Quantum Gates, Algorithms, and Error Correction with a Neutral Atom Qubit Array

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

Quantum computing is attracting great interest due to its potential for solving practical problems that are intractable on classical computers. Areas of application include cryptography, database searching, pattern classification, solving large systems of coupled equations, and design of new functional materials or chemical compounds. A quantum computer is built from quantum bits, or qubits, that store quantum information which is processed using quantum logic gates. This project will continue the development of quantum computing using qubits encoded in the internal states of individual atoms. While the current state of the art involves experiments with 10-20 qubits, it is believed that computers with many thousands of qubits will be needed to realize the promise of quantum computation. In this respect the atom based approach being developed in this project is particularly attractive since a large number of neutral atoms can be held and controlled in close proximity without undesired interference with each other. The project seeks to advance the state of the art of several key performance metrics: the ability of qubits to preserve their quantum states for a long time, the fidelity of logic gate operations and qubit measurements, and the number of qubits that can be prepared in a single system. These advances will then be used to demonstrate a quantum algorithm for database searching. The project will contribute to scientific workforce development through training of students and postdoctoral researchers. The training will be interdisciplinary, drawing on methods and ideas from atomic and laser physics, electronic and computer based control systems, and quantum information theory. Research results will be incorporated into the University teaching curriculum. The experimental approach will use a two-dimensional array of cesium and rubidium atoms trapped in potential wells defined by light. The potential wells will be prepared by combining laser light of several different frequencies in a way that protects stored quantum information from decoherence. A movable optical tweezer system will be implemented to arrange trapped atoms for 100% occupancy of an array with up to 50 qubit sites. Quantum logic gates to create entanglement will be performed by exciting atoms to Rydberg states with laser pulses. Adiabatic pulses with shaped temporal profiles will be used to improve the fidelity of the entangling operations. Atomic states will be measured without crosstalk to other qubits or loss of atoms using a two-species approach whereby one species (cesium) will be used for memory and quantum logic, and a second species (rubidium) will be used for measurements. The quantum states to be measured will be transferred from cesium to rubidium atoms using an interspecies Rydberg gate. These capabilities will then be leveraged to demonstrate a multi-qubit quantum algorithm that provides quadratic speedup for database searching. The team will also implement quantum error correction to protect qubits against bit flip or phase flip errors.