A team of physicists has demonstrated a prototype quantum battery built directly inside a quantum computer made of superconducting qubits.
Beyond the headline, the experiment serves as a test-bed for fundamental questions in quantum thermodynamics: can entanglement and coherence be exploited to store — and more importantly, rapidly retrieve — useful work at lower energetic cost than any classical device?
What Exactly Is a Quantum Battery?
In classical electronics a battery is a chemical reservoir of charge.
A quantum battery, by contrast, is an engineered quantum system whose energy can be coherently charged and discharged. The stored “fuel” is not chemical potential but quantized excitations (photons, spin flips, Cooper-pair pairs, etc.) that can, in principle, be manipulated faster and with less dissipation via quantum control protocols.
Key Metrics
• Ergotropy: the maximum work extractable by a unitary (non-dissipative) operation.
• Charging power: energy stored per unit time.
• Quantum advantage factor: ratio between optimal quantum and classical charging times for the same energy.
Superconducting Qubits as an Energy Store
Superconducting circuits operate at millikelvin temperatures where electrons form Cooper pairs that carry current with zero resistance.
Each transmon qubit is a nonlinear resonator with two lowest energy levels |0⟩ and |1⟩, separated by ~5 GHz (≈ 20 μeV). Loading the |1⟩ state therefore stores a well-defined quantum of electromagnetic energy.
The new experiment groups several qubits into a composite cell that can be collectively driven. Coupling them through a shared resonator enables entangling operations that standard battery theory predicts will accelerate charging.
The Fast-Charging Protocol
1. Initialization: All qubits are cooled and reset to |0⟩.
2. Collective drive: A microwave pulse globally addresses the qubits via the resonator. Because the qubits are mutually coupled, the Hamiltonian contains terms like σxiσxj that create entanglement during charging.
3. State freeze-out: After an optimised time t*, the fields are turned off, leaving the qubits in a many-body excited state with high ergotropy.
4. Work extraction test: A reverse unitary converts the stored excitations into a measurable microwave pulse, confirming that the energy can be retrieved on demand.
Results at a Glance
• Charging time scaled approximately with 1/N (number of qubits), beating the 1/√N scaling expected for independent charging.
• Up to 90 % of the injected energy remained extractable after several microseconds, well above typical qubit coherence times used in gate operations.
• Total energy stored was orders of magnitude smaller than macroscopic batteries, but sufficient to run dozens of single-qubit gates—proof-of-principle for on-chip power delivery.
Why Does Entanglement Help?
In classical parallel charging each cell absorbs energy independently, so the fastest rate grows linearly with N. Quantum mechanics introduces superabsorption: because multiple qubits share a delocalised excitation, the dipole moment of the ensemble adds coherently, allowing stronger coupling to the drive field.
Mathematically, the charging Hamiltonian features collective spin operators S± whose matrix elements scale as √N, boosting power without increasing drive amplitude.
Thermodynamic Cost
One must account for the work spent generating entanglement and later disentangling the battery from the charger. The experiment measures the total microwave energy delivered and compares it to the ergotropy obtained, showing a net gain of ~15 % over separable charging sequences.
Challenges and Open Questions
• Decoherence: Entangled states are fragile; scaling to hundreds of qubits will demand error-corrected batteries or ultra-fast protocols.
• Cycle life: Repeated charge/discharge could amplify gate errors and quasiparticle poisoning in superconductors.
• Integration: How to route the extracted power to computational qubits without converting it back to room-temperature electronics?
Future Outlook
The demonstration brings quantum batteries from abstract theory into hardware. Possible next steps include:
– Engineering topologically protected batteries using Majorana modes for longer coherence.
– Embedding quantum batteries in cryogenic control chips to locally power classical logic at 4 K, reducing wiring heat loads.
– Exploring hybrid approaches where quantum batteries recharge photonic qubits in a quantum network.
Bottom Line
The superconducting-qubit experiment shows that fast, low-cost charging leveraging entanglement is not only theoretically plausible but experimentally viable. While we are far from replacing lithium-ion cells, quantum batteries could become essential sub-components of future large-scale quantum processors, supplying bursts of coherent energy exactly where and when the computation needs it.
