High-fidelity Quantum Gates Controllable with New Technique

**Hamiltonian learning technique advances quantum spin register**

Points to note…

+ Researchers have developed an efficient way to characterize the effective many-body Hamiltonian of the solid-state spin system associated with a nitrogen-vacancy (NV) centre in diamond. The technique will be important for making and controlling high-fidelity quantum gates in this multi-spin quantum register, they say.

+ A quantum register is a system made up of many quantum bits (qubits) and it is the quantum equivalent of the classical processor register. Quantum computers work by manipulating qubits within such a register and making and understanding such systems is essential for quantum information processing.

* One way to overcome this problem is to fully characterize the many-body Hamiltonian of the system, which determines how it evolves and crosstalk interactions. “Once we have this information, we can predict the evolution of any initial states and, what is more, design optimized gate operations to reduce crosstalk errors and achieve high-fidelity gates in a multi-qubit register,” explains Panyu Hou of the Center for Quantum Information at Tsinghua University in Beijing, who is the lead author of this new study. “The Hamiltonian can often be fully described by some essential parameters, which characterize the coupling between the qubits.”*

+ Researchers have made quantum registers from many physical systems thus far, including trapped ions, solid-state spins, neutral atoms and superconducting qubits. Whatever their nature, however, they all suffer from the same problem: crosstalk between multiple qubits when an individual qubit is addressed during a measurement. This induces a state error to other qubits and reduces gate fidelity.

+ The researchers validated their technique by designing a universal quantum gate set that includes three single-qubit gates for each of the 11 qubits and the entangling two-qubit gate between the electron spin and each of the 10 nuclear spins. “In principle, we can realize any unitary operation on this 11-qubit register by combining the above gate set,” Hou tells Physics World.

+ “This learning technique could be a useful tool to characterize many-body Hamiltonians with a constantly-on interaction,” he adds. “The 11-qubit quantum register that we made might also be used as a proof-of-principle test of some quantum algorithms.”

Source:* physicsworld. Belle Dume, Hamiltonian learning technique advances quantum spin register… *

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