October 7, 2024 -- Entanglement is an important resource in quantum information, and refers to the phenomenon where two systems become linked, regardless of the distance between them. But often, that entanglement is hard to create, and even then, it is often unstable. This means that entanglement created in qubit platforms (whether using photons, atoms, or superconductors) often deteriorates over time, frequently because of interactions between the qubits and their environment.

A recent collaboration between the University of Illinois and the University of Chicago addresses this problem by using the using the environment as a help, instead of a hindrance. In using the environment of superconducting qubits, sometimes referred to as a reservoir, the researchers show that more stable entanglement can be created.

The theoretical work entitled “Loss resilience of driven-dissipative remote entanglement in chiral waveguide quantum electrodynamics”  is the result of a partnership between the groups of Illinois Assistant Professor of Physics Wolfgang Pfaff and University of Chicago Professor of Molecular Engineering Aashish Clerk. Several graduate students contributed to the research, including Abdullah Irfan, first author of the paper published in Physical Review Research a few weeks ago.

The paper is based on previous research that examines what is possible for superconducting qubits. Whereas previous experiments involving such qubits focus on generating entanglement in proximity, the goal of this research is to generate remote entanglement.

“Our work is based on previous results that show that you can generate stable, remote entanglement by coupling two superconducting qubits to a chiral waveguide, which is a waveguide in which photons move in only one direction,” says Irfan.

A waveguide is an instrument that transmits and directs energy. The energy is what controls the qubits. However, environmental interaction means that this entanglement is hard to store because of the close connection between the qubits and the waveguide. A chiral waveguide is made using a commercially available element called a circulator, which is typically very lossy, reducing the entanglement between the qubits.

“We realized that if at some point we want to store entanglement, this is not the best way of doing things since the qubits are coupled to a chiral waveguide which is really lossy,” says Irfan. “Our idea initially was to swap the entanglement into other pairs of qubits not directly connected to the waveguide. We were surprised to find out that continuously interacting another pair of qubits with the first pair was sufficient to create entanglement in the latter qubits.”

His group got in touch with the Clerk group, experts in modeling interactions like those between the waveguide and now two pairs of qubits. Later, the researchers extended this idea further, considering an arbitrary number of qubits.

Physicists can describe what happens in an experiment using a mathematical operator called a Hamiltonian. The Hamiltonian corresponds to the total energy of the system and contains all the parameters of the experiment, both parameters that cannot be changed and those that scientists can vary to get a desirable outcome. The researchers vary the Hamiltonian to see if changing the parameters, that is, the energy, will result in a new lowest-energy state, called a ground state.

“The point of quantum reservoir engineering is creating a new ground state of your system that is non-trivial. In this case, we’re looking for the new ground state to be the entangled state instead of the typical non-entangled ground state,” says Irfan.

The collaboration found that this ground state can include many layers of pairs of qubits. The research shows that for any number of additional pairs of qubits, a ground state that is entangled can be found. This entangled state is created by “driving” the system of qubits, generating a particular frequency of electromagnetic energy that interacts with the superconducting qubits, entangling them.

“The math shows that if we keep driving the qubits, we can add as many qubits as we want and they will all relax into Bell pairs,” says Irfan. Bell pairs are strongly entangled pairs of qubits. “As long as the drives stay on [just the first qubits], you keep your state entangled and so it is stabilized for as long as you want.”

This result, an example of entanglement replication, surprised even Irfan and his co-authors, and showcases the fascinating nature of quantum behaviors. The work of the Pfaff and Clerk groups in studying these phenomena continues. For the Pfaff group, which specializes in superconducting qubits, work continues to build these qubits in the lab, with a new two-qubit experiment currently in progress.

“Our first project, which will focus on implementing the initial theoretical result with just two qubits, will teach us a lot about how we tune up these experiments,” says Irfan. “Once we do that, the next natural step would to be add another pair of qubits, but this remains challenging experimentally.”