-level systems can in principle be superextensive: the power can scale faster than linearly in N when the charger drives the battery qubits coherently and entanglement builds up across them. A classically charged array of N independent batteries gains energy at a rate proportional to N. A quantum battery with global unitary charging can, for certain Hamiltonians, achieve a rate scaling as N^(3/2) or faster. This quantum charging advantage was identified theoretically around 2012 and has since been the subject of intense debate about whether it survives in open systems with realistic decoherence.
Ergotropy and coherence
Not all energy stored in a quantum state is extractable as work. The ergotropy of a state rho with Hamiltonian H is the maximum energy that can be drawn out by a unitary operation alone, without access to an ancillary heat bath. The difference between the internal energy Tr(rho H) and the ergotropy is the passive energy: energy trapped in the state that cannot be released unitarily. Coherence in the energy eigenbasis can increase ergotropy compared to an incoherent state with the same energy populations, meaning that maintaining quantum coherence during charging and storage directly increases the battery’s useful output.
This also highlights a fundamental challenge: decoherence, the unavoidable interaction of the battery with its environment, degrades both coherence and entanglement over time. In practice, a quantum battery must be shielded from its environment or operated on timescales short compared to its coherence time. For current physical platforms this means quantum batteries are best viewed as short-lived, ultrafast energy storage devices rather than long-term repositories.
Physical implementations
Several platforms have been proposed and, in some cases, demonstrated as quantum batteries. Superconducting qubit circuits, where microwave photons stored in resonators represent the energy, allow rapid coherent charging and have been used to demonstrate ergotropy extraction. Organic photovoltaic molecules have been proposed as room-temperature quantum batteries, with some experimental evidence of coherence-assisted energy transfer. Nuclear spin systems and nitrogen-vacancy centers in diamond offer long coherence times and precise control. Photonic systems, where energy is stored in optical modes, enable room-temperature operation and integration with quantum optical networks.
Open questions and outlook
Quantum battery research remains largely theoretical and proof-of-concept experimental. Key open questions include whether the superextensive charging advantage survives in realistic open system models with Markovian noise, how to measure ergotropy directly in laboratory systems, and whether any application requires a quantum battery rather than a classical capacitor or inductor operating on similar timescales. The field is closely connected to quantum thermodynamics more broadly: the laws of thermodynamics for quantum systems, the role of information in work extraction, and the ultimate limits on efficiency imposed by the uncertainty principle.