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تسجيل مستخدم جديدقياس غير مدمج للكم الذري للإلكترون القطبي
Quantum nondemolition measurement of an electron spin qubit
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تحديد النظامات الكمية بشكل ضروري يتضمن التشويش في أشكال مختلفة. ومع الحدود التي تقيدها قوانين كيمياء الكم الذري، يمكن تصميم قياس مشروعي مثالي لا يضيف تأثيرا عكسيا على المقياس المقياس، المعروف باسم قياس غير مدمر (QND). هنا نوضح قياس QND كل الكهربائي لإحدى الألياف الكمية الواحدة في نقطة كيميائية محددة بواسطة البوابة عبر كوبيت أنسيلا مرتبط بالتبادل. يتم ترميز كوبيت الأنسيلا في مجال الجزئين الثنائي الجزئين، ويتم تشابكه مع الإحدى الألياف الكمية وقراءتها في قياس مشروعي في دفعة واحدة بمعدل يكون أسرع من معدل تساقط الإحدى الألياف الكمية بنسبة مرتفعة بمقدار مرتفع بنسبتين. يثبت نوعية قياس QND لبروتوكول القياس من خلال ملاحظة زيادة مستمرة لدقة القراءة عبر مائة تقييمات متكررة ضد حالات المدخلات العشوائية. نخرج المعلومات من سجل القياس باستخدام طريقة التأكد المثالية، التي تتسامح مع وجود التساقط والتشوه. يسمح قياس QND لنا بملاحظة الانقلابات الذاتية المتحركة (القفزات الكمية) في نظام منفصل مع تشويش صغير. مزدوجاً مع السيطرة العالية الدقة على الألياف الكمية، تسهم هذه النتائج في تصميم العديد من التحكمات الكمية المستندة إلى القياس، بما في ذلك بروتوكولات تصحيح الأخطاء الكمية.
Measurement of quantum systems inevitably involves disturbance in various forms. Within the limits imposed by quantum mechanics, however, one can design an ideal projective measurement that does not introduce a back action on the measured observable, known as a quantum nondemolition (QND) measurement. Here we demonstrate an all-electrical QND measurement of a single electron spin in a gate-defined quantum dot via an exchange-coupled ancilla qubit. The ancilla qubit, encoded in the singlet-triplet two-electron subspace, is entangled with the single spin and subsequently read out in a single shot projective measurement at a rate two orders of magnitude faster than the spin relaxation. The QND nature of the measurement protocol is evidenced by observing a monotonic increase of the readout fidelity over one hundred repetitive measurements against arbitrary input states. We extract information from the measurement record using the method of optimal inference, which is tolerant to the presence of the relaxation and dephasing. The QND measurement allows us to observe spontaneous spin flips (quantum jumps) in an isolated system with small disturbance. Combined with the high-fidelity control of spin qubits, these results pave the way for various measurement-based quantum state manipulations including quantum error correction protocols.
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