Quantum Engine Harvests Energy from Spin, Not Just Heat Alone

Researchers are developing novel heat engines that move beyond traditional designs requiring two energy reservoirs. André R. R. Carvalho, Liam J. McClelland, and Erik W. Streed, from the Queensland Quantum and Advanced Technologies Research Institute and Griffith University, alongside Joan Vaccaro, demonstrate a quantum spin-heat engine (SHE) operating between energy and spin thermal reservoirs. Their proposed implementation utilises trapped ions and extracts work via Raman transitions, crucially resetting spin states using a spin reservoir without energetic cost. This represents a significant advance, potentially enabling the harnessing of quantum coherence in conserved quantities and opening avenues for entirely new thermodynamic devices.

This novel engine functions using both energy and spin thermal reservoirs, differing from traditional designs that rely on two energy reservoirs alone.

The work detailed in this study successfully converts heat into optical work via a two-photon Raman transition, utilising close-to energy degenerate spin states within the trapped ion. The engine operates in two distinct stages, beginning with the extraction of work from a thermal energy reservoir. This conversion is achieved through resonant optical transitions manipulating the ion’s spin states.
Subsequently, the internal spin states are reset using a spin reservoir in a process termed non-energetic information erasure. Crucially, this reset does not require energy input, instead occurring at the cost of angular momentum transferred from the spin bath acting as the thermal spin reservoir. The device leverages a three-level trapped ion, where two ground states are spin-degenerate, and vibrational degrees of freedom mediate heat exchange during the engine cycle.

Initial simulations utilized ions initialized in the state ρ = |↑⟩⟨↑| ⊗ρν(0), with ρν(0) representing a thermal vibrational state. With d ≥2 internal spin states and initial temperatures ranging between room temperature and typical power plant operating temperatures, this engine could potentially exceed the efficiency limits of a standard Carnot engine. The research paves the way for exploring devices that extract energetic work from a hot thermal bath using a cold, spin-polarized reservoir, opening possibilities for advanced thermodynamic applications.

Raman pulse driven energy transfer and spin reservoir reset within a trapped ion system enable high-fidelity quantum control

A three-level trapped ion system forms the basis for implementing an optical spin-heat engine (SHE), designed to operate between thermal energy and spin reservoirs. The working fluid consists of a trapped ion possessing two energy-degenerate ground states distinguished solely by their spin, held within a harmonic potential.

Initialisation occurs with the ion in the state ρ = |↑⟩⟨↑| ⊗ρν(0), where ρν(0) represents a thermal vibrational state, establishing the initial conditions for the engine cycle. Subsequently, the internal spin states are reset to their original configuration, utilising a spin reservoir to dissipate angular momentum as spintherm Q, completing the cycle. This methodology diverges from conventional heat engines requiring two thermal reservoirs, instead employing a spin reservoir to balance the cycle without energetic cost.

The study leverages the principles of non-Abelian thermal states and generalised Gibbs ensembles to describe the thermal state of subsystems exchanging non-commuting conserved quantities. By manipulating the ion’s spin and vibrational degrees of freedom, the research demonstrates the potential to surpass the Carnot efficiency limit, particularly with ions exhibiting d ≥2 internal spin states and operating between room temperature and typical power plant temperatures.

Spin-dependent work extraction and angular momentum dissipation in a trapped ion system represent a novel approach to quantum control

A three-level trapped ion system demonstrates the potential for a spin-heat engine (SHE) capable of converting heat into optical work via a two-photon Raman transition. SHE, operating between energy and spin thermal reservoirs.

During the work extraction stage, energy from motional states is transferred to the optical field as useful work, simultaneously utilising angular momentum from the initial spin distribution. The amount of work extracted corresponds directly to the vibrational energy converted in the process, calculated as ħδP↓, where δ represents the energy difference between the two Raman lasers and P↓ is the final population in the lower level.

Angular momentum changes during the transition necessitate dissipation of spin labor as spintherm into a spin reservoir. The change in total angular momentum, quantified as L = ħ 2(P↑−P↓) −ħ 2 = −ħP↓, accounts for this dissipation, where Jz is the z component of angular momentum. Energy and angular momentum are interconnected throughout the cycle, adhering to conservation laws where the absolute values of heat and work are equal, and the absolute values of spin labor and spintherm are also equal.

The system operates by initializing the ion in a state ρ = |↑⟩⟨↑| ⊗ρν(0), where ρν(0) represents a thermal vibrational state. Following the work extraction phase, the system contacts a spin reservoir to remove entropy from the working fluid, returning the electronic state to |↑⟩ and restoring the initial thermal motional state ρν(0), thus completing the cycle. This engine functions by converting heat into optical work through a Raman transition between nearly degenerate spin states, subsequently resetting the spin states via an energetically cost-free process utilising a spin reservoir.

The process involves an exchange of angular momentum from a spin bath, effectively acting as the thermal spin reservoir. This implementation serves as a crucial initial step towards realising heat engines that transcend the limitations of traditional systems. It establishes a mechanism for achieving energy efficiencies exceeding the Carnot limit in systems possessing multiple conserved quantities and illustrates the trade-offs inherent in quantum coherence between these quantities.

The engine’s operation explicitly demonstrates work exchange between distinct conserved quantities, highlighting a novel approach to energy conversion. The authors acknowledge that the current implementation relies on an ion trap and represents a simplified model. Further analytical solutions were derived to explore the system’s dynamics at varying temperatures, detailing the evolution of vibrational excitation and the influence of the Raman transition. Future research may focus on expanding this framework to more complex systems and exploring the potential for harnessing quantum coherence in a wider range of conserved quantities, potentially leading to advanced energy technologies.