Source-linked AI summary
Experimental Realization of Universal Geometric Quantum Gates with Solid-State Spins
C. Zu, W. -B. Wang, L. He, W. -G. Zhang, C. -Y. Dai, F. Wang, L. -M. Duan
TL;DR
The paper addresses the need for a scalable experimental platform supporting universal geometric quantum computation, beyond prior demonstrations in non-scalable liquid-molecule ensembles. It realizes geometric operations with solid-state spins in diamond defects and reports a platform with potential for room-temperature, noise-resilient quantum computation.
Problem
Universal geometric quantum computation has been observed in non-scalable liquid-molecule ensembles, motivating demonstrations in scalable solid-state platforms.
Method
The experiment uses optically controlled NV-center electron and nearby nuclear spins, fluorescence calibration, and quantum process tomography to implement and characterize geometric gates.
Results
The experiment realizes a universal set of geometric quantum gates with solid-state spins in diamond defects.
Takeaways & Limitations
Solid-state diamond defects provide an experimental platform with potential for room-temperature and potentially robust all-geometric quantum computation.
Abstract
from arXiv · showhide
Experimental realization of a universal set of quantum logic gates is the central requirement for implementation of a quantum computer. An all-geometric approach to quantum computation offered a paradigm for implementation where all the quantum gates are achieved based on the Berry phases and their non-abelian extensions, the holonomies, from geometric transformation of quantum states in the Hilbert space. Apart from its fundamental interest and rich mathematical structure, the geometric approach has some built-in noise-resilient features. On the experimental side, geometric phases and holonomies have been observed using nuclear magnetic resonance with thermal ensembles of liquid molecules, however, such systems are known to be non-scalable for quantum computing. There are proposals to implement geometric quantum computation in scalable experimental platforms such as trapped ions, superconducting qubits, or quantum dots, and a recent experiment has realized geometric single-bit gates with the superconducting system. Here, we report the experimental realization of a universal set of geometric quantum gates with solid-state spins of the diamond defects. The diamond defects provide a scalable experimental platform with the potential for room-temperature quantum computing, which has attracted strong interest in recent years. Based on advance of coherent control in this system, our experiment shows that all-geometric and potentially robust quantum computation can be realized with solid-state spin qubits.
Experimental setup
The experiment uses confocal optical addressing and microwave/radio-frequency control to prepare, manipulate, and detect individual NV-center spins. A 451 G magnetic field polarizes nearby nuclear spins and supports the experimental protocol.
- Experimental setup: A confocal microscope addresses and detects single NV centers in a synthetic diamond using laser excitation and fluorescence collection.The setup includes an oil-immersed objective, a 532 nm laser, photon detection, and piezoelectric scanning.
- Experimental setup: A 451 G magnetic field along the NV axis enables electron–nuclear spin flip-flops during optical pumping.The esLAC condition facilitates polarization of the host nitrogen and nearby C13 nuclear spins after 2 µs of green illumination.
- Experimental setup: Measurements are repeated at least 10^6 times, yielding about 3 × 10^4 photons whose Poissonian fluctuations determine the error bars.Monte Carlo sampling under a Poisson distribution is used to estimate each datum’s mean and standard deviation.
Calibration of fluorescence levels for different states
Fluorescence calibration assigns characteristic photon levels to individual electron–nuclear spin components before tomography. Calibrated microwave or radio-frequency π-pulses transfer population between the reference state and other components.
- Calibration of fluorescence levels for different states: The optically pumped state |m = 0, mn = ↑⟩ is used as the fluorescence reference for calibrating all spin components.The calibration accounts for different fluorescence levels caused by detection-induced flip-flops and imperfect initial polarization.
- Calibration of fluorescence levels for different states: Calibrated microwave or radio-frequency π-pulses transfer population from the reference state to any other |m, mn⟩ component.The π-pulses are calibrated through Rabi oscillations.
- Calibration of fluorescence levels for different states: The resulting fluorescence levels are used to read out post-gate states through quantum state tomography.This converts measured fluorescence into state information for the geometric-gate experiments.
Quantum Process tomography
Quantum process tomography reconstructs the operation of a quantum gate from its action on selected input states and compares the result with an ideal process. The experiment uses maximum-likelihood reconstruction and process fidelity to obtain average gate fidelity.
- Quantum Process tomography: A quantum process is represented as a completely positive map ε that transforms an initial state ρ_i into a final state ρ_f.The map is expanded in a fixed set of basis operators, with coefficients forming the process matrix χ.
- Quantum Process tomography: For single-qubit tomography, the experiment uses the operator basis I, X = σ_x, Y = −iσ_y, and Z = σ_z with four different input states.The corresponding output density operators are reconstructed through standard quantum state tomography.
- Quantum Process tomography: The experimentally reconstructed process matrix χ_e is compared with the ideal matrix χ_id using maximum likelihood and process fidelity.Process fidelity is calculated as F_P = Tr(χ_eχ_id).
- Quantum Process tomography: Average gate fidelity is obtained from process fidelity as F = (dF_P + 1)/(d + 1), with d = 2 for a single qubit.The fidelity averages performance over all possible input states with equal weight.