Publisher Correction: Resolving the positions of defects in superconducting quantum bits.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Resolving the positions of defects in superconducting quantum bits.

Solid-state quantum coherent devices are quickly progressing. Superconducting circuits, for instance, have already been used to demonstrate prototype quantum processors comprising a few tens of quantum bits. This development also revealed that a major part of decoherence and energy loss in such devices originates from a bath of parasitic material defects. However, neither the microscopic structure of defects nor the mechanisms by which they emerge during sample fabrication are understood. Here, we present a technique to obtain information on locations of defects relative to the thin film edge of the qubit circuit. Resonance frequencies of defects are tuned by exposing the qubit sample to electric fields generated by electrodes surrounding the chip. By determining the defect's coupling strength to each electrode and comparing it to a simulation of the field distribution, we obtain the probability at which location and at which interface the defect resides. This method is applicable to already existing samples of various qubit types, without further on-chip design changes. It provides a valuable tool for improving the material quality and nano-fabrication procedures towards more coherent quantum circuits.

Decoherence spectroscopy with individual two-level tunneling defects.

Recent progress with microfabricated quantum devices has revealed that an ubiquitous source of noise originates in tunneling material defects that give rise to a sparse bath of parasitic two-level systems (TLSs). For superconducting qubits, TLSs residing on electrode surfaces and in tunnel junctions account for a major part of decoherence and thus pose a serious roadblock to the realization of solid-state quantum processors. Here, we utilize a superconducting qubit to explore the quantum state evolution of coherently operated TLSs in order to shed new light on their individual properties and environmental interactions. We identify a frequency-dependence of TLS energy relaxation rates that can be explained by a coupling to phononic modes rather than by anticipated mutual TLS interactions. Most investigated TLSs are found to be free of pure dephasing at their energy degeneracy points, around which their Ramsey and spin-echo dephasing rates scale linearly and quadratically with asymmetry energy, respectively. We provide an explanation based on the standard tunneling model, and identify interaction with incoherent low-frequency (thermal) TLSs as the major mechanism of the pure dephasing in coherent high-frequency TLS.

Observation of directly interacting coherent two-level systems in an amorphous material.

Parasitic two-level tunnelling systems originating from structural material defects affect the functionality of various microfabricated devices by acting as a source of noise. In particular, superconducting quantum bits may be sensitive to even single defects when these reside in the tunnel barrier of the qubit's Josephson junctions, and this can be exploited to observe and manipulate the quantum states of individual tunnelling systems. Here, we detect and fully characterize a system of two strongly interacting defects using a novel technique for high-resolution spectroscopy. Mutual defect coupling has been conjectured to explain various anomalies of glasses, and was recently suggested as the origin of low-frequency noise in superconducting devices. Our study provides conclusive evidence of defect interactions with full access to the individual constituents, demonstrating the potential of superconducting qubits for studying material defects. All our observations are consistent with the assumption that defects are generated by atomic tunnelling.

Strain tuning of individual atomic tunneling systems detected by a superconducting qubit.

In structurally disordered solids, some atoms or small groups of atoms are able to quantum mechanically tunnel between two nearly equivalent sites. These atomic tunneling systems have been identified as the cause of various low-temperature anomalies of bulk glasses and as a source of decoherence of superconducting qubits where they are sparsely present in the disordered oxide barrier of Josephson junctions. We demonstrated experimentally that minute deformation of the oxide barrier changes the energies of the atomic tunneling systems, and we measured these changes by microwave spectroscopy of the superconducting qubit through coherent interactions between these two quantum systems. By measuring the dependence of the energy splitting of atomic tunneling states on external strain, we verify a central hypothesis of the two-level tunneling model for disordered solids.

Observational study of fibrinogen concentrate in massive hemorrhage: evaluation of a multicenter register.

In acute hemorrhage, a critical decrease in fibrinogen often induces acquired coagulopathy. Fibrinogen concentrate has been used to supplement fibrinogen during massive hemorrhage. However, there are limited data on the utilization of fibrinogen concentrate in this setting. This prospective, multicenter observational study analyzed clinical treatment with fibrinogen concentrate in acute bleeding. A prospective multicenter web-based register was developed to document closed cases of massive hemorrhage treated with fibrinogen concentrate perioperatively. Anonymized data including the cause and kinetics of the bleeding, coagulation parameters, coagulation therapy, clinical effects and adverse events were recorded. Two hundred and twenty-three cases entered between September 2008 and August 2009 were eligible for analysis. According to patient needs, additional common blood and coagulation products were administered. Fibrinogen substitution by fibrinogen concentrate and fresh frozen plasma (FFP) was initiated at a median blood loss of 2.0 l and plasma fibrinogen of 1.45 g/l. After a median dose of 12.0 g fibrinogen (4 g in fibrinogen concentrate and 8 g in FFP), plasma fibrinogen rose to 2.19 g/l at the end of surgery; corresponding to a median increment of 0.045 g/l per gram of fibrinogen administered. After substitution, 6% of patients had supra-physiological plasma fibrinogen levels. Three percent of patients sustained thromboembolic complications perioperatively. Logistic regression analysis showed positive correlation of postoperative plasma fibrinogen and survival (P < 0.05). Clinical application of fibrinogen concentrate in bleeding patients is included within a multimodal therapeutic concept. High levels of fibrinogen are necessary in order to reach therapeutic goals. In bleeding patients, higher plasma fibrinogen might be associated with higher rates of survival.

Postoperative changes in procoagulant factors after major surgery.

Surgical patients are primarily at an increased risk of perioperative bleeding; however, after surgery, these patients develop hypercoagulability that favors thrombotic events. Currently, the time course of postoperative coagulation is not well characterized. Thus, the aim of the present study was to provide a detailed description of the changes in procoagulant factors in patients after major surgery and to evaluate coagulation tests based on their ability to detect hypercoagulability. Fifty-one consecutive patients undergoing different types of major surgery were analyzed. Blood samples were taken preoperatively and on postoperative days (PODs) 1, 2, 3, and 6. In addition to prothrombin time (PT) and activated partial thromboplastin time (aPTT), all PT-dependent and aPTT-dependent clotting factors, von Willebrand factor (vWF), and fibrinogen were obtained, and thrombelastometry and multiplate electrode aggregometry (MEA) were performed. On POD 1, the majority of clotting factors, including factors II, VII, X, XI, and XII, showed a significant decrease from baseline. Factors II, X, XI, and XII remained significantly reduced until POD 3. In contrast, starting on POD 2, fibrinogen, factor VIII, and vWF continuously increased. No relevant changes were found for PT or aPTT. Thrombelastometry revealed a continuous increase in clot firmness, and MEA demonstrated an increase in platelet aggregation on POD 6. However, absolute values remained within normal ranges, and only serial measurements showed hypercoagulation. Beginning on POD 2 after major surgery, significant hypercoagulability developed in patients. However, clinically used global coagulation tests and point-of-care devices did not reliably reflect the hypercoagulatory state.