QnTM: EMT: Spin Bus for Quantum Information Processing

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

Quantum information forms a new concept in information technology, which has the potential to impact many aspects of modern life, from finance to secure communications to scientific research. The quantum mechanical property known as "spin" lies at the heart of many quantum information technologies. The spin of an electron, for example, can carry a quantum bit of information (known as a "qubit"), which takes the values 0 or 1, or a superposition of the two. Like an ordinary computer, the quantum computer performs operations on its qubits. This is accomplished by bringing the qubits (e.g. electrons) into proximity, and letting them interact over a prescribed time period. Typically, the interaction distances are very small, on the order of nanometers (one billionth of a meter). This distance requirement can cause communication problems for a quantum computer, which must access thousands of qubits. Indeed, in many architecture proposals, information must be transmitted to distant qubits by iterated swapping between neighboring spins in a one-dimensional array. There is considerable overhead associated with such swap protocols, both in time and efficiency. The present proposal puts forward a new concept for long-range, effective interactions between spin qubits, based on a "spin-bus." The bus is formed from a long chain of closely spaced spins, which are fixed in place so that their interactions are constant and strong. The bus acts as a fast conduit for quantum information. Additional, external spin qubits can be coupled to the bus, by means of controlled electrical gates. Thus, the bus provides a mechanism for indirect qubits interactions at large distances. The spin-bus architecture has pros and cons that should be weighed and optimized. A particular advantage of the spin-bus approach is that it does not require any resources besides those normally assumed for spin quantum computing. Further, it intentionally exploits the most robust feature of spin-based architectures: strong coupling at short distances. The objective of the work is to characterize and develop all aspects of the spin-bus architecture for quantum computing. The proposal involves three separate and interdisciplinary research components: Bus Operation. A thorough understanding of the spin-bus energy spectrum is needed, to characterize the possible operating modes. Inevitable, inequivalent couplings between the neighboring spins reduce the bus speed. Theoretical analysis is needed, to determine the importance of such variability, and to optimize the bus operation in terms of bus size and speed. Decoherence Analysis. Quantum information is particularly susceptible to environmental influences, which lead to errors in the quantum computer. Because the spin-bus is much larger than a single spin, unique types of errors may affect it. Analysis will focus on fluctuating and spatially varying magnetic and electrostatic fields, vibrations within the matrix containing the spins, and "leakage" of information from the bus into undesired spin states. Quantum Error Correction. Error correction in the spin-bus is quite different from conventional quantum computers. The special challenges to be addressed include trade-offs between long-range coupling and parallelizability, leakage control, and the tailoring of established error correction techniques for the spin-bus architecture.