Qubits, the fundamental units of quantum information, drive advancements in technologies like quantum computing, sensing, and communication. In a recent breakthrough, researchers at the IBS center for quantum nanoscience (QNS, Seoul, Korea) introduced a novel qubit platform that places individual magnetic atoms on a pristine insulator surface. Using a scanning tunneling microscope (STM) combined with electron spin resonance (ESR-STM), our team precisely positioned and manipulated atoms at the atomic level, enabling coherent control of multiple spin qubits (labeled "A" and "B" on the right) which are then read-out through a sensor spin located in the tunnel junction of the STM.

This innovative platform allows us to study crucial quantum properties such as superposition and coherence, both of which can be directly controlled with ESR-STM. By achieving simultaneous control over multiple qubits, we paved the way for scaling up to tens of qubits in a defect-free environment, allowing us to study the interaction of qubits in a pristine environment with extremely high precision.

Our next goal was to prove quantum entanglement—a phenomenon where the quantum states of multiple qubits become inseparable. The biggest challenge of this work was not to prove entanglement of two spins, which is nearly trivial in simulations, but to develop a protocol that can be implemented in an ESR-STM. Verifying entanglement experimentally is a significant challenge due to STM's highly localized measurements. STM excels at precise measurements at individual atomic sites, but proving entanglement, which involves multiple qubits, required us to find a  global measurement approach. Since entanglement is a cornerstone of quantum technologies, confirming this in our spin qubit platform became a top priority.

At this point, our collaboration with Sander Otte’s group at TU Delft in the Netherlands, especially his student Rik Broekhoven, became essential. During the 2023 DPG Spring Meeting in Germany, Rik and Christoph Wolf, the group leader for theory projects at QNS exchanged ideas on tackling the challenge of proving entanglement in a surface spin system. With Rik’s expertise in open quantum system simulations and our team's knowledge of driven quantum systems, we developed a protocol for studying their dynamics in just half a year. This collaborative work accelerated further when Rik visited QNS in Seoul for three months, where we worked closely together to find a proper solution.

The breakthrough came when we figured out how to measure the global dynamics of two entangled spins by using the additional “sensor” spin of the qubit platform. We mapped the dynamics of the entangled qubits onto the populations of this sensor spin, which we knew from earlier simulations and experiments could be read out with high fidelity. This "entanglement readout" is a strong function of temperature and in general a weaker function of coherence time of the two spin qubits as shown on the right. By creating a direct and unambiguous link between the time-evolution of entangled states and the population of the sensor spin we arrive at direct proof of entanglement in an ESR-STM. 

Now that we’ve succeeded in developing the theory and protocol, we are eager to see it implemented in actual experiments. We accounted for authentic experimental conditions and limitations in our simulations to ensure the protocol can be immediately adapted for experiments. We expect most promising results using our low temperature dilution fridge STM “BOB,” where high entanglement and stronger readout should be possible.

Looking ahead, our team is now studying the scaling of these systems to explore how many qubits can be realized on this surface-based spin qubit platform.