Quantum computation mediated by ancillary qudits and spin coherent states

Timothy J. Proctor, Shane Dooley, Viv Kendon

Research output: Contribution to journalArticlepeer-review


Models of universal quantum computation in which the required interactions between register (computational) qubits are mediated by some ancillary system are highly relevant to experimental realizations of a quantum computer. We introduce such a universal model that employs a d-dimensional ancillary qudit. The ancilla-register interactions take the form of controlled displacements operators, with a displacement operator defined on the periodic and discrete lattice phase space of a qudit. We show that these interactions can implement controlled phase gates on the register by utilizing geometric phases that are created when closed loops are traversed in this phase space. The extra degrees of freedom of the ancilla can be harnessed to reduce the number of operations required for certain gate sequences. In particular, we see that the computational advantages of the quantum bus (qubus) architecture, which employs a field-mode ancilla, are also applicable to this model. We then explore an alternative ancilla-mediated model which employs a spin ensemble as the ancillary system and again the interactions with the register qubits are via controlled displacement operators, with a displacement operator defined on the Bloch sphere phase space of the spin coherent states of the ensemble. We discuss the computational advantages of this model and its relationship with the qubus architecture. © 2015 American Physical Society.
Original languageEnglish
Article number012308
Number of pages11
JournalPhysical Review A - Atomic, Molecular, and Optical Physics
Issue number1
Publication statusPublished - 8 Jan 2015


  • degrees of freedom (mechanics)
  • phase space methods
  • quantum computers
  • ancillary system
  • computational advantages
  • controlled phase gate
  • discrete lattices
  • displacement operators
  • experimental realizations
  • geometric phasis
  • spin-coherent state
  • quantum theory


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