In mathematical physics, the primon gas or free Riemann gas is a toy model illustrating in a simple way some correspondences between number theory and ideas in quantum field theory and dynamical systems. It is a quantum field theory of a set of non-interacting particles, the primons; it is called a gas or a free model because the particles are non-interacting. The idea of the primon gas was independently discovered by Donald Spector and Bernard Julia. Later works by Bakas and Bowick and Spector  explored the connection of such systems to string theory.
The second-quantized version of this Hamiltonian converts states into particles, the primons. A multi-particle state is given by the numbers of primons in the single-particle states :
This corresponds to the factorization of into primes:
The labelling by the integer n is unique, since every number has a unique factorization into primes.
The energy of such a multi-particle state is clearly
with s = E0/kBT where kB is Boltzmann's constant and T is the absolute temperature. The divergence of the zeta function at s = 1 corresponds to the divergence of the partition function at a Hagedorn temperature of TH = E0/kB.
The supersymmetric model
The above second-quantized model takes the particles to be bosons. If the particles are taken to be fermions, then the Pauli exclusion principle prohibits multi-particle states which include squares of primes. By the spin-statistics theorem, field states with an even number of particles are bosons, while those with an odd number of particles are fermions. The fermion operator (−1)F has a very concrete realization in this model as the Möbius function , in that the Möbius function is positive for bosons, negative for fermions, and zero on exclusion-principle-prohibited states.
More complex models
The connections between number theory and quantum field theory can be somewhat further extended into connections between topological field theory and K-theory, where, corresponding to the example above, the spectrum of a ring takes the role of the spectrum of energy eigenvalues, the prime ideals take the role of the prime numbers, the group representations take the role of integers, group characters taking the place the Dirichlet characters, and so on.
- D. Spector, Supersymmetry and the Möbius Inversion Function, Communications in Mathematical Physics 127 (1990) pp. 239–252.
- Bernard L. Julia, Statistical theory of numbers, in Number Theory and Physics, eds. J. M. Luck, P. Moussa, and M. Waldschmidt, Springer Proceedings in Physics, Vol. 47, Springer-Verlag, Berlin, 1990, pp. 276–293.
- I. Bakas and M.J. Bowick, Curiosities of Arithmetic Gases, J. Math. Phys. 32 (1991) p. 1881
- D. Spector, Duality, Partial Supersymmetry, and Arithmetic Number Theory, J. Math. Phys. 39 (1998) pp. 1919–1927