Correlations in a quantum switch-based heat engine with measurements: A proof-of-principle demonstration

Allowing the order of quantum operations to exist in superposition is known to open new routes for thermodynamic tasks. We investigate a quantum heat engine where energy exchanges are driven by generalized measurements, and the sequence of these operations is coherently controlled in a superposition of causal orders. Our analysis explores how initial correlations between the working medium and the controller affect the engine’s performance. Considering uncorrelated, classically correlated, and entangled initial states, we show that entanglement enables the superposed causal order to generate coherence in the working medium, thereby enhancing work extraction and efficiency beyond the separable and uncorrelated cases. Finally, we present a proof-of-principle simulation on the IBM Quantum Experience platform, realizing a quantum switch of two measurement channels with tunable strengths and experimentally confirming the predicted efficiency enhancement enabled by correlation-assisted superposed causal order.

💡 Research Summary

The paper investigates a measurement‑driven quantum heat engine whose two generalized measurement channels are placed in a superposition of causal orders using a quantum SWITCH. The central question is how initial correlations between the working medium (a qubit Q) and the control qubit C that determines the order affect the engine’s performance. Three classes of initial joint states are considered: (i) uncorrelated product states, (ii) classically correlated separable states, and (iii) entangled states (e.g., Bell pairs).

First, the authors review the standard measurement‑based engine with a definite order. The working qubit, initially in a Gibbs state at inverse temperature β, undergoes two non‑selective measurements with tunable strengths a and b. The first measurement (strength a) acts as a heat‑injection stroke, the second (strength b) is chosen to be isentropic and thus extracts work, and a final thermalization stroke closes the cycle. Analytic expressions for the internal energy change, entropy change, extracted work W_ext, and efficiency η are derived (Eqs. 6–11). Positive work and non‑zero efficiency are only possible when the measurement strength a lies within a narrow interval that depends on β; otherwise the engine stalls.

The core contribution is the extension of this model to a superposed causal order (SCO). An additional measurement apparatus A′ with strength a′ is introduced, and the order of A and A′ is coherently controlled by the state of C via the quantum SWITCH. The SWITCH implements the map
S(M_a,M_{a′})