Abstract
We propose a scheme for universal quantum computing based on Kramers rare-earth ions. Their nuclear spins in the presence of a Zeeman-split electronic crystal field ground state act as “passive” qubits that store quantum information. The “active” qubits are switched on optically by fast coherent transitions to excited crystal field states with a magnetic moment, and the magnetic dipole interaction between these states is used to implement controlled not (cnot) gates. We compare our proposal with others, noting particularly the much improved cnot gate time as compared with a : proposal, also relying on magnetic dipole interactions between active qubits, and rare-earth schemes depending on the dipole blockade for qubits spaced by more than of the order of .
3 More- Received 24 July 2020
- Revised 6 December 2020
- Accepted 17 December 2020
DOI:https://doi.org/10.1103/PRXQuantum.2.010312
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Nuclear spins and electronic crystal-field excitations of rare-earth ions in insulating crystals have a long history of exploitation for quantum science and technology. Here we show that they are ideal qubit hosts for a programmable quantum processor, and we propose a scheme to implement a universal set of quantum gates between them.
The scheme builds on the extremely long lifetime of nuclear spins in the presence of rare-earth electronic spins to realize “passive” memory qubits. A magnetic field polarizes the electronic spins, which efficiently prevents entanglement spreading between the qubits, even though the electronic interactions are not shielded. At first sight, this looks like a bug, whereas in fact, it is a feature, allowing qubits to be effectively “shielded” (on demand) locally without the need for complicated operations. We use optical pulses to efficiently map the quantum information from nuclear spins to excited, long-lived crystal field states with electronic magnetism. This implements single-qubit gates up to three orders of magnitude faster than direct nuclear spin manipulations. Two such “activated” qubits can interact via strong magnetic dipolar interactions, enabling the implementation of fast two-qubit gates, up to two orders of magnitude faster than previous proposals exploiting optical activation of atoms in crystals. The long-range nature of the dipolar interaction does not limit gates to nearest-neighbor qubits. Instead, it allows for a large number of qubits to be connected, even while crosstalk with nontargeted qubits in their neighborhood is robustly suppressed.
The next step is to implement our concepts in experiments. We suggest that rare-earth ions implanted in isotopically enriched silicon could meet the requirements for a manufacturable quantum processor. Here, as for other platforms, hybrid architectures including photonic cavity resonators will need to be explored for efficient coupling of photons to the individual qubits.