Demo of two-qubit fault-tolerant universal holonomic quantum gates could enable faster quantum computers

Japases researchers have demonstrated holonomic quantum gates under zero-magnetic field at room temperature, which will enable the realization of fast and fault-tolerant universal quantum computers.

A quantum computer is a powerful machine with the potential to solve complex problems much faster than today’s conventional computer can. Researchers are currently working on the next step in quantum computing: building a universal quantum computer.

Although the H, S, T, and CZ gates offer the elementary discrete set required for the universal quantum gates to construct arbitrary unitary quantum gates, the availability of arbitrary phase gates would be beneficial for fast gating in many practical applications, such as quantum Fourier transformation and blind quantum computing. The scheme allows a purely holonomic gate without requiring an energy gap, which would have induced dynamic phase interference to degrade the gate fidelity, and thus enables precise and fast control over long-lived quantum memories for realising quantum repeaters interfacing between universal quantum computers and secure communication networks.

Nature Communications – Universal holonomic quantum gates over geometric spin qubits with polarised microwaves. Kodai Nagata, Kouyou Kuramitani, Yuhei Sekiguchi & Hideo Kosaka Nature Communicationsvolume 9, Article number: 3227 (2018)

It was an experimental demonstration of non-adiabatic and non-abelian holonomic quantum gates over a geometric spin qubit on an electron or nitrogen nucleus, which paves the way to realizing a universal quantum computer.

The geometric phase is currently a key issue in quantum physics. A holonomic quantum gate manipulating purely the geometric phase in the degenerate ground state system is believed to be an ideal way to build a fault-tolerant universal quantum computer. The geometric phase gate or holonomic quantum gate has been experimentally demonstrated in several quantum systems including nitrogen-vacancy (NV) centers in diamond. However, previous experiments required microwaves or light waves to manipulate the non-degenerate subspace, leading to the degradation of gate fidelity due to unwanted interference of the dynamic phase.

“To avoid unwanted interference, they used a degenerate subspace of the triplet spin qutrit to form an ideal logical qubit, which we call a geometric spin qubit, in an NV center. This method facilitated fast and precise geometric gates at a temperature below 10 K, and the gate fidelity was limited by radiative relaxation,” says the corresponding author Hideo Kosaka, Professor, Yokohama National University. “Based on this method, in combination with polarized microwaves, we succeeded in manipulation of the geometric phase in an NV center in diamond under a zero-magnetic field at room temperature.”

The group also demonstrated a two-qubit holonomic gate to show universality by manipulating the electron-nucleus entanglement. The scheme renders a purely holonomic gate without requiring an energy gap, which would have induced dynamic phase interference to degrade the gate fidelity, and thus enables precise and fast control over long-lived quantum memories, for realizing quantum repeaters interfacing between universal quantum computers and secure communication networks.

Abstract – Universal holonomic quantum gates over geometric spin qubits with polarised microwaves

A microwave shares a nonintuitive phase called the geometric phase with an interacting electron spin after an elastic scattering. The geometric phase, generally discarded as a global phase, allows universal holonomic gating of an ideal logical qubit, which we call a geometric spin qubit, defined in the degenerate subspace of the triplet spin qutrit. We here experimentally demonstrate nonadiabatic and non-abelian holonomic quantum gates over the geometric spin qubit on an electron or nitrogen nucleus. We manipulate purely the geometric phase with a polarised microwave in a nitrogen-vacancy centre in diamond under a zero-magnetic field at room temperature. We also demonstrate a two-qubit holonomic gate to show universality by manipulating the electron−nucleus entanglement. The universal holonomic gates enable fast and fault-tolerant manipulation for realising quantum repeaters interfacing between universal quantum computers and secure communication networks.

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