Quantum entanglement at room temperature! CSU's Du Jiangfeng team successfully entangles two qutrit

Quantum correlation is an important problem in quantum physics, and recently, high-dimensional quantum correlation has caused a huge scientific research frenzy. Recently, the team of academician Du Jiangfeng at CSU reported the experimental study of quantum correlation in a double qutrit spin system with a single nitrogen-vacancy (NV) color center in diamond at room temperature [1]. Quantum entanglement between the two qutrit is observed at room temperature and reveals the existence of non-classical correlations beyond entanglement in the qutrit case. This experiment demonstrates the potential of NV color centers to perform quantum information tasks as multi-qutrit systems, providing a powerful experimental platform for future studies of the underlying physics of high-dimensional quantum systems.

It should be noted that qutrit is the basic unit of quantum triplet, while quantum bit (qubit) is binary. a qutrit represents more states than qubit, and as the number increases, the represented states grow exponentially. rit

01Quantum correlation: revealing quantum disassociation, quantum entanglement

Quantum correlation elucidates the most fundamental features that distinguish quantum correlated systems from those fully attributed to joint classical probability distributions and may reveal the origin of quantum enhancements in various quantum information processing (QIP). QIP with quantum bits may provide higher Hilbert space, leading to higher efficiency and flexibility in quantum computing, larger channel capacity and better noise tolerance in quantum communication, and looser constraints in nature's fundamental tests.

Quantum discord, which describes non-classical correlations even in separable states, may contribute to quantum enhancement when these states are applied to QIP. The phenomenon of quantum discord has been intensively studied in the past decades, but experiments are limited to systems with four bits.

02Measuring the quantum correlation of "double qutrit isotropic states

In this experiment, Jiangfeng Du's team reported an experimental study of quantum correlations in a two-qutrit system with an NV color center with electron spin and nuclear spin, which can be used as a two-qutrit system to study high-dimensional quantum correlations. In the experiment, the quantum correlations of two-qutrit isotropic states, which are crucial in the study of quantum correlations, are prepared and measured: the quantum decoupling and quantum entanglement characteristics are revealed for different values of state parameters. It is experimentally demonstrated that there is a threshold at which quantum entanglement disappears and quantum disassociation remains.

Fig. 1 Double qutrit system constructed from NV color centers. (a) Schematic diagram of the atomic structure. (b) The ground state energy level of the NV color center. The nine different states of electron spin and nuclear spin constitute a double qutrit system. The transitions between the different electron (nuclear) spin states can be guided by the microwave (RF) pulses shown by the purple (red) arrows.

Figure 2 Experimental pulse sequence and reconstructed density matrix. (a) Schematic of the pulse sequence, including polarization, state preparation and tomography. During state preparation, selective MW (purple) and RF (red) pulses are used to generate the isotropic state ρiso (ρ). The frequencies of these pulses correspond to the transition frequencies between energy levels shown in Figure 1(b). The gray boxes represent the MW and RF pulse sequences for tomography. (b) Experimental density matrix of entangled states for ρ = 0.94. The bars show the experimental results, while the line grid represents the corresponding simulated results. The experimental results are in excellent agreement with the simulated results, with 96% state fidelity.

Fig. 3 Quantum correlation of isotropic states. (a) and (b) show the experimental results of quantum entanglement and quantum disassociation, respectively. The dots are the experimental data and the solid lines are the simulation results. The vertical error bars of the data points are calculated using Monte Carlo simulations with Gaussian statistics; the missing data points and their error bars are obtained by comparing the experimental results with the simulations to find the most probable ρ for each Monte Carlo run. The dashed line corresponds to ρ = 1/4, which is the dividing line between the separable state and the entropy.

03NV color centers have great potential: promising applications for high-dimensional quantum information processing

This experiment investigates the quantum correlation between two qutrit in a single NV color center and observes the entanglement of the two qutrit systems at room temperature. In particular, the team demonstrated that non-classical correlations beyond entanglement hold in the case of qutrit. These results show that NV color centers are a powerful platform for further studies of fundamental properties such as high-dimensional quantum correlations: for example, the nature of quantum correlations, their relation to quantum superposition and nonlocality.

Moreover, qutrit systems have advantages in QIP, and experimental studies of key procedures in QIP are necessary. NV color center is a natural high-dimensional system with long coherence times even at room temperature. However, it is used as a quantum bit in most studies, which does limit its potential. This work promotes the NV color center as a high-dimensional system to perform quantum computing and quantum sensing tasks. "NV qutrit systems may play an important role in high-dimensional quantum information processing: further, two NV color centers will be coupled on top of quantum bits as qutrit, although this remains challenging."

Link to paper：

[1]https://arxiv.org/abs/2208.05618