Abstract
Near-term quantum computers are limited by the decoherence of qubits to only being able to run low-depth quantum circuits with acceptable fidelity. This severely restricts what quantum algorithms can be compiled and implemented on such devices. One way to overcome these limitations is to expand the available gate set from single- and two-qubit gates to multiqubit gates, which entangle three or more qubits in a single step. Here, we show that such multiqubit gates can be realized by the simultaneous application of multiple two-qubit gates to a group of qubits where at least one qubit is involved in two or more of the two-qubit gates. Multiqubit gates implemented in this way are as fast as, or sometimes even faster than, the constituent two-qubit gates. Furthermore, these multiqubit gates do not require any modification of the quantum processor, but are ready to be used in current quantum-computing platforms. We demonstrate this idea for two specific cases: simultaneous controlled- gates and simultaneous iswap gates. We show how the resulting multiqubit gates relate to other well-known multiqubit gates and demonstrate through numerical simulations that they would work well in available quantum hardware, reaching gate fidelities well above 99%. We also present schemes for using these simultaneous two-qubit gates to swiftly create large entangled states like Dicke and Greenberger-Horne-Zeilinger states.
9 More- Received 4 July 2021
- Accepted 8 November 2021
DOI:https://doi.org/10.1103/PRXQuantum.2.040348
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
Quantum algorithms may outperform classical ones on important computational tasks in chemistry, optimization, and many other fields. However, to run on current quantum computers, these algorithms must be compiled into long sequences of elementary operations (gates) on one or two qubits. Since the available quantum hardware still struggles to protect qubits from noise, it is desirable to execute the algorithms swiftly to have a hope for quantum advantage. Here we show how two-qubit gates on existing quantum hardware can be run simultaneously to create new multi-qubit gates, which are both fast and powerful: they entangle more qubits and take less time to execute than the two-qubit gates.
The key to creating three-qubit gates in our scheme is two-qubit gates that rely on swapping excitations between neighboring qubits. When the middle qubit in a chain of three qubits interacts in that manner with both its neighbors, a pathway is created for swapping excitations between the two outer qubits, conditioned on the state of the middle qubit. We demonstrate the construction of such three-qubit gates for different types of two-qubit gates and for multiple architectures, showing through extensive numerical simulations that they can be implemented with high fidelity in available experimental setups.
Our results suggest that additional multi-qubit gates can be discovered using similar constructions with other two-qubit gates. Having such new multi-qubit gates on current quantum computers opens up for re-compiling many quantum algorithms into shorter gate sequences, enhancing the performance of the quantum computers without needing to upgrade the hardware.