The measurement of the quantum states of huge systems made of many interacting particles is one of the fundamental goals of quantum physics research. This might be very beneficial in the development of quantum computers and other quantum data processing devices.
Researchers at the Cavendish Laboratory at the University of Cambridge have developed a novel method for monitoring the spin states of a nuclear ensemble, which is a system made up of many interacting particles with long-lived quantum features. This approach, which was reported in Nature Physics, operates by taking advantage of the system's reaction to collective spin excitations.
"For a dense ensemble of quantum objects, such as spins, it isn't possible to measure each individually, to learn how they interacted with each other," Claire Le Gall and Mete Atatüre, two of the researchers who carried out the study, told Phys.org. "Instead, one can look for tell-tale signals in the collective response of the ensemble; a bit like the behavior of a flock of birds might say something about how the birds engage with each other. Our system of interest is a large flock, or ensemble, of nuclear spins in a semiconductor quantum dot."
Three Harvard University physicists discovered that massive ensembles of nuclear spins in a silicon quantum dot might be possible hosts for solid-state quantum memory in 2002, and their findings were published a year later. As detailed in their most recent publication, Le Gall, Atatüre, and their colleagues investigated this form of nuclear ensemble utilizing a 'proxy' quantum bit, an electron spin that relates to all nuclear spins concurrently.
"We achieved a significant milestone recently, when we showed that collective modes of the nuclear ensemble (i.e., spin waves) could be excited coherently via the electron," According to Dorian Gangloff, the paper's original author. "In our new study, we set out to use these electron-activated spin waves to change the state of the nuclear ensemble and to read it out. This would demonstrate a basic form of 'write-in' and 'read-out' via the electron spin."
The Cambridge scientists' technique is based on the premise that the sort of nuclear spin-wave mode that can be activated by an electron spin is determined by the state of the nuclear ensemble being studied. Some spin-wave modes, for example, enhance and others reduce an ensemble's polarization (i.e., how much all spins point 'up'). The relative intensity of these two types of spin-wave modes is determined by how far an ensemble has previously 'pointed up' or 'pointed down.' Measuring both can help researchers figure out how much each nuclear spin is already pointing up or down on average, allowing them to extrapolate spin populations.
"But there's more: If the nuclear spins had previously interacted and built up some mutual information, which in this case may be quantum in character," Atatüre explained, "then the electron, as a quantum entity with one-to-all coupling with these nuclei, will experience this pre-existing contact." "This changes the power of the spin-wave modes it may trigger, and this is what makes our method distinctive. As a consequence, we were able to employ the electron as a 'witness' for entanglement among the nuclei in the ensemble by integrating observations of different spin-wave modes."
The researchers' way of studying many-body systems using a 'proxy' electron spin qubit offers up new and exciting possibilities for investigating nuclear ensembles without depending on individual spin readouts. Unlike previously suggested approaches, their strategy takes use of a proxy qubit's intrinsic connection in contact with a dense nuclear ensemble, allowing them to extract useful information from these systems, such as quantum characteristics.
"Perhaps an analogy to our approach could be an orchestra, where one can tell if musicians are performing well together without prior knowledge of every instrument separately," Le Gall said. "Our study also showed for the first time that a nuclear spin ensemble in a semiconductor quantum dot (amongst the very best single photon sources in the world) can host many-spin entanglement and can therefore be used as a large quantum resource efficiently connected to light."
The innovative approach for investigating the spin states of nuclear ensembles might open the path for new quantum technology development in the future. The researchers are currently working to improve the spin ensembles of the quantum dots studied in their publication so that they have more coherence and quantum characteristics.
"This will be critical if we want to use quantum dot nuclei for a quantum memory," Gangloff said. "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."
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