Explaining the Greenberger-Horne-Zeilinger (GHZ) state of three or more qubits to a layperson can be challenging due to the abstract nature of quantum mechanics. However, I'll try to provide a simplified explanation:
Imagine you have three particles, which we'll call qubits. In classical systems, each qubit can exist in one of two possible states, like flipping a coin where it can be heads or tails. However, in quantum mechanics, qubits can exist in a superposition of states, meaning they can be in a combination of both states simultaneously.
The GHZ state is a special type of quantum state where all the qubits are highly entangled with each other. In the GHZ state, the three qubits are in a superposition that connects their states together in a particular way.
Let's use an analogy with a group of three friends, Alice, Bob, and Carol. In a classical scenario, you can imagine that they each have a red or blue ball, and you randomly give them one. If you separate them and then ask each person about the color of their ball, they might have different colors.
Now, in the quantum GHZ state, it's as if their balls are magically connected. If you prepare the GHZ state, and then separate the three qubits (friends) without looking at them, it turns out that when you measure the color of the balls (the qubits' states), you'll find that all three qubits have the same color, either all red or all blue.
This phenomenon, where the states of the qubits are perfectly correlated even when they are far apart, is what we call entanglement. It's as if the qubits share some kind of mysterious connection that allows them to instantaneously "communicate" their states to each other.
The GHZ state is particularly interesting because it demonstrates this entanglement in a highly pronounced way with multiple qubits. It has been experimentally demonstrated, and its properties have implications for various applications in quantum information processing and foundational tests of quantum mechanics.
While this explanation simplifies the concept for a layperson, it's important to note that understanding the full details of quantum entanglement and the GHZ state requires a deeper understanding of quantum mechanics. Nonetheless, this analogy provides a glimpse into the strange and fascinating world of quantum phenomena.