Quantum partition noise in a superconducting tunnel junction

The theory of charge partition noise of quasiparticles in a superconducting tunnel junction is developed. The charge fluctuations are shown to have a significant contribution from partition noise that arises from the quantum superposition of the electron and hole character of the quasiparticles. These fluctuations are dominant at small bias voltage. The charge fluctuations are compared to the usual Poisson ``shot noise'' of the current. The implications for the design of energy-resolving single-photon detectors are explored.