A new method for measuring quantum entanglement in a set of nuclear spins

These are the three-dimensional spectral data the team obtained from the electronic proxy qubit, with spin wave modes corresponding to each “peak”. Horizontally, the qubit probes a fixed state of the nuclear assembly. Vertically, the state of the nuclear assembly is regulated by the qubit. Spectral asymmetry is a witness to quantum correlations between nuclei. It’s also somewhat symbolic as this work is the result of nearly two decades of continuous research efforts, by Cambridge researchers and many other teams, to achieve this demonstration of an entangled nuclear assembly. Credit: Gangloff et al.

One of the main goals of quantum physics studies is to measure the quantum states of large systems made up of many interacting particles. This could be particularly useful for the development of quantum computers and other quantum information processing devices.

Researchers at Cambridge University’s Cavendish Laboratory recently introduced a new approach to measure the spin states of a nuclear assembly, a system made up of many particles interacting with long-lived quantum properties. This method, presented in an article published in Physics of nature, works by exploiting the response of this system to collective spin excitations.

“For a dense set of quantum objects, like spins, it is not possible to measure each one individually, to learn how they interact with each other”, Claire Le Gall and Mete Atatüre, two of the researchers who have conducted the study, says Phys.org. “Instead, one can look for telltale signals in the collective response of the whole; much like the behavior of a flock of birds might say something about how birds engage with each other. others. Our system of interest is a large herd, or collection, of nuclear spins in a semiconductor quantum dot. “

In 2002, three Harvard University physicists discovered that large arrays of nuclear spins in a semiconductor quantum dot could be potential hosts for semiconductor quantum memories, and then published their work a year later. 19 years later, Le Gall, Atatüre and their colleagues probed this type of nuclear assembly using a quantum “proxy” bit, an electronic spin that couples simultaneously to all nuclear spins, as shown in their last article.

“We recently took an important step when we showed that the collective modes of the nuclear assembly (i.e. spin waves) can be coherently excited via the electron,” said Dorian Gangloff. , the first author of the article. “In our new study, we decided to use these electron-activated spin waves to change the state of the nuclear assembly and to read it. This would demonstrate a basic form of ‘writing’ and ‘reading’. via the spin of the electron. “

The idea behind the approach proposed by the scientists at Cambridge is that the type of nuclear spin wave mode that can be activated by electron spin depends on the state of the nuclear assembly that is being examined. For example, some spin wave modes increase the polarization of an ensemble (i.e. by how much all the spins point up) and others decrease it. The relative strength of these two different types of spin wave modes depends on how much an assembly is already “pointing up” or “pointing down”. up or down, ultimately allowing researchers to infer spin populations.

“But there is more: if the nuclear spins have interacted beforehand and accumulated some mutual information, which in this case may be quantum in nature, then the electron, as a quantum object with a one-to-one coupling. everything with those nuclei, feeling that pre-existing interaction, ”Atatüre said.“ It changes the strength of the spin wave modes that it can activate, and that’s what is very unique about our approach. As a result, by combining the measurements of several spin wave modes, we were able to use the electron as a “witness” for the entanglement among the nuclei in the set. “

The researchers’ method of observing multi-body systems using a “proxy” electronic spin qubit opens up new and interesting possibilities for probing nuclear assemblies without relying on individual spin readings. Unlike the previously proposed methods, their approach takes advantage of the native connectivity of a proxy qubit interacting with a dense nuclear assembly, ultimately extracting interesting information from these systems, including their quantum properties.

“Perhaps an analogy to our approach could be an orchestra, where we can tell if musicians perform well together without prior knowledge of each instrument separately,” said Le Gall. “Our study also showed for the first time that a set of nuclear spins in a semiconductor quantum dot (among the best single photon sources in the world) can harbor a multi-spin entanglement and therefore can be used as a large quantum resource efficiently connected to lightweight. “

In the future, the new technique for probing the spin states of nuclear assemblies could pave the way for the development of new quantum technologies. The research team are now trying to design the quantum dots examined in their paper to ensure that their sets of spins have greater coherence and exhibit more quantum properties.

“This will be essential if we are to use quantum dot nuclei for quantum memory,” Gangloff said. “Once we achieve more consistency, especially with a new generation of quantum dots, based on a different growth method, which shows a very promising hundredfold improvement over the quantum dots used so far – our plans are to shape nuclei into increasingly controlled quantum states, understand how entanglement is lost and can be preserved in this multibody system, and demonstrate that this resource can be used in quantum computing and computing. quantum communication. ”

Light used to detect quantum information stored in 100,000 nuclear quantum bits

More information:
Dorian A. Gangloff et al, Evidence of quantum correlations in a nuclear assembly via an electronic spin qubit, Physics of nature (2021). DOI: 10.1038 / s41567-021-01344-7

JM Taylor et al, Long-term memory for mesoscopic quantum bits, Physical examination letters (2003). DOI: 10.1103 / PhysRevLett.90.206803

DA Gangloff et al, Quantum interface of an electron and a nuclear assembly, Science (2019). DOI: 10.1126 / science.aaw2906

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Quote: A new method for measuring quantum entanglement in a set of nuclear spins (2021, November 12) retrieved on November 15, 2021 from https://phys.org/news/2021-11-method-quantum-entanglement-nuclear- set.html

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