A new method to measure quantum entanglement in a nuclear spin ensemble

This is the three-dimensional data that the team obtained using the proxy electron qubit. Each "peak" corresponds to spin-wave modes. Horizontally the qubit probes the fixed state of nuclear ensemble. The qubit controls the vertical state of the nuclei. The qubit can adjust the state of the nuclear ensemble vertically to show quantum correlations between nuclei. This work is symbolic in that it is the culmination of nearly two decades of research by researchers at Cambridge and other teams to demonstrate an entangled nuclear ensemble. Credit: Gangloff et al.
Quantum physics studies aim to determine the quantum states of large systems made up of many interacting particles. This is one of their primary goals. This could prove to be especially useful in the development of quantum computers or other quantum information processing devices.

Cavendish Laboratory at the University of Cambridge has recently developed a new method to measure the spin states of a nucleus ensemble. This is a system that includes many interconnected particles with long-lasting quantum properties. The method was presented in Nature Physics and is described in a paper. It works by analyzing the system's response to collective spin excitations.

Claire Le Gall, Mete Atature and Mete Atature, who were two of the researchers involved in the study, said that it was impossible to measure individual quantum objects such as spins to determine how they interact with one another. Instead, one can examine the collective response of the ensemble for tell-tale signs. This is a bit like observing the behavior of a flock. It might reveal something about the interaction between the birds and each other. Our system of interest is a large group, or ensemble of nuclear spins in the semiconductor quantum dot.

Three Harvard University physicists discovered that large numbers of nuclei in a semiconductor quantum dots could be hosts for solid-state memory systems. They published their findings a year later. Le Gall, Atature and their colleagues explored this type of nuclear ensemble 19 years later using a "proxy" quantum bit, which is an electron spin that couples to all the nuclear spins.

Dorian Gangloff was the first author of this paper. He stated that "we achieved a significant milestone when we demonstrated that collective modes in the nuclear ensemble (i.e. spin waves) can be excited coherently through the electron." "In our new study we set out to use electron-activated spinning waves to change nuclear ensemble's state and to read it out. This would allow for a simple form of write-in and read-out via electron spin.

The Cambridge scientists propose an approach that assumes that the type and nature of the nuclear spin-wave modes that can be activated with an electron spin is dependent on the state of the nuclear group being studied. Some spin-wave modes, for example, increase the ensemble's polarization, i.e. how many spins point up, while others decrease it. The relative strength of the two types of spinwave modes is determined by how much an ensemble 'points up/down'. Both can be used to measure how strong each spin-wave mode is pointing up and down on an ensemble, which in turn allows researchers to infer the spin populations.

Atature stated that there was more. "If the nuclear spins interact beforehand and build up some mutual information which in this instance can be quantum in nature then the electron will feel this pre-existing interaction as a quantum object having one-to-all coupling to these nuclei." This modifies the strength and activation of spin-wave modes that it can activate. This is what makes our approach unique. We were able to combine measurements from multiple spin-wave modes and use the electron as a "witness" for entanglement among the nuclei of the ensemble.

Researchers' method of studying many-body systems with a proxy electron spin qubit offers new and exciting possibilities to probe nuclear ensembles, without having to rely on individual spin readouts. Their approach is different from other methods. It leverages the native connectivity and interaction of a proxy qubit with a dense nucleus ensemble to extract interesting information, including quantum properties.

Le Gall stated, "Perhaps an analogy for our approach could be an Orchestra, where one can determine if musicians perform well together without having to know each instrument individually." "Our study also demonstrated that the semiconductor quantum dot, which is one of the most efficient single photon sources around the globe, can host nuclear spin ensembles and can be used to efficiently connect to light.

The new technique to probe the spin states in nuclear ensembles may open the door for new quantum technology. Now, the research team is trying to engineer the quantum dots that they have examined in their paper to improve their coherence and to exhibit more quantum properties.

Gangloff stated that this is crucial if we are to use quantum dot nuclei as a quantum memory. "Once we achieve more coherence--particularly with a new generation of quantum dots, based on a different growth method, that show a very promising hundredfold improvement over the quantum dots used thus far--our plans involve crafting the nuclei into evermore controlled quantum states, understanding how entanglement is lost and can be preserved in this many-body system, and demonstrating that this resource can be used in quantum computing and quantum communication."

Further reading Light was used to detect quantum information stored within 100,000 nuclear quantum bits

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