Comment on "Charge-qubit operation of an isolated double quantum dot".
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Comment↗
Biomedical subjects
J S Tsai
Publications and source records attributed to J S Tsai.
Decoherence of flux qubits due to 1/f flux noise.
We have investigated decoherence in Josephson-junction flux qubits. Based on the measurements of decoherence at various bias conditions, we discriminate contributions of different noise sources. We present a Gaussian decay function extracted from the echo signal as evidence of dephasing due to 1/f flux noise whose spectral density is evaluated to be about (10(-6)Phi0)2/Hz at 1 Hz. We also demonstrate that, at an optimal bias condition where the noise sources are well decoupled, the coherence observed in the echo measurement is limited mainly by energy relaxation of the qubit.
Journal Article↗
Temperature square dependence of the low frequency charge noise in the Josephson junction qubits.
To verify the hypothesis about the common origin of the low-frequency 1/f noise and the quantum f noise recently measured in the Josephson charge qubits, we study the temperature dependence of the 1/f noise and decay of coherent oscillations. The T2 dependence of the 1/f noise is experimentally demonstrated, which supports the hypothesis. We also show that dephasing in the Josephson charge qubits off the electrostatic energy degeneracy point is consistently explained by the same low-frequency 1/f noise that is observed in the transport measurements.
Journal Article↗
Controllable coupling between flux qubits.
We propose an experimentally realizable method to control the coupling between two flux qubits. In our proposal, the bias fluxes are always fixed for these two inductively coupled qubits. The detuning of these two qubits can be initially chosen to be sufficiently large, so that their initial interbit coupling is almost negligible. When a variable frequency or time-dependent magnetic flux (TDMF) is applied to one of the qubits, a well-chosen frequency of the TDMF can be used to compensate the initial detuning and to couple two qubits. This proposed method avoids fast changes of either qubit frequencies or the amplitudes of the bias magnetic fluxes through the qubit loops, and also offers a remarkable way to implement any logic gate, as well as tomographically measure flux qubit states.
Journal Article↗
Quantum noise in the josephson charge qubit.
We study decoherence of the Josephson charge qubit by measuring energy relaxation and dephasing with help of the single-shot readout. We found that the dominant energy relaxation process is a spontaneous emission induced by quantum noise coupled to the charge degree of freedom. Spectral density of the noise at high frequencies is roughly proportional to the qubit excitation energy.
Journal Article↗
Demonstration of conditional gate operation using superconducting charge qubits.
Following the demonstration of coherent control of the quantum state of a superconducting charge qubit, a variety of qubits based on Josephson junctions have been implemented. Although such solid-state devices are not currently as advanced as microscopic qubits based on nuclear magnetic resonance and ion trap technologies, the potential scalability of the former systems--together with progress in their coherence times and read-out schemes--makes them strong candidates for the building block of a quantum computer. Recently, coherent oscillations and microwave spectroscopy of capacitively coupled superconducting qubits have been reported; the next challenging step towards quantum computation is the realization of logic gates. Here we demonstrate conditional gate operation using a pair of coupled superconducting charge qubits. Using a pulse technique, we prepare different input states and show that their amplitude can be transformed by controlled-NOT (C-NOT) gate operation, although the phase evolution during the gate operation remains to be clarified.
Journal Article↗
Quantum oscillations in two coupled charge qubits.
A practical quantum computer, if built, would consist of a set of coupled two-level quantum systems (qubits). Among the variety of qubits implemented, solid-state qubits are of particular interest because of their potential suitability for integrated devices. A variety of qubits based on Josephson junctions have been implemented; these exploit the coherence of Cooper-pair tunnelling in the superconducting state. Despite apparent progress in the implementation of individual solid-state qubits, there have been no experimental reports of multiple qubit gates--a basic requirement for building a real quantum computer. Here we demonstrate a Josephson circuit consisting of two coupled charge qubits. Using a pulse technique, we coherently mix quantum states and observe quantum oscillations, the spectrum of which reflects interaction between the qubits. Our results demonstrate the feasibility of coupling multiple solid-state qubits, and indicate the existence of entangled two-qubit states.
Journal Article↗
Scalable quantum computing with Josephson charge qubits.
A goal of quantum information technology is to control the quantum state of a system, including its preparation, manipulation, and measurement. However, scalability to many qubits and controlled con-nectivity between any selected qubits are two of the major stumbling blocks to achieve quantum com-puting (QC). Here we propose an experimental method, using Josephson charge qubits, to efficiently solve these two central problems. The proposed QC architecture is scalable since any two charge qubits can be effectively coupled by an experimentally accessible inductance. More importantly, we formulate an efficient and realizable QC scheme that requires only one (instead of two or more) two-bit operation to implement conditional gates.
Journal Article↗
Charge echo in a cooper-pair box.
A spin-echo-type technique is applied to an artificial two-level system that utilizes a charge degree of freedom in a small superconducting electrode. Gate-voltage pulses are used to produce the necessary pulse sequence in order to eliminate the inhomogeneity effect in the time-ensemble measurement and to obtain refocused echo signals. Comparison of the decay time of the observed echo signal with an estimated decoherence time suggests that low-frequency energy-level fluctuations due to the 1/f charge noise dominate the dephasing in the system.
Journal Article↗
Rabi oscillations in a Josephson-junction charge two-level system.
We investigated temporal behavior of an artificial two-level system driven by a strong oscillating field; namely, quantum-state evolution between two charge states in a small Josephson-junction circuit irradiated with microwaves. Rabi oscillations corresponding to 0-, 1-, and 2-photon resonances were observed. As a function of microwave amplitude, the Rabi frequencies followed a first-kind Bessel function of the corresponding order to the number of photons.
Journal Article↗
Comparison of radiobiologic modeling for one- and two-isocenter dose distributions applied to ellipsoidal radiosurgery targets.
PURPOSE: RTOG protocol 90-05 determined the maximum acutely tolerated dose of single-fraction radiosurgery in patients receiving prior fractionated large volume cranial irradiation. Data from 90-05 have suggested that patients treated with a gamma unit, compared to linac-based therapy, have a tumor control advantage and lower rates of severe complications. This study was performed to investigate the radiobiologic effect of using one vs. two isocenters in single-fraction radiosurgery of ellipsoidal targets. METHODS AND MATERIALS: For a series of ellipsoidal targets that varied by volume and radiosensitivity, single and two-isocenter treatment plans were generated to approximate those typically employed for gamma unit and linac radiosurgery. Tumor control probabilities (TCP) and normal tissue complication probabilities (NTCP) were generated automatically by the treatment planning system based on established parameter values. RESULTS: The modeling data showed that multiple-isocenter plans resulted in improved TCP with equivalent or lesser NTCP, particularly for larger, radioresistant targets. Multiple-isocenter plans reduce the amount of normal tissue that receives high dose. Also, areas within the tumor receive significantly higher doses than the prescription dose, which contributes to increased tumor cell inactivation. CONCLUSION: For ellipsoidal targets, radiobiologic modeling data are consistent with the clinical findings of the RTOG 90-05 trial, as they predict improved outcome with a multiple-isocenter plan relative to a single-isocenter plan. The benefit is most apparent with increasing target volume and decreasing tumor radiosensitivity.
Algorithms↗
Effectiveness of chemotherapy for advanced lung cancer in the elderly: instrumental variable and propensity analysis.
PURPOSE: To compare the effectiveness of chemotherapy given to elderly patients in routine practice for stage IV non-small-cell lung cancer (NSCLC) with the efficacy observed in randomized trials. PATIENTS AND METHODS: We used instrumental variable analysis (IVA) and propensity scores (PS) to simulate the conditions of a randomized trial in a retrospective cohort of patients over age 65 from the Survival, Epidemiology, and End Results (SEER) tumor registry. Geographic variation in chemotherapy use served as the instrument for the IVA analysis, and propensity scores were calculated with a logistic model based on patient disease and sociodemographic characteristics. RESULTS: Among 6,232 elderly patients, the instrumental variable estimate indicated an increase in median survival of 33 days and an improvement in 1-year survival of 9% attributable to chemotherapy. In a Cox regression model, chemotherapy administration was associated with a hazard ratio of 0.81 (95% confidence interval, 0.76 to 0.85). When survival was analyzed separately within propensity score quintiles, the hazard ratios were all similar, ranging from 0.78 to 0.85. These results are comparable with those of a large meta-analysis, which found a hazard ratio of 0.87 in the subgroup of patients over age 65. CONCLUSION: Chemotherapy for stage IV NSCLC seems to have effectiveness for elderly patients and those with comorbid conditions that is similar to the efficacy seen in randomized trials containing mostly younger, highly selected patients. All suitable patients should be given the opportunity to consider palliative chemotherapy for metastatic NSCLC.
Aged↗
A formalism for independent checking of Gamma Knife dose calculations.
For stereotactic radiosurgery using the Leksell Gamma Knife system, it is important to perform a pre-treatment verification of the maximum dose calculated with the Leksell GammaPlan (DLGP) stereotactic radiosurgery system. This verification can be incorporated as part of a routine quality assurance (QA) procedure to minimize the chance of a hazardous overdose. To implement this procedure, a formalism has been developed to calculate the dose DCAL(X,Y,Z,dav,t) using the following parameters: average target depth (dav), coordinates (X,Y,Z) of the maximum dose location or any other dose point(s) to be verified, 3-dimensional (3-dim) beam profiles or off-centerratios (OCR) of the four helmets, helmet size i, output factor Oi, plug factor Pi, each shot j coordinates (x,y,z)i,j, and shot treatment time (ti,j). The average depth of the target dav was obtained either from MRI/CT images or ruler measurements of the Gamma Knife Bubble Head Frame. DCAL and DLGP were then compared to evaluate the accuracy of this independent calculation. The proposed calculation for an independent check of DLGP has been demonstrated to be accurate and reliable, and thus serves as a QA tool for Gamma Knife stereotactic radiosurgery.
Biophysical Phenomena↗
Hyperinsulinemia and insulin resistance related metabolic syndrome.
BACKGROUND: A cross-sectional study in northern Taiwan was conducted to investigate the role of serum insulin level on the development of hyperglycemia, hypertension, obesity and hyperlipidemia. METHODS: Demographic data (age, gender), body mass index, blood pressure, and laboratory blood tests (uric acid, total cholesterol, triglycerides, glucose and immunoreactive insulin when fasting and two hours after loading 75 gm glucose) were collected. A logistic model or polychotomous model, treating each chronic disease (hyperglycemia, hypertension, obesity and hyperlipidemia) as a dependent variable, were fitted to study the effect of serum insulin level. RESULTS: Four hundred and twenty one volunteers (women:men = 237:184) were recruited from 1991 to 1993. Women were more obese and had more hyperglycemia, while the frequency of hyperuricemia was lower than of men. Women with a higher level of the sum of fasting and 2-hour post glucose load insulin (SIRI) levels had higher frequencies of glucose intolerance, hypertension, obesity and hyperlipidemia, whereas SIRI was related to only obesity and hyperlipidemia in men. The high plasma SIRI was a risk factor for both impaired glucose tolerance (IGT) and obesity in women. Men with a high plasma SIRI showed a more than three-fold risk of obesity. CONCLUSIONS: We have observed that a higher serum insulin level was significantly associated with the development of metabolic manifestations (glycemic status and obesity) in a suburban community in northern Taiwan. Men and women with a higher serum insulin level probably had a greater chance of developing obesity, and women had a greater chance of developing IGT.
Adolescent↗
Dependence of linac output on the switch rate of an intensity-modulated tomotherapy collimator.
The electro-mechanical, multivane intensity modulated collimator ("MIMiC") slit collimator with 40 vanes has been applied in the delivery of inversely planned sequential tomotherapy to over 4,000 patients. The collimator is binary in that each vane switches between fully open or closed status. Resulting beamlet patterns provide the intensity distributions imparting dose to the patient. The bouncing and damping of vanes at the two ends of their travel cause transient dose perturbations near and at the borders of the treatment field. These perturbations are not explicitly modeled by the planning system. Clinical beamlet profiles and output factors may then differ from those in the planning system and as a function of the vane switch period. A mechanical model of vane switching was developed to describe this dependency. Dose output and distribution of seven simple vane patterns with different switch times were measured with ionization chambers and radiographic films in polystyrene and anthropomorphic phantoms. Linac output dependence on switch time relative to vane open time was determined for four intensity modulated radiotherapy (IMRT) patients from measurements of an ionization chamber embedded in a cylindrical polystyrene phantom. Results demonstrate output dependence on switch time and, accordingly, on the servo mechanism for monitor units, arc length, dose rate, and gantry speed. In conclusion, the output dependence borders on clinical significance-improvements to collimator, dose calculation, commissioning, and quality assurance (QA) are suggested.
Humans↗
Demonstration of reserved anterior pituitary function among patients with amenorrhea after postpartum hemorrhage.
To demonstrate the residual pituitary function of patients with Sheehan's syndrome years after the obstetric complication, 14 patients with postpartum hemorrhage followed by secondary amenorrhea and agalactia were included in this review. Due to their unfamiliarity with the clinical symptoms, these patients did not receive pretreatment hormonal therapy. The mean age at their last delivery was 29 years (range 21-38 years). The mean duration between postpartum hemorrhage and the subsequent clinical manifestations leading to the endocrine investigation was 18 years (range 1-33 years). Eight patients presented with symptoms of severe hyponatremia (serum sodium less than 125 mmol/l) more than 16 years (mean 23 +/- 10) after the occurrence of postpartum hemorrhage. The electrolyte abnormality was primarily due to adrenal dysfunction. Seven out of 14 patients had normal basal luteinizing hormone (LH) levels and adequate LH responses to gonadotropin releasing hormone stimulation. Administration of thyrotropin releasing hormone provoked thyrotropin release and/or prolactin secretion in four cases. The manifestation of clinical hypopituitarism and the degree of empty sella on computed tomography scanning did not accurately indicate the secreting ability of the pituitary in patients with Sheehan's syndrome. Although all the patients had amenorrhea, the gonadotropic functions of the pituitary still remain in some patients. Various degrees of other pituitary functions can also been demonstrated even several decades after the occurrence of obstetric complications. Our data suggest that the amenorrhea of Sheehan's patients is not simply due to a dysfunction of the pituitary gonadotrophs.
Adult↗
Gamma knife radiosurgery as a primary treatment for prolactinomas.
OBJECT: The purpose of this study was to estimate the efficacy of gamma knife radiosurgery (GKS) in controlling tumor growth and endocrinopathy associated with prolactinomas. METHODS: Between 1993 and 1997, 164 of 469 patients with pituitary adenomas treated by GKS harbored prolactinomas. The dose to the tumor margin ranged from 9 to 35 Gy (mean 31.2 Gy), and the visual pathways were exposed to a dose of less than 10 Gy. The mean tumor diameter was 13.4 mm. The mean follow-up time for 128 cases was 33.2 months (range 6-72 months). Tumor control was observed in all but two patients who underwent surgery 18 and 36 months, respectively, after GKS. Clinical cure was achieved in 67 cases. Clinical improvement was noted with a decrease in the hyperprolactinemia after GKS. Nonetheless, in 31 (29%) of 108 patients who were followed for more than 2 years no improvement in serum prolactin levels was demonstrated, although this could be normalized by bromocriptine administration after treatment. Nine infertile women became pregnant 2 to 13 months after GKS and all gave birth to normal children. There was no visual deterioration related to GKS. Five women experienced premature menopause. In these patients there was subtotal disappearance of the tumor and an empty sella developed. CONCLUSIONS: Gamma knife radiosurgery as a primary treatment for prolactinomas can be safe and effective both for controlling tumor growth and for normalization of prolactin hypersecretion. A higher margin dose (> or = 30 Gy) seemed to be associated with a better clinical outcome. Gamma knife radiosurgery may make prolactinomas more sensitive to the bromocriptine.
Humans↗
A non-invasive immobilization system and related quality assurance for dynamic intensity modulated radiation therapy of intracranial and head and neck disease.
PURPOSE: To develop and implement a non-invasive immobilization system guided by a dedicated quality assurance (QA) program for dynamic intensity-modulated radiotherapy (IMRT) of intracranial and head and neck disease, with IMRT delivered using the NOMOS Corporation's Peacock System and MIMiC collimator. METHODS AND MATERIALS: Thermoplastic face masks are combined with cradle-shaped polyurethane foaming agents and a dedicated quality assurance program to create a customized headholder system (CHS). Plastic shrinkage was studied to understand its effect on immobilization. Fiducial points for computerized tomography (CT) are obtained by placing multiple dabs of barium paste on mask surfaces at intersections of laser projections used for patient positioning. Fiducial lines are drawn on the cradle along laser projections aligned with nasal surfaces. Lateral CT topograms are annotated with a crosshair indicating the origin of the treatment planning and delivery coordinate system, and with lines delineating the projections of superior-inferior field borders of the linear accelerator's secondary collimators, or with those of the fully open MIMiC. Port films exposed with and without the MIMIC are compared to annotated topograms to measure positional variance (PV) in superior-inferior (SI), right-left (RL), and anterior posterior (AP) directions. MIMiC vane patterns superposed on port films are applied to verify planned patterns. A 12-patient study of PV was performed by analyzing positions of 10 anatomic points on repeat CT topograms, plotting histograms of PV, and determining average PV. RESULTS AND DISCUSSION: A 1.5+/-0.3 mm SD shrinkage per 70 cm of thermoplastic was observed over 24 h. Average PV of 1.0+/-0.8, 1.2+/-1.1, and 1.3+/-0.8 mm were measured in SI, AP, and RL directions, respectively. Lateral port films exposed with and without the MIMiC showed PV of 0.2+/-1.3 and 0.8+/-2.2 mm in AP and SI directions. Vane patterns superimposed on port films consistently verified the planned patterns. CONCLUSION: The CHS provided adequately reproducible immobilization for dynamic IMRT, and may be applicable to decrease PV for other cranial and head and neck external beam radiation therapy.
Brain Neoplasms↗