Polyakov loop

In quantum field theory, the Polyakov loop is the thermal version of Wilson loop, acting as an order parameter for confinement in pure gauge theories at nonzero temperatures. Their vacuum expectation value must vanish in the confined phase due to their noninvariance under centre gauge transformations. Equivalently, they are directly related to the free energy of individual quarks, which diverges in the confined phase. This also makes them useful for studying the potential between pairs of quarks at nonzero temperatures. Introduced by Alexander M. Polaykov in 1977, they are Wilson loops that wind around the compactified Euclidean temporal direction in thermal quantum field theories.