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 multi-qubit gates, which entangle three or more qubits in a single step. Here, we show that such multi-qubit 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. Multi-qubit gates implemented in this way are as fast as, or sometimes even faster than, the constituent two-qubit gates. Furthermore, these multi-qubit 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-Z gates and simultaneous iSWAP gates. We show how the resulting multi-qubit gates relate to other well-known multi-qubit 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 Greenberg-Horne-Zeilinger states.
Highlights
Quantum computers [1,2] hold a promise of eventually being able to tackle complex problems in chemistry [3,4], materials science [5], finance [6,7], simulation of quantum systems [8], and many other fields [9,10,11,12]
To further put the time gained by performing the threequbit gate in perspective, we show in Fig. 8 the decomposition of the CCZS(π/2, π, 0) gate into single-qubit gates and two-qubit CZ gates between the middle qubit and its neighbors
Since our scheme for multiqubit gates only relies on control operations corresponding to two-qubit gates, our ideas are ready to be implemented in existing quantum hardware without the need for any additional components, complicated pulse shapes, hardware redesign, or other changes beyond some recalibration of the lengths of the control pulses already optimized for two-qubit gates
Summary
Quantum computers [1,2] hold a promise of eventually being able to tackle complex problems in chemistry [3,4], materials science [5], finance [6,7], simulation of quantum systems [8], and many other fields [9,10,11,12]. By changing the relative strengths of the constituent iSWAP gates and the gate time, a family of different three-qubit DIV gates can be realized Since both the DIV gates and the CCZS gates conserve the number of excitations, they may find applications in quantum-chemistry calculations with a fixed number of electrons [85] or in the mixing layer of the quantum alternating operator ansatz [86] for constrained combinatorial optimization with conserved Hamming weights [87]. Some further analytical calculations for the simultaneous CZ gates with an additional coupling between the outer qubits in the linear chain are given in Appendix A and additional data from the numerical simulations of possible experimental applications are presented in Appendix B
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