Suppressing qubit dephasing using real-time Hamiltonian estimation.

Unwanted interaction between a quantum system and its fluctuating environment leads to decoherence and is the primary obstacle to establishing a scalable quantum information processing architecture. Strategies such as environmental and materials engineering, quantum error correction and dynamical decoupling can mitigate decoherence, but generally increase experimental complexity. Here we improve coherence in a qubit using real-time Hamiltonian parameter estimation. Using a rapidly converging Bayesian approach, we precisely measure the splitting in a singlet-triplet spin qubit faster than the surrounding nuclear bath fluctuates. We continuously adjust qubit control parameters based on this information, thereby improving the inhomogenously broadened coherence time (T2*) from tens of nanoseconds to >2 µs. Because the technique demonstrated here is compatible with arbitrary qubit operations, it is a natural complement to quantum error correction and can be used to improve the performance of a wide variety of qubits in both meteorological and quantum information processing applications.

Charge noise spectroscopy using coherent exchange oscillations in a singlet-triplet qubit.

Two level systems that can be reliably controlled and measured hold promise as qubits both for metrology and for quantum information science. Since a fluctuating environment limits the performance of qubits in both capacities, understanding environmental coupling and dynamics is key to improving qubit performance. We show measurements of the level splitting and dephasing due to the voltage noise of a GaAs singlet-triplet qubit during exchange oscillations. Unexpectedly, the voltage fluctuations are non-Markovian even at high frequencies and exhibit a strong temperature dependence. This finding has impacts beyond singlet-triplet qubits since nearly all solid state qubits suffer from some kind of charge noise. The magnitude of the fluctuations allows the qubit to be used as a charge sensor with a sensitivity of 2 × 10(-8)e/sqrt[Hz], 2 orders of magnitude better than a quantum-limited rf single electron transistor. Based on these measurements, we provide recommendations for improving qubit coherence, allowing for higher fidelity operations and improved charge sensitivity.

Demonstration of entanglement of electrostatically coupled singlet-triplet qubits.

Quantum computers have the potential to solve certain problems faster than classical computers. To exploit their power, it is necessary to perform interqubit operations and generate entangled states. Spin qubits are a promising candidate for implementing a quantum processor because of their potential for scalability and miniaturization. However, their weak interactions with the environment, which lead to their long coherence times, make interqubit operations challenging. We performed a controlled two-qubit operation between singlet-triplet qubits using a dynamically decoupled sequence that maintains the two-qubit coupling while decoupling each qubit from its fluctuating environment. Using state tomography, we measured the full density matrix of the system and determined the concurrence and the fidelity of the generated state, providing proof of entanglement.

Judgments of grammaticality by normal and language-disordered children.

Fifteen linguistically normal children and 15 linguistically deviant children were presented with three types of agrammatical sentences. The subjects were asked to judge the sentences as right or wrong and to change the sentences judged as wrong, rendering them correct. The three types of agrammatical sentences represented rule violations of syntactic agreement (Type A), lexical restrictions (Type B), and word order (Type C). The two groups of children were compared in terms of the number of sentences of each type that were agrammatical. Those productions which represented the child's correction of agrammatical sentences were subjected to descriptive analyses (percentages) with specific reference to the number of attempted changes and the number of those changes which demonstrated corrections of the specific deviation from well formedness. Results indicated that the two groups of subjects were significantly different in their ability to recognize grammatical errors in sentence Types A and C, but did not differ in their ability to recognize errors in sentence Type B. The descriptive comparison of the groups' verbal corrections reflected this trend, in that the language-disordered subjects made corrections specific to the error on more of the Type B sentences (for example, "The dog writes the food.") than on Types A (for example, "She will pick some flowers last week.") or C (for example, "Get and come your dinner."1.) Linguistically normal children accurately corrected 90.7% of the sentences judges as agrammatical; this percentage did not vary more than 1% across sentence types.