GHZ state generation at large scale

Short answer: A GHZ state is a highly entangled state of many particles that is easy to verify but hard to prepare because it is very sensitive to noise. A 2024 Nature Communications paper reports GHZ states of up to 60 superconducting qubits, an October 2025 arXiv preprint reports a 120-qubit GHZ state with a fidelity of 0.56(3) after post-selection, and a 2022 Physical Review Letters paper reports four- and six-photon GHZ states with higher generation rates. These are laboratory results, and the 120-qubit paper is a preprint.

Scale and Platform Limitations

GHZ states, also known as two-component Schrödinger cat states, are central to quantum physics foundations and potential applications. The 2024 authors say they show the strongest nonlocal entanglement for N particles but are the most fragile entangled states, and that enlargement and coherent control of GHZ states pose tremendous challenges because of their vulnerability to noise. They describe the ability to generate, preserve, and manipulate these states as a long-term goal for building practical quantum computers that can outperform classical machines.

On size, the 2025 preprint reports the largest GHZ state prepared to date, consisting of 120 superconducting qubits. It used a combination of optimized compilation, low-overhead error detection, and temporary uncomputation. The 2024 authors report a scalable protocol to create genuinely entangled GHZ states with up to 60 qubits, which they say almost doubled the previous size record. They describe superconducting processors as a promising platform for exploring nonequilibrium quantum matters and emerging applications.

Photonic Scalability and Feed-Forward

Generating multiphoton entangled states remains an experimental challenge because current methods often rely on stitching together photons from probabilistic sources. In standard nonmultiplexed approaches, state generation rates drop exponentially as the number of photons increases. The 2022 authors implement a system based on active feed-forward and multiplexing. They report scalable generation of four-photon and six-photon GHZ states.

They report increasing generation rates by factors of 9 and 35, respectively, for the four-photon and six-photon states. They say this is consistent with the exponential enhancement predicted by their theory compared with the standard nonmultiplexed approach.

Protection and Manipulation Strategies

Because GHZ states are extremely sensitive to imperfections, preserving their coherence is critical. The 2024 authors propose embedding a GHZ state into the eigenstates of a tailor-made discrete time crystal (DTC). They say DTCs were originally for exploring exotic nonequilibrium quantum matters and that this embedding extends the lifetime of the GHZ state, in experiments built on high-fidelity digital quantum circuits.

For manipulation, the system allows switching between DTC eigenstates using in-situ quantum gates. This capability modifies the effectiveness of the GHZ protection. The authors say their findings establish a viable path towards coherent operations on large-scale entanglement.

Verification and Fidelity Metrics

Verifying the quality of large entangled states is essential for benchmarking hardware and control. In the 120-qubit experiment, the preprint authors measured a GHZ fidelity of 0.56(3) with a post-selection rate of 28%. They say they certify the fidelity using multiple methods and show that they are all equivalent, albeit with different practical considerations.

The ability to faithfully entangle many distinct particles is often used as a benchmark for the quality of hardware and control in a quantum computer. GHZ states are easy to verify but difficult to prepare due to their high sensitivity to noise. The preprint says successful preparation of large GHZ states can serve as a demonstration of good control and suppression of noise. These states have historically been used as benchmarks in various quantum platforms, including ions, superconductors, neutral atoms, and photons.

FAQ

What is the maximum qubit count reported?

The preprint reports the largest GHZ state prepared to date, consisting of 120 superconducting qubits. This state was protected by an additional 8 qubits. The experiment utilized optimized compilation and low-overhead error detection to achieve this scale.

How does active feed-forward improve photonic generation?

Active feed-forward and multiplexing increase generation rates for multiphoton GHZ states. For four-photon and six-photon states, rates increased by factors of 9 and 35, respectively. This addresses the exponential drop in rates seen in standard nonmultiplexed approaches.

Why are GHZ states considered fragile?

GHZ states are the most fragile entangled states and are extremely sensitive to imperfections in the experiment. This high sensitivity to noise makes them difficult to prepare. However, this same sensitivity allows them to be used for quantum sensing at the Heisenberg limit.

What role do discrete time crystals play?

Discrete time crystals are used to extend the lifetime of GHZ states. By embedding the GHZ state into the eigenstates of a tailor-made DTC, the 2024 authors extend its lifetime. In-situ quantum gates can then switch these eigenstates to modify the protection effectiveness.

How is the fidelity of these states verified?

The preprint authors say they certify fidelity with multiple methods that are all equivalent. In the 120-qubit experiment, a GHZ fidelity of 0.56(3) was measured. These states are easy to verify, making them useful benchmarks for assessing hardware quality and control capabilities.