The Carnot cycle is an ideal reversible cycle operating between thermal reservoirs at absolute temperatures $T_H$ and $T_L$.

For a heat engine, it contains four reversible processes:

1. isothermal expansion at $T_H$, absorbing heat $Q_H$;
2. isentropic expansion from $T_H$ to $T_L$;
3. isothermal compression at $T_L$, rejecting heat $Q_L$;
4. isentropic compression back to the initial state.

Because the reversible heat transfers satisfy

$$\frac{Q_L}{Q_H}=\frac{T_L}{T_H},$$

the maximum possible engine efficiency between those reservoirs is

$$\eta_{Carnot}=1-\frac{T_L}{T_H}.$$

Reversing the cycle gives the maximum refrigerator and heat-pump performance:

$$COP_{R,Carnot}=\frac{T_L}{T_H-T_L},\qquad COP_{HP,Carnot}=\frac{T_H}{T_H-T_L}.$$

For $T_H=600\ \mathrm K$ and $T_L=300\ \mathrm K$, $\eta_{Carnot}=0.50$.

Carnot performance is a second-law benchmark, not a practical machine prescription: real heat transfer requires finite temperature differences and real components generate entropy.