<\/p>\n A three-step optical protocol developed by Gangloff\u00a0et al.<\/em>\u00a0allows an electron spin in a quantum dot to couple to the nuclear-spin bath.<\/p>\n <\/p>\n <\/p>\n The most promising candidates for quantum bits in the solid state are defects (3<\/em><\/a>), such as nitrogen vacancies in diamond, which are well isolated from their surroundings (4<\/em><\/a>). Atom-like or, more precisely, defect-like localization can be mimicked with quantum dot structures with threedimensional carrier confinement (5<\/em><\/a>). This approach has led to valuable QI hardware achievements, such as efficient and highquality single or entangled photon sources (6<\/em><\/a>). As stationary qubits, quantum dots offer charge coherence times in the nanosecond range and spin coherence times in the microsecond range, but these values are still orders of magnitude shorter than other solid-state competitors (7<\/em><\/a>). The nuclei of the quantum dot are responsible for this shortcoming. At cryogenic temperatures of a few kelvin, the lattice vibrations are frozen out, so that the main interaction that quantum dot carrier spins undergo is the hyperfine coupling with the lattice nuclei.<\/p>\n Long-lived spin coherence of lattice nuclei is well known from nuclear magnetic resonance studies, so nuclei have been considered as a resource into which QI from the carrier spins can be shuffled and stored with possible subsequent recovery through this coupling. However, the nuclear spin bath is highly complex. In the quantum dots studied by Gangloff\u00a0et al.<\/em>, this bath is formed by tens of thousands of nuclei of indium, gallium, and arsenic atoms that form a very inhomogeneous ensemble. This inhomogeneity is the result of the spins being strongly perturbed by electric fields created by strain in these structures that are grown by molecular beam epitaxy. Further, even at low temperatures of a few kelvin, the nuclear system is \u201chot\u201d and disordered. Thus, coupling of an electron spin to the nuclei is generally considered to be detrimental, as QI carried by carrier spins quickly dissipates, which limits the carrier spin coherence in quantum dots (7).<\/p>\n In order to use the nuclear bath as a quantum resource, a strong, nondissipative coupling would need to be established between carrier and nuclear spins, which until now has been considered almost impossible for quantum dots. However, hints that the hyperfine interaction does not necessarily have to act destructively could be found, such as in studies of the electron spin precession in a quantum dot ensemble in a magnetic field. Here, a nuclear magnetic field is established in each dot through electron-nuclei flip-flop processes so that the otherwise very inhomogeneous, broadly distributed precession frequencies are focused onto very few modes or even a single mode (8<\/em><\/a>).<\/p>\n The success of the approach taken by Gangloff\u00a0et al.<\/em>\u00a0depended on several breakthroughs. The authors initially cooled the nuclear spin systems by using optical pulses to trigger Raman transitions of the system. The electron spin in a single quantum dot was oriented, and then its polarization was transferred to the nuclei. For sufficiently long pumping under optimized conditions, a nuclear temperature as low as 200 \u00b5K was achieved, far below the 2 K temperature of the hot crystal.<\/p>\n In this regime, the authors found that the electron spin and nuclear ensemble formed a strongly coupled state. They could map out spectroscopically a change of total nuclear spin by a single unit of angular momentum. This change corresponded to a single nuclear magnon, the elementary quantum unit of a collective nuclear wave (see the figure).<\/p>\n Finally, Gangloff\u00a0et al.<\/em>\u00a0coherently manipulated the coupled electron-nuclei entity in which a large fraction of quantum dot nuclei are involved, which launched a nuclear magnon by all-optical means. This process corresponds to a coherent, nondissipative exchange between the electron spin and the collective of nuclei\u2014a prerequisite for a quantum memory.<\/p>\n These results could be the first step toward the development of a quantum memory interface with sufficiently long coherence time with quantum dots. The coherence benefits from the many-body nature of the nuclear system, which provides robustness. Moreover, every quantum bit would be associated with its dedicated inherent memory by default. This step has been the missing piece of the puzzle for a semiconductor nanostructure QI platform. As for carrier spin quantum bits, other key demonstrations (6<\/em><\/a>) have been provided already, such as efficient initialization and manipulation on time scales of nanoseconds or even shorter as well as the efficient interconversion with photons for information transfer. Also, at a fundamental level, these results are highly interesting because a quantum many-body state for the nuclei has been established that can be coherently manipulated optically through the electron spin. It should be possible to create specific nonclassical nuclear states, such as Schr\u00f6dinger cat states.<\/p>\n <\/p>\n <\/p>\n (\uc6d0\ubb38: \uc5ec\uae30<\/a>\ub97c \ud074\ub9ad\ud558\uc138\uc694~)<\/p>\n <\/p>\n <\/p>\n <\/p>\n","protected":false},"excerpt":{"rendered":"![](\"http:\/\/science.sciencemag.org\/content\/sci\/364\/6435\/30\/F1.medium.gif\")
<\/span><\/a><\/div>\n\n
GRAPHIC: C. BICKEL\/SCIENCE<\/em><\/q><\/p>\n
![<\/i>\u00a0Download high-res image<\/span><\/a><\/li>\n<\/i>\u00a0Open in new tab<\/span><\/a><\/li>\n<\/i>\u00a0Download Powerpoint<\/span><\/a><\/li>\n<\/ul>\n<\/div>\n<\/div>The cooled crowd does the magnon wave<\/span><\/p>\nA three-step optical protocol developed by Gangloff\u00a0et al.<\/em>\u00a0allows an electron spin in a quantum dot to couple to the nuclear-spin bath.<\/p>\nGRAPHIC: C. BICKEL\/SCIENCE<\/em><\/q><\/p>\n<\/div>\n<\/figcaption><\/figure>\n <\/p>\n <\/p>\nThe most promising candidates for quantum bits in the solid state are defects (3<\/em><\/a>), such as nitrogen vacancies in diamond, which are well isolated from their surroundings (4<\/em><\/a>). Atom-like or, more precisely, defect-like localization can be mimicked with quantum dot structures with threedimensional carrier confinement (5<\/em><\/a>). This approach has led to valuable QI hardware achievements, such as efficient and highquality single or entangled photon sources (6<\/em><\/a>). As stationary qubits, quantum dots offer charge coherence times in the nanosecond range and spin coherence times in the microsecond range, but these values are still orders of magnitude shorter than other solid-state competitors (7<\/em><\/a>). The nuclei of the quantum dot are responsible for this shortcoming. At cryogenic temperatures of a few kelvin, the lattice vibrations are frozen out, so that the main interaction that quantum dot carrier spins undergo is the hyperfine coupling with the lattice nuclei.<\/p>\nLong-lived spin coherence of lattice nuclei is well known from nuclear magnetic resonance studies, so nuclei have been considered as a resource into which QI from the carrier spins can be shuffled and stored with possible subsequent recovery through this coupling. However, the nuclear spin bath is highly complex. In the quantum dots studied by Gangloff\u00a0et al.<\/em>, this bath is formed by tens of thousands of nuclei of indium, gallium, and arsenic atoms that form a very inhomogeneous ensemble. This inhomogeneity is the result of the spins being strongly perturbed by electric fields created by strain in these structures that are grown by molecular beam epitaxy. Further, even at low temperatures of a few kelvin, the nuclear system is \u201chot\u201d and disordered. Thus, coupling of an electron spin to the nuclei is generally considered to be detrimental, as QI carried by carrier spins quickly dissipates, which limits the carrier spin coherence in quantum dots (7).<\/p>\nIn order to use the nuclear bath as a quantum resource, a strong, nondissipative coupling would need to be established between carrier and nuclear spins, which until now has been considered almost impossible for quantum dots. However, hints that the hyperfine interaction does not necessarily have to act destructively could be found, such as in studies of the electron spin precession in a quantum dot ensemble in a magnetic field. Here, a nuclear magnetic field is established in each dot through electron-nuclei flip-flop processes so that the otherwise very inhomogeneous, broadly distributed precession frequencies are focused onto very few modes or even a single mode (8<\/em><\/a>).<\/p>\nThe success of the approach taken by Gangloff\u00a0et al.<\/em>\u00a0depended on several breakthroughs. The authors initially cooled the nuclear spin systems by using optical pulses to trigger Raman transitions of the system. The electron spin in a single quantum dot was oriented, and then its polarization was transferred to the nuclei. For sufficiently long pumping under optimized conditions, a nuclear temperature as low as 200 \u00b5K was achieved, far below the 2 K temperature of the hot crystal.<\/p>\nIn this regime, the authors found that the electron spin and nuclear ensemble formed a strongly coupled state. They could map out spectroscopically a change of total nuclear spin by a single unit of angular momentum. This change corresponded to a single nuclear magnon, the elementary quantum unit of a collective nuclear wave (see the figure).<\/p>\nFinally, Gangloff\u00a0et al.<\/em>\u00a0coherently manipulated the coupled electron-nuclei entity in which a large fraction of quantum dot nuclei are involved, which launched a nuclear magnon by all-optical means. This process corresponds to a coherent, nondissipative exchange between the electron spin and the collective of nuclei\u2014a prerequisite for a quantum memory.<\/p>\nThese results could be the first step toward the development of a quantum memory interface with sufficiently long coherence time with quantum dots. The coherence benefits from the many-body nature of the nuclear system, which provides robustness. Moreover, every quantum bit would be associated with its dedicated inherent memory by default. This step has been the missing piece of the puzzle for a semiconductor nanostructure QI platform. As for carrier spin quantum bits, other key demonstrations (6<\/em><\/a>) have been provided already, such as efficient initialization and manipulation on time scales of nanoseconds or even shorter as well as the efficient interconversion with photons for information transfer. Also, at a fundamental level, these results are highly interesting because a quantum many-body state for the nuclei has been established that can be coherently manipulated optically through the electron spin. It should be possible to create specific nonclassical nuclear states, such as Schr\u00f6dinger cat states.<\/p>\n <\/p>\n <\/p>\n(\uc6d0\ubb38: \uc5ec\uae30<\/a>\ub97c \ud074\ub9ad\ud558\uc138\uc694~)<\/p>\n <\/p>\n <\/p>\n <\/p>\n","protected":false},"excerpt":{"rendered":" Quantum information (QI) has become a focus of research during the past two decades, with the goal of exploiting the potentialities offered by(more…)<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false},"categories":[32,34,35,29,30],"tags":[],"class_list":["post-3255","post","type-post","status-publish","format-standard","hentry","category-essays-on-science","category-lets-do-chemistry","category-lets-do-computer-science","category-lets-do-science","category-recent-science-news"],"aioseo_notices":[],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack-related-posts":[{"id":4159,"url":"https:\/\/biochemistry.khu.ac.kr\/lab\/?p=4159","url_meta":{"origin":3255,"position":0},"title":"Google claims quantum computing milestone","author":"biochemistry","date":"September 27, 2019","format":false,"excerpt":"\u00a0 \u00a0 The age of quantum computing may have begun not with a flashy press conference, but with an internet leak. According to a paper posted briefly\u2014and presumably mistakenly\u2014to a lab website, physicists at Google have used a quantum computer to perform a calculation that would overwhelm the world's best\u2026","rel":"","context":"In "Essays on Science"","block_context":{"text":"Essays on Science","link":"https:\/\/biochemistry.khu.ac.kr\/lab\/?cat=32"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":3241,"url":"https:\/\/biochemistry.khu.ac.kr\/lab\/?p=3241","url_meta":{"origin":3255,"position":1},"title":"Walk the line (QUANTUM TECHNOLOGY)","author":"biochemistry","date":"April 8, 2019","format":false,"excerpt":"\u00a0 \u00a0 Qubits made from semiconductor quantum dots are a potential platform for future quantum computing. Although quantum gates with high fidelity have been demonstrated, the coupling of such qubits over distances, for example for use in quantum registers, remains a challenge. Mills et al. now show how they can\u2026","rel":"","context":"In "Let's Do Chemistry!"","block_context":{"text":"Let's Do Chemistry!","link":"https:\/\/biochemistry.khu.ac.kr\/lab\/?cat=34"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":1525,"url":"https:\/\/biochemistry.khu.ac.kr\/lab\/?p=1525","url_meta":{"origin":3255,"position":2},"title":"\ucc45 \uc18c\uac1c – Understanding the double slit","author":"biochemistry","date":"September 2, 2018","format":false,"excerpt":"\u00a0 \u00a0 (\uc6d0\ubb38: \uc5ec\uae30\ub97c \ud074\ub9ad\ud558\uc138\uc694~) \u00a0 \u00a0 Science\u00a0\u00a031 Aug 2018: Vol. 361, Issue 6405, pp. 855 DOI: 10.1126\/science.aav0128 \u00a0 \u00a0 In his famous\u00a0Lectures on Physics, Richard Feynman argued that nothing more is needed to get a solid grasp of the behavior of quantum objects than the simple double-slit experiment, in\u2026","rel":"","context":"In "Essays on Science"","block_context":{"text":"Essays on Science","link":"https:\/\/biochemistry.khu.ac.kr\/lab\/?cat=32"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":4545,"url":"https:\/\/biochemistry.khu.ac.kr\/lab\/?p=4545","url_meta":{"origin":3255,"position":3},"title":"Quantum computing takes flight & A precarious milestone for quantum computing & Hello quantum world! Google publishes landmark quantum supremacy claim","author":"biochemistry","date":"October 23, 2019","format":false,"excerpt":"\u00a0 \u00a0 \ub17c\ub780\uc758 \uad6c\uae00 \uc591\uc790\ucef4\ud4e8\ud130 \uce69 \ub4dc\ub514\uc5b4 \uacf5\uac1c...\"\uc591\uc790\uc6b0\uc6d4\uc131 \ub2ec\uc131\ud588\ub2e4\" \u00a0 \u00a0 \uad6c\uae00 '\ub124\uc774\ucc98' \ub17c\ubb38 \ubc1c\ud45c...\ub09c\uc218 \uc99d\uba85 \ud2b9\uc815 \uacfc\uc81c\uc5d0\uc11c \uc288\ud37c\ucef4 \ub2a5\uac00 \ud655\uc778 \u00a0 \uad6c\uae00\uc774 \uc591\uc790\ucef4\ud4e8\ud130\ub85c \uae30\uc874 \ucef4\ud4e8\ud130\ub97c \ub2a5\uac00\ud558\ub294 \uc5f0\uc0b0 \uc131\ub2a5\uc744 \ubcf4\uc774\ub294 \uc774\ub978\ubc14 '\uc591\uc790\uc6b0\uc6d4\uc131'\uc744 \ub2ec\uc131\ud588\ub2e4\ub294 \ub17c\ubb38\uc744 \uc815\uc2dd \ubc1c\ud45c\ud588\ub2e4. 9\uc6d4 \uc911\uc21c\ubd80\ud130 \ud55c \ub2ec \uc774\uc0c1 \uc9c0\uc18d\ub3fc \uc628 \ub17c\ub780\uc774 \uc77c\ub2e8 \uac00\ub77c\uc549\uc744 \uac83\uc73c\ub85c \ubcf4\uc778\ub2e4. \uad6c\uae00\u00a0AI\u00a0\ube14\ub85c\uadf8 \uc81c\uacf5\u2026","rel":"","context":"In "'06. \uc5d0\ub108\uc9c0\uc640 \uc5d4\ud2b8\ub85c\ud53c'\uc640 '07. \uacfc\ud559\uacfc \ubb38\uba85' \uad00\ub828"","block_context":{"text":"'06. \uc5d0\ub108\uc9c0\uc640 \uc5d4\ud2b8\ub85c\ud53c'\uc640 '07. \uacfc\ud559\uacfc \ubb38\uba85' \uad00\ub828","link":"https:\/\/biochemistry.khu.ac.kr\/lab\/?cat=42"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":1821,"url":"https:\/\/biochemistry.khu.ac.kr\/lab\/?p=1821","url_meta":{"origin":3255,"position":4},"title":"Reimagining of Schr\u00f6dinger\u2019s cat breaks quantum mechanics \u2014 and stumps physicists","author":"biochemistry","date":"September 23, 2018","format":false,"excerpt":"\u00a0 \u00a0 (\uc6d0\ubb38: \uc5ec\uae30\ub97c \ud074\ub9ad\ud558\uc138\uc694~) \u00a0 \u00a0 In a multi-\u2018cat\u2019 experiment, the textbook interpretation of quantum theory seems to lead to contradictory pictures of reality, physicists claim. \u00a0 \u00a0 Credit: Aleksei Isachenko\/Alamy \u00a0 \u00a0 In the world\u2019s most famous thought experiment, physicist Erwin Schr\u00f6dinger described how a cat in a\u2026","rel":"","context":"In "Let's Do Chemistry!"","block_context":{"text":"Let's Do Chemistry!","link":"https:\/\/biochemistry.khu.ac.kr\/lab\/?cat=34"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":4092,"url":"https:\/\/biochemistry.khu.ac.kr\/lab\/?p=4092","url_meta":{"origin":3255,"position":5},"title":"Making perfectly controlled arrays of molecules at rest","author":"biochemistry","date":"September 18, 2019","format":false,"excerpt":"\u00a0 \u00a0 Since their invention in the early 1970s, optical tweezers have evolved from enabling simple manipulation to applying calibrated forces on\u2014and measuring nanometer-level displacements of\u2014optically trapped objects. Optical tweezers use laser light to create a force trap that can hold nanometer- to micrometer-sized dielectric objects (1). They can noninvasively\u2026","rel":"","context":"In "Let's Do Chemistry!"","block_context":{"text":"Let's Do Chemistry!","link":"https:\/\/biochemistry.khu.ac.kr\/lab\/?cat=34"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]}],"jetpack_sharing_enabled":false,"jetpack_shortlink":"https:\/\/wp.me\/p9Xo1j-Qv","_links":{"self":[{"href":"https:\/\/biochemistry.khu.ac.kr\/lab\/index.php?rest_route=\/wp\/v2\/posts\/3255","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/biochemistry.khu.ac.kr\/lab\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/biochemistry.khu.ac.kr\/lab\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/biochemistry.khu.ac.kr\/lab\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/biochemistry.khu.ac.kr\/lab\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=3255"}],"version-history":[{"count":1,"href":"https:\/\/biochemistry.khu.ac.kr\/lab\/index.php?rest_route=\/wp\/v2\/posts\/3255\/revisions"}],"predecessor-version":[{"id":3256,"href":"https:\/\/biochemistry.khu.ac.kr\/lab\/index.php?rest_route=\/wp\/v2\/posts\/3255\/revisions\/3256"}],"wp:attachment":[{"href":"https:\/\/biochemistry.khu.ac.kr\/lab\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=3255"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/biochemistry.khu.ac.kr\/lab\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=3255"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/biochemistry.khu.ac.kr\/lab\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=3255"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}](\"http:\/\/science.sciencemag.org\/content\/sci\/364\/6435\/30\/F1.large.jpg?download=true\"){kind=link}