Towards a Particle-Number Conserving Theory of Majorana Zero Modes in p+ip Superfluids

Majorana zero modes are predicted to exist in p+ip (either inherent or effective due to proximity effect) superfluids and are proposed to be used for constructing topological qubits for topologically protected quantum computing. Existing theories on the subject are mostly based on BCS mean-field theory which breaks particle number conservation (U(1) symmetry). More specifically, Bogoliubov-de Gennes (BdG) equations are used to derive Majorana zero modes and their braiding properties. The broken particle number conservation is, on the other hand, respected in any fermionic condensed matter system and therefore may be crucial in studying quantum coherence, essential for quantum computing. In this paper, we work towards a particle-number conserving theory by examining the role played by particle number conservation in topological properties of Majorana zero modes. We conclude that the latter may be affected by the former and new theoretical framework that respects particle number conservation is needed in establishing (or dismissing) the existence of Majorana zero modes in p+ip superfluids and their application to topological quantum computing.