Fidelity of the surface code in the presence of a bosonic bath

We study the resilience of the surface code to decoherence caused by the presence of a bosonic bath. This approach allows us to go beyond the standard stochastic error model commonly used to quantify decoherence and error threshold probabilities in this system. The full quantum mechanical system-bath dynamics is computed exactly over one quantum error correction cycle. Since all physical qubits interact with the bath, space-time correlations between errors are taken into account. We compute the fidelity of the surface code as a function of the quantum error correction time. The calculation allows us to map the problem onto an Ising-like statistical spin model with two-body interactions and a fictitious temperature which is related to the inverse bath coupling constant. The model departs from the usual Ising model in the sense that interactions can be long ranged and can involve complex exchange couplings; in addition, the number of allowed configurations is restricted by the syndrome extraction. Using analytical estimates and numerical calculations, we argue that, in the limit of an infinite number of physical qubits, the spin model sustain a phase transition which can be associated to the existence of an error threshold in the surface code. An estimate of the transition point is given for the case of nearest-neighbor interactions.Comment: 15 pages, 5 figure

Fidelity threshold of the surface code beyond single-qubit error models

The surface code is a promising alternative for implementing fault-tolerant, large-scale quantum information processing. Its high threshold for single-qubit errors under stochastic noise is one of its most attractive features. We develop an exact formulation for the fidelity of the surface code that allows us to probe much further on this promise of strong protection. This formulation goes beyond the stochastic single-qubit error model approximation and can take into account both correlated errors and inhomogeneities in the coupling between physical qubits and the environment. For the case of a bit-flipping environment, we map the complete evolution after one quantum error correction cycle onto the problem of computing correlation functions of a two-dimensional Ising model with boundary fields. Exact results for the fidelity threshold of the surface code are then obtained for several relevant types of noise. Analytical predictions for a representative case are confirmed by Monte Carlo simulations

Laser-assisted surgery addressing snoring long-term outcome comparing CO2 laser vs. CO2 laser combined with diode laser.

Our study encompasses 61 patients (49 men--80.3%; 12 women--19.7%) treated from September 1998 through August 2001. Mean follow-up covers 25 months (range: 7-43 months). Our CO2-LAUP technique involves vaporizing the palatine mucosa along a rectangular surface from the palatal dimple to the base of the uvula; trimming the palatine arches under the velum; and resecting the uvula. For 22 patients, we employed intravelar diode laser coagulation in the expectation of reducing the postoperative pain whilst achieving the same therapeutic effectiveness. There is no pain difference between the CO2-LAUP technique versus that combining intravelar diode laser coagulation with CO2 laser uvula resection and trimming of the palatine arches. Mean maximal pain reaches 6.93 +/- 3.55 with CO2 laser and 6.95 +/- 3.64 with CO2 laser plus diode laser. Similarly, both techniques involve the same mean algesic period of 22 days with the day of maximal pain at 1 week after surgery. Associating base of tongue vaporization significantly increases the algesic period (p = 0.042). No long-term complications were observed in relation to intravelar coagulation or LAUP, whether combined or not with base of tongue vaporization. In terms of patients satisfaction, no significant difference exists between the various surgical techniques of the velum alone. The satisfaction rate reaches 5.26 +/- 3.92 with CO2 laser and 5.82 +/- 2.67 with the CO2 laser plus diode laser. Satisfaction is statistically identical when base of tongue vaporization is included in the procedure

Transcanalicular diode laser assisted dacryocystorhinostomy.

AIM OF THE STUDY: To assess the validity of transcanalicular diode laser assisted dacryocystorhinostomy (TLADCR) in the treatment of distal obstruction of the lachrymal pathways. MATERIAL AND METHOD: Between June 1999 and February 2000, 29 TLADCR were performed in the ENT department on 26 patients. Surgical indications included chronic dacryocystitis in 19 cases and dacryostenosis in 10 cases. In all cases but one, the surgery was carried out under a standard general anesthetic. The diode laser was used with a power setting of 10 watts. RESULTS: Seventeen out of 29 TLADCR were regarded as having successful outcomes. 2 out of 29 TLADCR had a little persisting tearing regarded as non debilitating and much less than which was present preoperatively. In 10 out of 29 TLADCR, the procedure was deemed a failure. These failures included 4 out of the 10 primary dacryostenosis and 6 out of the 19 chronic dacryocystitis. Two patients refused further treatment. In the other 8, surgical revision was undertaken via the traditional endonasal approach. In 7 of these cases, the lachrymal system was full of purulent secretions. COMPLICATIONS: Two patients developed a partial stenosis of one canaliculus that was diagnosed during the surgical revision. Another patient had a small fistula between the canalicular system and the skin of the corner of the eye that disappeared after the revision surgery. In one other case, the superior canaliculus was cauterised by the denudated portion of a resculpted laser fibre. Two bicanalicular nasal stent required reinsertion. Three patients developed a granuloma around the stent. Four patients developed an episode of infection whilst the stent was still in situ. CONCLUSION: Transcanalicular diode laser assisted dacryocystorhinostomy (TLADCR) performed with a 600-micron contact fibre is a relatively simple, elegant and effective procedure when used in conjunction with nasal endoscopy, to treat distal obstruction of the lachrymal pathways. But, the cost, the high prevalence of minor complications and the currently higher reported failure rate would favour external or endonasal approaches over this approach

Transnasal endoscopic orbital decompression and Graves' ophtalmopathy.

AIM OF THE STUDY: To assess the validity and the limits of endoscopic endonasal orbital decompression for Graves' ophtalmopathy resistant to the medical theapy. MATERIAL AND PATIENTS: Between September 1994 and May 1998, 16 patients with Graves' ophtalmopathy resistant to the medical treatment underwent an orbital decompression transnasally. 27 orbits were decompressed. The surgery was bilateral in 11 patients. In the 5 remaining cases, the surgery was unilateral. It was carried out on the left side in 2 cases and on the right side in 3 cases. RESULTS: Preoperatively, the average visual acuity was 8/10. Postoperatively, the visual acuity was 9.5/10. The average preoperative exophtalmometry measurement was 25.04 mm and the average postoperative measurement was 21.83 mm. The average retrodisplacement was 3.17 mm (range: 2-8). Preoperatively, 3 patients had mild diplopia whereas 5 others had moderate to severe extraocular muscle dysfunction. Postoperatively, 6 patients had mild diplopia whereas 10 patients required squint surgery for moderate to severe extraocular muscle dysfunction. CONCLUSION: Endoscopic orbital decompression improve all the symptoms of Graves' ophtalmopathy but one: the extraocular muscle dysfunction. Its cardinal indication is the treatment of compressive optic neuropathy whereas this surgical approach provides an excellent control of the medial wall of the orbit and the orbital apex. But the average reduction of proptosis of 3.17 mm is not high enough to propose this approach alone for the treatment of disfiguring proptosis. In such cases, a 2 or 3 wall orbital decompression should be performed to get marked cosmetic and functional improvement. In all cases, the patient should be informed about the risk of postoperative diplopia

Low temperature atomic layer deposition of cobalt using dicobalt hexacarbonyl-1-heptyne as precursor

In this work, we present the development of an atomic layer deposition (ALD) process for metallic cobalt. The process operates at low temperatures using dicobalt hexacarbonyl-1-heptyne [Co2(CO)6HC≡CC5H11] and hydrogen plasma. For this precursor an ALD window in the temperature range between 50 and 110 °C was determined with a constant deposition rate of approximately 0.1 Å/cycle. The upper limit of the ALD window is defined by the onset of the decomposition of the precursor. In our case, decomposition occurs at temperatures of 125 °C and above, resulting in a film growth in chemical vapour deposition mode. The lower limit of the ALD window is around 35 °C, where the reduction of the precursor is incomplete. The saturation behaviour of the process was investigated. X-ray photoelectron spectroscopy measurements could show that the deposited cobalt is in the metallic state. The finally established process in ALD mode shows a homogeneous coating at the wafer level

Sclerosing cholangitis associated to cryptosporidiosis in liver-transplanted children.

Three children of a series of 461 pediatric liver transplant recipients developed diffuse cholangitis associated with intestinal cryptosporidium carriage. All three received immunosuppression consisting of tacrolimus and prednisone. Cryprosporidium carriage was treated with paramomycin, while immunosuppression was decreased according to graft tolerance. No other infectious pathogens were found, and no vascular problems were detected. Bile duct anastomosis was reoperated in all three, but biliary cirrhosis developed in one patient, requiring retransplantation. All three patients are alive and well, and free of intestinal parasites on follow-up. CONCLUSION: Cryptosporidium intestinal infection may play a role in some cases of otherwise unexplained cholangiopathies in pediatric liver transplant recipients. This may lead to significant morbidity, including need for retransplantation

Fidelity threshold of the surface code beyond single-qubit error models

The surface code is one the most promising alternatives for implementing fault-tolerant, large-scale quantum information processing. Its high threshold for single-qubit errors under stochastic noise is one of its most attrative features. We develop an exact formulation for the fidelity of the surface code that allows us to probe much further on this promise of strong protection. This formulation goes beyond the stochastic single-qubit error model approximation and can take into account both correlated errors and inhomogeneities in the coupling between physical qubits and the environment. For the case of a bit-flipping environment, we map the complete evolution after one quantum error correction cycle onto the problem of computing correlation functions of a two-dimensional Ising model with boundary fields. Exact results for the fidelity threshold of the surface code are then obtained for several relevant types of noise. Analytical predictions for a representative case are confirmed by Monte Carlo simulations.Comment: 12 pages, 6 figures; revised and extended versio