Hybrid Quantum Computers combining topological qubits and conventional superconducting qubits

Quantum computers come in two forms, depending on how localized the information is. Researchers are looking into hybrid models that could combine the best of both systems. Two papers in Physical Review Letters describe possible interfaces for relaying information between the two quantum platforms.

Quantum computers store information in “qubits,” which are quantum combinations of ones and zeros. Traditionally, qubits have been recorded in some intrinsic property of an isolated system, like the spin of a trapped electron. The trouble is that any slight interaction with the environment will force the qubit to collapse into a specific state and lose information.

One possible solution to this problem is to use topological quantum computers, which store information in intertwined particle states, called anyons. If one imagines these anyons as checkers on a board, a computation would consist of swapping the pieces in a precise sequence. The quantum information is not localized on individual checkers, but is instead encoded in the way the anyon trajectories weave around each other in spacetime.

Topological qubits have yet to be fully realized, but they show promise as robust quantum storage units. By contrast, conventional qubits appear better suited for a variety of logic gates. To take advantage of both types of “quantum hardware,” Liang Jiang, from Caltech, and his colleagues have proposed a way to link topological qubits on the surface of a topological insulator to conventional qubits made from superconducting wire loops. Parsa Bonderson and Roman Lutchyn at the University of California, Santa Barbara, have devised a similar sort of quantum bus to connect anyon-carrying nanowires with conventional quantum dots

Topological Quantum Buses: Coherent Quantum Information Transfer between Topological and Conventional Qubits

Arxiv – Topological Quantum Buses: Coherent Quantum Information Transfer between Topological and Conventional Qubits (5 pages)

We propose computing bus devices that enable quantum information to be coherently transferred between topological and conventional qubits. We describe a concrete realization of such a topological quantum bus acting between a topological qubit in a Majorana wire network and a conventional semiconductor double quantum dot qubit. Specifically, this device measures the joint (fermion) parity of these two different qubits by using the Aharonov-Casher effect in conjunction with an ancilliary superconducting flux qubit that facilitates the measurement. Such a parity measurement, together with the ability to apply Hadamard gates to the two qubits, allows one to produce states in which the topological and conventional qubits are maximally entangled and to teleport quantum states between the topological and conventional quantum systems.

Interface between Topological and Superconducting Qubits

Arxiv -Interface between Topological and Superconducting Qubits (12 pages)

We propose and analyze an interface between a topological qubit and a superconducting flux qubit. In our scheme, the interaction between Majorana fermions in a topological insulator is coherently controlled by a superconducting phase that depends on the quantum state of the flux qubit. A controlled-phase gate, achieved by pulsing this interaction on and off, can transfer quantum information between the topological qubit and the superconducting qubit.

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