Thermally induced creation of quantum coherence

We describe and quantify the emergence of coherence between quantum systems that are initially at thermal equilibrium. We show that the coherence can be induced by a conditional locking method based on a feasible joint detection of photons spontaneously emitted by the systems. The energy employed by the locking method comes solely from the initial thermal excitation of the systems, which makes the method suitable for a wide range of experimental platforms. We show that the same method can be applied to a pair of two-level systems or to a pair of quantum oscillators and we compare the results with regard to their first- and second-order correlation functions. We also propose feasible experimental tests to verify the thermally induced coherence.

Figure 1

Conditional birth of quantum coherence of matter systems in four steps. (a) Matter systems $A$ and $R$ are thermally excited by the interaction with independent environments. (b) After the interaction is switched off, the systems spontaneously emit light, which is directed towards the joint detection scheme. (c) Photons are registered by the detectors and systems are postselected. (d) Coherent quantum systems $A$ and $R$.

Figure 2

First-order and second-order coherence generated from thermal equilibrium states as a function of the number of detected photons $M$ in a single step of the procedure for ideal optical interference $V_0=1$ (top) and as a function of visibility $V_0$ of the optical interference for $M=20$ steps of the procedure with single-photon detection (bottom): local $g_{A,R}^{\left(2\right)}\left(0\right)$ (dotted line), mutual $g_{AR}^{\left(2\right)}\left(0\right)$ (dashed line), and mutual visibility $V_{AR}$ (solid line).

Figure 3

Mutual visibility $V_{AR}$ relative to visibility of individual steps $V_0$ and their number $M$.