The frontier between quantum and classical physics

Cartoon depiction of coherence time. 
The sphere surrounded by a bubble represents an isolated quantum state. 
Environmental disruptions cause a quantum superposition to dissipate. 
Here the final system has two distinguishable states, 
represented by blue and yellow poles and is no longer in a coherent quantum superposition.

Two of the main differences 

between classical and quantum physics are 

causality and quantification. 

However, the relationship between classical and quantum theory is way more complex than that.

To understand this relationship it is necessary to look at the intuitive ideas of the founders of quantum theory.  One such idea is Heisenberg’s quantization theory, and  another is the Bohr’s correspondence principle or the Schrodinger’s wave packets (or coherent states). They continue to be of great importance in understanding classical behavior from quantum mechanics.
On the other hand, no consensus has been reached on the Copenhagen Interpretation.

The properties of quantum mechanics have led to the identification and quantification of many nonclassical quantum properties such as quantum entanglement, nonlocality or quantum discord (measure of nonclassical correlations between two subsystems of a quantum system). With many new applications that exploit these nonclassical properties it became more and more important to understand the frontier between classical and quantum physics through the theory of quantum coherence. This resource theory is similar to the study of entanglement.

Recently, a new study was presented that showed how to quantify the amount of coherence present in an arbitrary superposition of coherent states. This was achieved by unifying two well-known notions in quantum information theory and quantum optics: the concept of quantum coherence that was recently developed based on the framework of quantum resource theories, and the notion of nonclassicality of light that has been established since the 1960s based on the quantum theory of light. Kok Chuan and his team suggested that identical quantum resources underlie both quantum coherence in the discrete finite dimensional case and the nonclassicality of quantum light. They showed mathematically that by doing this it will be possible to measure and quantify the coherence between coherent states which will ease the development of quantum applications.

"Our hope is to be able to create synergy, where the tools and insights we gain from coherence can be used to achieve greater insight into the inner workings of nonclassical light and vice versa. For instance, our work suggests that the fact that both coherence and nonclassical light can both be converted to entanglement is no mere accident."
Kok Chuan Tan from Department of Physics and Astronomy in Seoul