How Researchers Entangled Six Qubits Across Two Quantum Nodes—and Why It Matters for Scaling Quantum Computers

In a landmark experiment published in Nature on February 24, 2021, a University of Chicago team led by Professor Andrew Cleland created a single entangled state spanning six qubits distributed across two physically separate quantum nodes, linked by a communication cable. The achievement was twofold: the researchers sent entangled qubit states through that cable between nodes, and then extended the entanglement until all six qubits formed one globally entangled state. The demonstration offers a concrete, step-by-step picture of how future quantum computers might be assembled from networked modules rather than single, monolithic processors.

The Challenge of Scaling Quantum Computers

Qubits, or quantum bits, are the basic units of quantum information. Their power comes from quantum properties such as superposition and their capacity to be entangled with one another—linked so that the state of one cannot be described independently of the others. As the University of Chicago's Pritzker School of Molecular Engineering explains, exploiting these properties is what scientists hope will allow next-generation machines to solve problems beyond the reach of classical computers.

The difficulty is scale. A useful quantum computer needs far more qubits than any single processor can comfortably host, and the field's emerging answer is modular: build machines out of modules, where families of entangled qubits conduct a computation, then network many such modules together—analogous to how today's supercomputers run parallel computations across many connected processing units. That architecture only works if qubits in different modules, or nodes, can be remotely entangled. As Cleland put it, "Developing methods that allow us to transfer entangled states will be essential to scaling quantum computing."

A Landmark 2021 Experiment: Entangling Six Qubits Across Two Nodes

The experiment, conducted in Cleland's lab at the Pritzker School of Molecular Engineering, used two quantum network nodes, each containing three qubits, connected by a communication cable. Researchers based at the school reported that they sent entangled qubit states through the cable linking the first node to the second—a first for the field and groundwork, in their words, for future quantum communication networks.

The published results went beyond transferring states between nodes. In the paper, "Deterministic multi-qubit entanglement in a quantum network," the team reported that it deterministically generated a globally distributed two-node, six-qubit GHZ state—a specific kind of highly entangled multipartite state—with a fidelity of 0.722 ± 0.021. Crucially, the process was deterministic rather than probabilistic: the entangled states were produced on demand rather than succeeding only occasionally, a distinction that matters for any practical computing architecture.

How the Entanglement Was Achieved: Step-by-Step

The method described by the researchers unfolds as a sequence, starting from ordinary, unentangled qubits and ending with a single state spanning both nodes. The key insight is that the cable is used twice over: first as a channel to create entanglement between the nodes, and then as a conduit for transferring already-entangled states.

The 2021 Multi-Qubit Entanglement Experiment Explained

The following chronology reconstructs the demonstrated procedure from the experiment's reported stages, showing how the six-qubit state was built progressively.

Setup of two separate nodes. The experiment began with two physically separate quantum nodes, each holding three qubits, connected by a communication cable.
Initial two-qubit entanglement across the cable. Researchers first used the cable to entangle two qubits—one in each node—establishing the foundational link between the two locations.
Amplification of entanglement within each node. The initially entangled qubits were then entangled further with the other qubits inside their respective nodes, spreading the correlation locally.
Formation of the globally entangled six-qubit state. When this process was finished, all six qubits across the two nodes were entangled in a single globally entangled state—a two-node, six-qubit GHZ state generated deterministically.
Transfer of a prepared entangled state. Separately, the team prepared a three-qubit GHZ state in one node and deterministically transferred it through the cable to the other node, demonstrating that complex entangled states—not just individual qubits—can move across a network link.

Each stage builds on the previous one: the cable creates the first cross-node entanglement, local operations extend it, and the final state is a single resource shared by both nodes. The state-transfer step shows the same infrastructure can also move entanglement around a network after it has been created.