Adiabatic phase gates in parity-based quantum computers
Example methods and mechanisms are described herein for implementing and adiabatically operating a topological quantum computing (TQC) phase gate that complements the existing Clifford operations, and thereby allows universal quantum computation with Majorana systems. Further embodiments include a testing system for the phase gate that is feasible with Majorana zero modes and demonstrates violations of the CHSH-Bell inequality. Further, the design used for the testing of the inequality leads directly to a practical platforms for performing universal TQC with Majorana wires in which explicit braiding need never occur. Thus, certain embodiments of the disclosed technology involve three synergistically connected aspects of anyonic TQC the context of the currently active area of using MZMs for topological quantum computation): a practical phase gate for universal topological quantum computation using MZMs, a precise protocol (using CHSH inequality) for testing that the desired gate operation has been achieved, and bypassing the necessity of MZM braiding (and so avoiding, e.g., problems of nonadiabaticity in the braids).
1. A method for operating an adjustable phase gate in a quantum circuit, the adjustable phase gate comprising a stationary pair of Ising anyons, comprising:
causing a mobile Ising anyon to pass through the adjustable phase gate, thereby creating a targeted phase change in the stationary pair of Ising anyons; and
controlling the mobile Ising anyon as the mobile Ising anyon passes through the adjustable phase gate so that the mobile Ising anyon has a complex amplitude during passage, wherein the controlling is performed such that the targeted phase change experienced by the stationary pair of Ising anyons is between 0 and π or 0 and −π.
2. The method of claim 1 , wherein the controlling comprises performing one or more of: (a) adjusting a gate voltage of one or more gates capacitively coupled to one or more superconducting islands that form the adjustable phase gate, (b) adjusting a capacitance of one or more of the superconducting islands; or (c) adjusting a strength of at least one Josephson junction relative to at least one other Josephson junction, the Josephson junctions connecting two or more of the superconducting islands to one another.
3. The method of claim 1 , further comprising:
in a first operational stage, altering a magnetic flux through the adjustable phase gate in a first direction, the first direction comprising either increasing the magnetic flux or decreasing the magnetic flux; and
in a second operational stage, altering the magnetic flux through the adjustable phase gate in a second direction that is opposite of the first direction.
4. The method of claim 3 , further comprising:
during either or both operational stages, adjusting a strength of a first Josephson junction at a first end of a superconducting region in the adjustable phase gate; and
during either or both operational stages, adjusting a strength of a second Josephson junction at a second end of the superconducting region in the adjustable phase gate,
wherein the strengths of the first and second Josephson junctions are selected to create a relative difference that produces the targeted phase change in the Ising pair.
5. The method of claim 1 , wherein the adjustable phase gate comprises a plurality of superconducting regions arranged to form a superconducting loop, wherein adjacent ones of the superconducting regions are connected to one another via respective Josephson junctions, and wherein one of the superconducting regions comprises a Majorana wire located on the one of the superconducting regions.
6. The method of claim 1 , wherein the adjustable phase gate is implemented in a transmission line resonator comprising three Majorana wires arranged to form a qubit between a bus and a phase ground of the transmission line resonator.
7. The method of claim 1 , further comprising measuring one or more Pauli operators of the qubit by selectively controlling: (a) Josephson couplings between a first superconducting region on which a first Majorana wire is located and a bus and phase ground; and (b) Josephson couplings between a second superconducting region on which a second Majorana wire is located and the bus and the phase ground, and wherein a superconducting loop is formed by the second superconducting region, the phase ground, and a third superconducting region containing no Majorana wire.