Doublers

Doublers is a term used in various scientific and engineering contexts to describe devices or phenomena that produce a doubling effect, most commonly a doubling of frequency or of distinct particle species in a lattice. The specifics depend on the field, but the underlying idea is the appearance of an additional copy or harmonic that is not present in the original system.

In electronics and signal processing, a frequency doubler is a circuit that outputs a signal at approximately twice the input frequency. This is typically achieved by exploiting nonlinearities in components such as diodes, transistors, or varactors to generate harmonics, followed by filtering or mixing to isolate the second harmonic. Frequency doublers are used in RF and microwave applications for signal generation and upconversion, as well as in test and measurement equipment. Key performance factors include conversion efficiency, harmonic suppression, input power handling, phase noise, and spectral purity.

In lattice gauge theory and high-energy physics, a fermion doubler refers to extra, unphysical fermion modes that arise when fermion fields are discretized on a space-time lattice. Naive discretization yields 2^d fermion species in d dimensions, rather than a single physical fermion, due to lattice artifacts. This doubling complicates simulations of quantum field theories. Remedies include the Wilson formulation, which adds a term to lift the doublers’ masses; staggered (Kogut-Susskind) fermions, which reduce the number of doublers; and more modern approaches such as domain-wall and overlap fermions, which aim to preserve chiral symmetry with higher computational cost. The issue is related to the Nielsen-Ninomiya no-go theorem, which constrains how locality, chiral symmetry, and a single fermion species can be achieved on a lattice.