Abstract
We introduce a quantum heat engine performing an Otto cycle by using the thermal properties of the quantum vacuum. Since Hawking and Unruh, it has been established that the vacuum space, either near a black hole or for an accelerated observer, behaves as a bath of thermal radiation. In this work, we present a fully quantum Otto cycle, which relies on the Unruh effect for a single quantum bit (qubit) in contact with quantum vacuum fluctuations. By using the notions of quantum thermodynamics and perturbation theory we obtain that the quantum vacuum can exchange heat and produce work on the qubit. Moreover, we obtain the efficiency and derive the conditions to have both a thermodynamic and a kinematic cycle in terms of the initial populations of the excited state, which define a range of allowed accelerations for the Unruh engine.
Highlights
Insufficient for a heat engine to produce work in individual thermodynamic cycles
We present a fully quantum Otto cycle, which relies on the Unruh effect for a single quantum bit in contact with quantum vacuum fluctuations
Even though an experimental realization of the Unruh effect remains as a challenge, it has recently appeared a number of proposals of physical observations, such as in superconducting qubits [17], ion traps [18], quantum metrology [19], oscillating neutrinos [20], and classical electrodynamics [21]
Summary
We briefly review some notions of quantum thermodynamics and quantum heat engines. The four stages of a quantum Otto cycle are (i) The qubit begins at the initial state ρin = p|e e|+(1−p)|g g| with an energy gap ω1 and undergoes an adiabatic expansion to a larger value ω2. This expansion is at the cost of some work over the system. The probabilistic character of a quantum engine implies that we need to enforce the cyclicity of the engine through the constraint that δpH + δpC = 0, which ensures that the state of the system at the end of stage (iv) is equal to the initial state at the beginning of stage (i) By imposing such a constraint, we obtain that the total amount of work performed by the heat engine is. It can be understood by the fact that the system is expected to receive heat from the hot bath if TH > ω2 and to transfer heat to the cold bath if TC < ω1
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