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.
We present a measurement protocol for a flux qubit coupled to a dc-Superconducting QUantum Interference Device (SQUID), representative of any two-state system with a controllable coupling to an harmonic oscillator quadrature, which consists of two steps. First, the qubit state is imprinted onto the SQUID via a very short and strong interaction. We show that at the end of this step the qubit dephases completely, although the perturbation of the measured qubit observable during this step is weak. In the second step, information about the qubit is extracted by measuring the SQUID. This step can have arbitrarily long duration, since it no longer induces qubit errors.
We report initialization, complete electrical control, and single-shot readout of an exchange-only spin qubit. Full control via the exchange interaction is fast, yielding a demonstrated 75 qubit rotations in under 2 ns. Measurement and state tomography are performed using a maximum-likelihood estimator method, allowing decoherence, leakage out of the qubit state space, and measurement fidelity to be quantified. The methods developed here are generally applicable to systems with state leakage, noisy measurements, and non-orthogonal control axes.
Electron spin s in semiconductor quantum dot s have been intensively studied for implementing quantum computation and high fidelity single and two qubit operation s have recently been achieved . Quantum teleportation is a three qubit protocol exploiting quantum entanglement and it serv es as a n essential primitive for more sophisticated quantum algorithm s Here, we demonstrate a scheme for quantum teleportation based on direct Bell measurement for a single electron spin qubit in a triple quantum dot utilizing the Pauli exclusion principle to create and detect maximally entangled state s . T he single spin polarization is teleported from the input qubit to the output qubit with a fidelity of 0.9 1 We find this fidelity is primarily limited by singlet triplet mixing which can be improved by optimizing the device parameters Our results may be extended to quantum algorithms with a larger number of se miconductor spin qubit s
We investigate qubit measurements using a single electron transistor (SET). Applying the Schrodinger equation to the entire system we find that an asymmetric SET is considerably more efficient than a symmetric SET. The asymmetric SET becomes close to an ideal detector in the large asymmetry limit. We also compared the SET detector with a point-contact detector. This comparison allows us to illuminate the relation between information gain in the measurement process and the decoherence generated by these measurement devices.
Initialization, manipulation, and measurement of a three-spin qubit are demonstrated using a few-electron triple quantum dot, where all operations can be driven by tuning the nearest-neighbor exchange interaction. Multiplexed reflectometry, applied to two nearby charge sensors, allows for qubit readout. Decoherence is found to be consistent with predictions based on gate voltage noise with a uniform power spectrum. The theory of the exchange-only qubit is developed and it is shown that initialization of only two spins suffices for operation. Requirements for full multi-qubit control using only exchange and electrostatic interactions are outlined.