Quasiparticle Poisoning of Superconducting Qubits from Resonant Absorption of Pair-Breaking Photons.

The ideal superconductor provides a pristine environment for the delicate states of a quantum computer: because there is an energy gap to excitations, there are no spurious modes with which the qubits can interact, causing irreversible decay of the quantum state. As a practical matter, however, there exists a high density of excitations out of the superconducting ground state even at ultralow temperature; these are known as quasiparticles. Observed quasiparticle densities are of order 1 µm^{-3}, tens of orders of magnitude greater than the equilibrium density expected from theory. Nonequilibrium quasiparticles extract energy from the qubit mode and can induce dephasing. Here we show that a dominant mechanism for quasiparticle poisoning is direct absorption of high-energy photons at the qubit junction. We use a Josephson junction-based photon source to controllably dose qubit circuits with millimeter-wave radiation, and we use an interferometric quantum gate sequence to reconstruct the charge parity of the qubit. We find that the structure of the qubit itself acts as a resonant antenna for millimeter-wave radiation, providing an efficient path for photons to generate quasiparticles. A deep understanding of this physics will pave the way to realization of next-generation superconducting qubits that are robust against quasiparticle poisoning.

Phonon downconversion to suppress correlated errors in superconducting qubits.

Quantum error correction can preserve quantum information in the presence of local errors, but correlated errors are fatal. For superconducting qubits, high-energy particle impacts from background radioactivity produce energetic phonons that travel throughout the substrate and create excitations above the superconducting ground state, known as quasiparticles, which can poison all qubits on the chip. We use normal metal reservoirs on the chip back side to downconvert phonons to low energies where they can no longer poison qubits. We introduce a pump-probe scheme involving controlled injection of pair-breaking phonons into the qubit chips. We examine quasiparticle poisoning on chips with and without back-side metallization and demonstrate a reduction in the flux of pair-breaking phonons by over a factor of 20. We use a Ramsey interferometer scheme to simultaneously monitor quasiparticle parity on three qubits for each chip and observe a two-order of magnitude reduction in correlated poisoning due to background radiation.

The effects of long-term rehabilitation therapy on poststroke hemiplegic patients.

BACKGROUND AND PURPOSE: Rehabilitation therapy is believed to be useful during the first few months after stroke when recovery usually takes place. However, evidence exists that this may not be the rule for all stroke victims. Therefore, we investigated, in a selected group of poststroke patients, the profile of recovery in response to long-term rehabilitation therapy. METHODS: Fifty-one hemiplegic subjects unable to walk 3 months after stroke were enrolled in this study. Patients underwent consecutive periods of rehabilitation up to 2 years after the cerebrovascular accident. Autonomy in daily living activities and the degree of neurological compromission were periodically assessed with the Barthel Index and a neurological scale designed for hemiplegic subjects. The main features of the patients were also evaluated as a possible predictor of outcome. RESULTS: In a consistent percentage of the patients, significant gains in gait and daily living abilities were observed during the first year and, in individual cases, during the second year after stroke. At the end of the study, 74% of the patients regained their capacity to walk without assistance, and up to 79% had a Barthel Index score above 70. Sphincter function, level of neurological impairments, and capacity in daily living activities are significantly related to the outcome of the patients as a whole but were not useful to anticipate the outcome of each patient. CONCLUSIONS: These results suggest that disabled poststroke subjects may attain significative functional improvements in response to prolonged restorative therapy. However, the possibility of predicting the outcome of individual patients appears the major problem to solve in order to assign to long-term rehabilitation programs only patients who will benefit from the therapy.

Subclavian steal diagnosis: Doppler sonography vs. angiography.

Continuous-wave Doppler sonography is a reliable technique for the diagnosis of subclavian steal. This tool also allows us to identify incipient steals, characterized by either alternating flow or systolic flow deceleration in the vertebral artery, which might escape detection by conventional angiography. Diagnostic criteria are reported and results of a 48-patient study are compared with angiographic data. The different patterns of vertebral flow depend on the degree of proximal subclavian or innominate artery functional stenosis. The Doppler examination is a useful method because it is non invasive, harmless to the patient and allows for the evaluation of associated lesions in other extracranial vessels. It is of particular interest for the diagnosis when the clinical picture is still apparently normal and in the absence of both neurological findings and asymmetrical blood pressure in the arms.

[Reversible cerebral ischemia: psychological reactions to the disease]. / L'ischemia cerebrale reversibile: reazioni psicologiche alla malattia.

Two different psychological tests were applied to 40 patients affected by reversible cerebral ischemic attacks in order to evaluate the presence of depression or neurotic symptoms. 40 normal volunteers were used as a control group. While no statistical differences emerged in the mean values of the control tests, significant psychological impairments were observed in the group of patients. Such psychological disturbances were marked in the series with minor neurological deficits. Similar reactions were noted in subjects still employed and in patients submitted to carotid surgery. The authors emphasize the importance of psychological reactions following organic disease for a better therapy in reversible cerebrovascular disorders.