Physicists Demonstrate ‘Teleportation’ of Quantum Gate between Two Qubits

A team of scientists at Yale University has experimentally demonstrated one of the key steps in building the architecture for quantum computers — the ‘teleportation’ of a quantum gate between two logical qubits, on demand.

A chip designed to operate as a 128-qubit superconducting adiabatic quantum optimization processor. Image credit: D-Wave Systems, Inc.

A chip designed to operate as a 128-qubit superconducting adiabatic quantum optimization processor. Image credit: D-Wave Systems, Inc.

A quantum computer has the potential to efficiently solve problems that are intractable for classical computers.

However, constructing a large-scale quantum processor is challenging because of the errors and noise that are inherent in real-world quantum systems.

One approach to addressing this challenge is to utilize modularity — a strategy used frequently in nature and engineering to build complex systems robustly.

“A quantum modular architecture consists of a collection of modules that function as small quantum processors connected into a larger network,” explained team leaders Professor Robert Schoelkopf and Dr. Kevin Chou and their colleagues.

“Modules in this architecture have a natural isolation from each other, which reduces unwanted interactions through the larger system.”

“Yet this isolation also makes performing operations between modules a distinct challenge. Teleported gates are a way to implement inter-module operations.”

Construction of a modular architecture and teleported gate: (a) network overview of the modular quantum architecture; modules are represented as nodes of a quantum network and are composed of data qubit(s) (magenta) and communication qubit(s) (cyan); coupling between modules is generated through reconfigurable communication channels that may be enabled (dark purple lines) or disabled (light purple lines); (b) quantum modules; each module houses a small quantum processor that is capable of high-fidelity operations among data qubits and communication qubits; in the experiment, Chou et al create two modules, each consisting of one data qubit (D1 and D2) and one communication qubit (C1 and C2); (c) teleported circuit between D1 and D2; the teleported circuit requires: (1) entanglement between C1 and C2 (purple meander), (2) local operations, (3) measurement of C1 in the Z basis and C2 in the X basis, where X and Z are Pauli operators, and (4) classical communication (double lines) and feedforward operations; (d) experimental realization (schematic top view) in a 3D circuit-quantum-electrodynamics implementation; each module consists of a data qubit defined as a coaxial quarter-wavelength (λ/4) 3D cavity (magenta), a communication qubit defined as a Y-shaped transmon qubit (cyan) and a Purcell-filtered, quasi-planar, λ/2 stripline readout resonator (black); in this experiment, the two modules are linked by an additional mode realized as a coaxial λ/4 3D cavity (purple) that serves as a bus mode. Image credit: Chou et al, doi: 10.1038/s41586-018-0470-y.

Construction of a modular architecture and teleported gate: (a) network overview of the modular quantum architecture; modules are represented as nodes of a quantum network and are composed of data qubit(s) (magenta) and communication qubit(s) (cyan); coupling between modules is generated through reconfigurable communication channels that may be enabled (dark purple lines) or disabled (light purple lines); (b) quantum modules; each module houses a small quantum processor that is capable of high-fidelity operations among data qubits and communication qubits; in the experiment, Chou et al create two modules, each consisting of one data qubit (D1 and D2) and one communication qubit (C1 and C2); (c) teleported circuit between D1 and D2; the teleported circuit requires: (1) entanglement between C1 and C2 (purple meander), (2) local operations, (3) measurement of C1 in the Z basis and C2 in the X basis, where X and Z are Pauli operators, and (4) classical communication (double lines) and feedforward operations; (d) experimental realization (schematic top view) in a 3D circuit-quantum-electrodynamics implementation; each module consists of a data qubit defined as a coaxial quarter-wavelength (λ/4) 3D cavity (magenta), a communication qubit defined as a Y-shaped transmon qubit (cyan) and a Purcell-filtered, quasi-planar, λ/2 stripline readout resonator (black); in this experiment, the two modules are linked by an additional mode realized as a coaxial λ/4 3D cavity (purple) that serves as a bus mode. Image credit: Chou et al, doi: 10.1038/s41586-018-0470-y.

Using a theoretical protocol developed in the 1990s, Professor Schoelkopf, Dr. Chou and co-authors demonstrated a quantum operation, or ‘gate,’ without relying on any direct interaction.

“Our work is the first time that this protocol has been demonstrated where the classical communication occurs in real-time, allowing us to implement a ‘deterministic’ operation that performs the desired operation every time,” Dr. Chou said.

“Quantum calculations are done via delicate bits of data called qubits, which are prone to errors,” Professor Schoelkopf added.

“In experimental quantum systems, ‘logical’ qubits are monitored by ‘ancillary’ qubits in order to detect and correct errors immediately.”

“Our experiment is also the first demonstration of a two-qubit operation between logical qubits.”

“It is a milestone toward quantum information processing using error-correctable qubits.”

The team’s results are published in the journal Nature.

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Kevin S. Chou et al. Deterministic teleportation of a quantum gate between two logical qubits. Nature, published online September 5, 2018; doi: 10.1038/s41586-018-0470-y

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