In April 2025, a team of physicists from the Czech Republic, France, and Singapore accomplished something that had been debated for over a decade: they experimentally proved that quantum coherence can make a heat engine run faster. Published in Nature Physics, their demonstration of a quantum speed-up in an Otto cycle using a superconducting transmon qubit represents a watershed moment for quantum thermodynamics — and a critical milestone on the road to practical quantum energy technologies.

The result didn't just confirm a theoretical prediction. It established a fundamental bound on the minimum cycle time for quantum heat engines, revealing a deep connection between quantum mechanics and the ultimate limits of thermodynamic performance. For the Floquet engineering community, where periodic driving of quantum systems is the central technique, this work opens the door to a new generation of coherence-enhanced thermal machines.

The Classical Speed Limit Problem

Heat engines — from the steam engines of the Industrial Revolution to the turbines powering modern grids — all face a fundamental trade-off: efficiency versus speed. Run an engine slowly, and it can approach the theoretical Carnot efficiency. Run it fast, and friction, turbulence, and irreversible losses eat into performance.

At the quantum scale, this trade-off takes on new dimensions. A quantum Otto engine operates by cycling a quantum system (the "working substance") between two thermal reservoirs — one hot, one cold — through a series of strokes that alternately change the system's energy levels and allow it to thermalize. The classical Otto cycle, familiar from internal combustion engines, has a quantum analogue where energy levels of atoms or qubits replace the compression and expansion of gas.

What Is a Quantum Otto Cycle?

A quantum Otto cycle consists of four strokes: (1) an isochoric (constant-volume analogue) heating stroke where the system contacts a hot reservoir, (2) a work stroke where energy levels are shifted, (3) an isochoric cooling stroke where the system contacts a cold reservoir, and (4) a return work stroke. At the quantum scale, the "working substance" can be a single qubit, an ion, or even a superconducting circuit — and quantum effects like coherence and entanglement can fundamentally alter performance.

The key question has been: when you shrink a heat engine down to the quantum realm, can you actually extract more power — not just the same power from a smaller device, but genuinely better performance per cycle? Or are quantum effects merely cosmetic, leaving the classical speed-efficiency trade-off intact?

The Breakthrough: Coherent Pulses on a Transmon Qubit

The experiment, led by Jiří Minář and Artem Kovalenko at Palacký University in Olomouc, together with Léa Bresque and Alexia Auffèves from CNRS/Institut Néel in Grenoble, used a superconducting transmon qubit — the same type of quantum bit that powers Google's and IBM's quantum computers — as their engine's working substance.

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Measured power increase from coherent driving compared to incoherent driving of the same average energy — a genuinely quantum advantage verified in a superconducting circuit.

The experimental protocol was elegant in its directness. The team drove their Otto cycle with coherent pulses — carefully phase-controlled microwave signals that create quantum superpositions in the qubit. They then measured the power output and compared it against a critical benchmark: an incoherent drive delivering exactly the same average energy, but without quantum coherence.

The result was unambiguous. The coherent drive produced measurably more power. And because the comparison was made at equal average energy, the excess power could only come from one source: quantum coherence itself acting as a thermodynamic resource.

"We demonstrate that quantum coherence can provide a provable and measurable thermodynamic speed-up in a quantum Otto cycle... We verify its quantum origin by comparing the power with a benchmark given by incoherent drive of same average energy." — Minář et al., Nature Physics 21, 939–943 (2025)

Why Coherence Matters: The Quantum Speed-Up Explained

To understand why coherence provides a speed-up, we need to think about what happens during the work strokes of a quantum Otto cycle. In a classical engine, the compression and expansion strokes must proceed slowly enough for the system to remain close to equilibrium — this is the "adiabatic" requirement. Go too fast, and you generate unwanted excitations that waste energy.

In a quantum engine, the analogous constraint is even more severe. The quantum adiabatic theorem says that if you change the system's Hamiltonian (its energy landscape) too quickly, you'll excite transitions between energy levels, reducing the work output. This sets a quantum speed limit on how fast the cycle can run.

But coherence changes the game. When the driving pulses maintain quantum coherence — a well-defined phase relationship between quantum states — the system can navigate its energy landscape more efficiently. Think of it as the difference between pushing a ball through a maze blindly (incoherent) versus having a GPS that knows all the shortcuts (coherent). The coherent drive doesn't just add energy; it adds information that the quantum system can exploit.

Coherence as a Thermodynamic Resource

In quantum thermodynamics, coherence occupies a special status. Unlike classical correlations, quantum coherence in the energy eigenbasis represents genuine "quantumness" that has no classical analogue. The 2025 experiments prove that this coherence is not just a theoretical curiosity — it provides measurable operational advantages that translate directly into increased power output.

Crucially, the Czech-French team also showed that this quantum speed-up is connected to a fundamental bound on the minimal cycle time. There exists a quantum speed limit — a minimum time below which even coherent driving cannot push the cycle. But this limit is shorter than the limit for incoherent driving. Quantum mechanics literally raises the thermodynamic speed limit.

A Wave of Quantum Otto Experiments in 2025

The Nature Physics result didn't emerge in isolation. The year 2025 has seen an extraordinary convergence of quantum Otto engine experiments across multiple physical platforms, each contributing a different piece of the puzzle.

Single-Ion Engines: Coherence in a Calcium Trap

In March 2025, a team published in Nature Communications a demonstration of a coherence-enhanced Otto engine using a single trapped calcium-40 ion. Using a cycling transition as the working medium, they showed that coherent control of the ion's quantum states similarly boosted engine performance. The trapped-ion platform offers exquisite control over individual quantum systems, providing complementary evidence to the superconducting-qubit results.

Quantum Otto on a Silicon Surface

Also in March 2025, researchers demonstrated a quantum Otto engine on a silicon surface using scanning tunneling microscopy and single-molecule manipulation, published in Communications Physics. This brought quantum heat engines into the realm of surface science, showing that even molecular-scale systems on solid substrates can execute thermodynamic cycles.

Superconducting Qubit Implementations

Multiple groups independently implemented quantum Otto cycles on superconducting qubits in early 2025. One notable effort, led by Parth Jatakia and collaborators, provided detailed characterization of gate errors, unintended coherences, and their effects on extracted work — crucial engineering knowledge for building practical quantum thermal machines.

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Independent experimental demonstrations of quantum Otto engines published in 2025 alone, spanning superconducting qubits, trapped ions, and molecular systems — an unprecedented surge in quantum heat engine research.

The Floquet Connection: Periodic Driving as Engine Fuel

For readers of Floquet Research, the connection between these results and Floquet engineering should be immediately apparent. A coherent drive applied to a quantum system at a fixed frequency is, by definition, periodic driving — the central tool of Floquet engineering. The quantum speed-up in Otto engines is, at its heart, a Floquet phenomenon.

This connection has been made explicit in a series of theoretical papers published alongside the experimental results. In June 2025, Sanaa Abaach, Zakaria Mzaouali, and Morad El Baz published a detailed analysis titled "Floquet-enhanced quantum Otto cycle through quantum coherence" (arXiv: 2506.01261). Their key finding: when periodic driving is applied during the thermalization strokes of an Otto cycle, it creates Floquet-modified thermal states that contain coherence in the energy eigenbasis. This coherence then acts as a thermodynamic resource, enabling enhanced work extraction compared to the standard Otto cycle.

"The periodic driving generates coherence in the energy eigenbasis, which acts as a thermodynamic resource, enabling enhanced work extraction compared to the standard Otto cycle." — Abaach et al., arXiv: 2506.01261 (2025)

By tuning the driving frequency and amplitude — standard Floquet engineering parameters — the engine's performance can be continuously controlled, with specific optimal regimes identified for maximum power and efficiency. This is Floquet engineering applied directly to thermodynamics: using the quasi-energy structure of periodically driven systems to design better thermal machines.

From Engines to Refrigerators: Floquet Cooling

The Floquet thermodynamics story extends beyond heat engines into quantum refrigeration — a technology with direct applications in quantum computing, where maintaining qubits at millikelvin temperatures is essential.

In April 2025, Anu Kv and B. Sharmila published a theory of Floquet-driven quantum Otto refrigerators (arXiv: 2504.19707) showing that periodic modulation of a harmonic oscillator working medium significantly enhances refrigeration performance. The cooling window broadens, and the coefficient of performance (COP) can approach and even surpass the static Otto limit under optimal driving conditions.

Earlier in the year, general principles of Floquet quantum refrigerators were established (arXiv: 2502.12279), creating a theoretical framework for understanding how periodically driven systems extract heat from cold reservoirs. Together with work on continuous Floquet quantum refrigerators (arXiv: 2501.09389), a comprehensive picture of Floquet-enhanced cooling is emerging.

Why Quantum Refrigerators Matter

Quantum computers require operating temperatures near absolute zero — typically around 15 millikelvin for superconducting qubits. Current dilution refrigerators are expensive, power-hungry, and difficult to scale. Floquet-enhanced quantum refrigerators could potentially provide more efficient cooling at the quantum level, reducing the overhead for quantum computing and enabling new cryogenic technologies.

The Stochastic Floquet Engine: Noise as an Ally

One of the most surprising developments came in late 2024, when A. Levy, M. Göb, and collaborators introduced the stochastic Floquet quantum heat engine, published in Physical Review Letters (PRL 133, 260402). This novel thermodynamic machine combines Floquet periodic driving with random noise — essentially asking: what happens when your periodic drive isn't perfectly periodic?

The answer challenges conventional wisdom. Rather than degrading performance, stochastic fluctuations in the drive can actually be harnessed as an additional thermodynamic resource. The stochastic Floquet engine represents a new paradigm where imperfection in quantum control isn't a bug — it's a feature. This is particularly relevant for practical implementations, where perfect periodic driving is never achievable.

Topological Heat Transport: Directing Heat Flow with Floquet Engineering

Beyond individual engines and refrigerators, 2025 has also seen major advances in using Floquet engineering to control the flow of heat through quantum systems — the thermal analogue of electronic circuit design.

Three significant papers have appeared:

The thermal diode result is particularly striking. Just as an electronic diode allows current to flow in only one direction, a Floquet-engineered thermal diode allows heat to flow preferentially in one direction. This could enable entirely new architectures for thermal management at the quantum scale — directing heat away from sensitive quantum components or channeling it toward energy harvesting systems.

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Major papers published in 2025 directly connecting Floquet engineering to quantum thermodynamic applications — from engines and refrigerators to topological heat transport and thermal diodes.

Implications for Quantum Energy Technology

What does all this mean for the future of energy technology? Several threads are converging:

1. Quantum coherence is a proven thermodynamic resource. The debate is over. Multiple experimental platforms have now confirmed that quantum coherence provides measurable, operational advantages in heat engines. The question shifts from "does it work?" to "how do we scale it?"

2. Floquet engineering provides the control knobs. By tuning driving frequency, amplitude, and waveform, researchers can continuously optimize quantum thermal machines. This makes Floquet techniques the natural toolkit for designing next-generation quantum energy devices.

3. The quantum advantage grows at extreme miniaturization. As devices shrink toward the quantum regime — in computing, sensing, and energy harvesting — the relative advantage of coherence-enhanced thermal machines increases. Classical thermodynamic limits become increasingly restrictive at small scales, precisely where quantum effects shine.

4. Noise tolerance is achievable. The stochastic Floquet engine results suggest that quantum thermal machines don't need perfect conditions to outperform classical ones. This is essential for practical applications outside the pristine environment of a physics laboratory.

The Road Ahead

The 2025 quantum Otto engine revolution is just the beginning. Several key challenges remain:

For the Floquet engineering community, the message from 2025 is clear: periodic driving isn't just a tool for creating exotic topological phases or time crystals. It's becoming the foundation of a new approach to thermodynamics — one where quantum mechanics doesn't just add complications to classical physics, but provides genuinely new capabilities that have no classical analogue.

The quantum speed limit has been rewritten. The engine of the future runs on coherence.

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