Optimized quantum sensing with a single electron spin using real-time adaptive measurements.

Quantum sensors based on single solid-state spins promise a unique combination of sensitivity and spatial resolution. The key challenge in sensing is to achieve minimum estimation uncertainty within a given time and with high dynamic range. Adaptive strategies have been proposed to achieve optimal performance, but their implementation in solid-state systems has been hindered by the demanding experimental requirements. Here, we realize adaptive d.c. sensing by combining single-shot readout of an electron spin in diamond with fast feedback. By adapting the spin readout basis in real time based on previous outcomes, we demonstrate a sensitivity in Ramsey interferometry surpassing the standard measurement limit. Furthermore, we find by simulations and experiments that adaptive protocols offer a distinctive advantage over the best known non-adaptive protocols when overhead and limited estimation time are taken into account. Using an optimized adaptive protocol we achieve a magnetic field sensitivity of 6.1 ± 1.7 nT Hz(-1/2) over a wide range of 1.78 mT. These results open up a new class of experiments for solid-state sensors in which real-time knowledge of the measurement history is exploited to obtain optimal performance.

Adaptive optical phase estimation using time-symmetric quantum smoothing.

Quantum parameter estimation has many applications, from gravitational wave detection to quantum key distribution. The most commonly used technique for this type of estimation is quantum filtering, using only past observations. We present the first experimental demonstration of quantum smoothing, a time-symmetric technique that uses past and future observations, for quantum parameter estimation. We consider both adaptive and nonadaptive quantum smoothing, and show that both are better than their filtered counterparts. For the problem of estimating a stochastically varying phase shift on a coherent beam, our theory predicts that adaptive quantum smoothing (the best scheme) gives an estimate with a mean-square error up to 2sqrt[2] times smaller than nonadaptive filtering (the standard quantum limit). The experimentally measured improvement is 2.24+/-0.14.

Linear-optical processing cannot increase photon efficiency.

We answer the question whether linear-optical processing of the states produced by one or multiple imperfect single-photon sources can improve the single-photon fidelity. This processing can include arbitrary interferometers, coherent states, feedforward, and conditioning on results of detections. We show that without introducing multiphoton components, the single-photon fraction in any of the single-mode states resulting from such processing cannot be made to exceed the efficiency of the best available photon source. If multiphoton components are allowed, the single-photon fidelity cannot be increased beyond 1/2. We propose a natural general definition of the quantum-optical state efficiency, and show that it cannot increase under linear-optical processing.

Entanglement-free Heisenberg-limited phase estimation.

Measurement underpins all quantitative science. A key example is the measurement of optical phase, used in length metrology and many other applications. Advances in precision measurement have consistently led to important scientific discoveries. At the fundamental level, measurement precision is limited by the number N of quantum resources (such as photons) that are used. Standard measurement schemes, using each resource independently, lead to a phase uncertainty that scales as 1/square root N-known as the standard quantum limit. However, it has long been conjectured that it should be possible to achieve a precision limited only by the Heisenberg uncertainty principle, dramatically improving the scaling to 1/N (ref. 3). It is commonly thought that achieving this improvement requires the use of exotic quantum entangled states, such as the NOON state. These states are extremely difficult to generate. Measurement schemes with counted photons or ions have been performed with N < or = 6 (refs 6-15), but few have surpassed the standard quantum limit and none have shown Heisenberg-limited scaling. Here we demonstrate experimentally a Heisenberg-limited phase estimation procedure. We replace entangled input states with multiple applications of the phase shift on unentangled single-photon states. We generalize Kitaev's phase estimation algorithm using adaptive measurement theory to achieve a standard deviation scaling at the Heisenberg limit. For the largest number of resources used (N = 378), we estimate an unknown phase with a variance more than 10 dB below the standard quantum limit; achieving this variance would require more than 4,000 resources using standard interferometry. Our results represent a drastic reduction in the complexity of achieving quantum-enhanced measurement precision.

A crisis team for the mentally ill: the effect on patients, relatives and admissions.

A Crisis Team staffed by eight psychiatric nurses provided a 24-hour service to the seriously and chronically mentally ill who were experiencing psychiatric crisis. Immediate and continuing help was provided to patients and their relatives in their homes. Effects on patients, relatives and hospital admissions were studied. The sample consisted of 69 patients seen during a three-month study period. Six months later, 80% of patients and 73% of their relatives were interviewed by an independent psychologist. Of the sample, 80% had previous psychiatric admissions and were diagnosed as suffering from one of the functional psychoses--mainly schizophrenia or manic depression. The crises that led to the Team's interventions were mostly psychotic episodes and aggressive or suicidal behaviours; 47% occurred at night. Hospital records confirmed reports from staff, patients and relatives that the Team had halved admissions. Most patients and relatives were "very satisfied" with the treatment received from the Team and considered it had helped "greatly". They especially liked the home visits, the 24-hour availability and the friendly staff who provided them with support, medication and where needed, accommodation. In spite of the interventions of the Team, some 40% of patients were admitted during the research period, and at follow-up most were still experiencing symptoms and difficulties. Relatives, more than patients, expressed a need for additional rehabilitation and easier access to hospital, especially for those severely ill patients whom they found too difficult to live with. While the work of the Crisis Team proved highly beneficial for both patients and relatives, and reduced hospital admissions by half during the study period, it is clear that there is still a need for long-term support and rehabilitation.

The approach to large left ventricular aneurysms. A report of three difficult cases.

We report the management and evaluation of three difficult left ventricular aneurysms from a series of 246 cases treated between 1970 and 1985. These three cases reflect our present approach to left ventricular aneurysms after previous aortocoronary bypass (case 1), aneurysms with distorted ventricular geometry and severe mitral regurgitation (case 2), and large, minimally symptomatic left ventricular aneurysms (case 3). All three cases had very little or no coronary artery disease, and the main purpose of surgery in them was primarily to correct ventricular pathology.

Left ventricular aneurysm repair with myocardial revascularization: an analysis of 246 consecutive patients over 15 years.

From 1970 to 1985, 246 consecutive patients with left ventricular (LV) aneurysm underwent repair and concomitant myocardial revascularization at Ochsner Foundation Hospital. The overall incidence of perioperative death was 7.3%. Although the deaths were mainly cardiac related (10/18) with congestive heart failure (CHF) as the leading cause (6/10), 8 deaths were of noncardiac origin. Perioperative mortality increased significantly in patients with mitral regurgitation (MR) (22%; p = 0.0008); perioperative mortality for patients without MR was 4.8%. The overall 5-year survival was 69%. Late deaths were caused most commonly by myocardial infarction (20/32) with only 7 due to CHF. Predictors of long-term survival were related to LV function preoperatively: absence of CHF (p = 0.001); LV end-diastolic pressure less than or equal to 20 mm Hg (p = 0.03); and ejection fraction greater than or equal to 35% (p = 0.02). Factors that did not significantly affect long-term survival were type of aneurysm repair (resection or plication), morphology of left anterior descending coronary artery (occlusion or stenosis), and size of the aneurysm.

Forebrain monoamines and associative learning: I. Latent inhibition and conditioned inhibition.

Two experiments evaluated the effects of depletion of forebrain norepinephrine (NE) or serotonin (5-HT) on two forms of internal inhibition. Experiment 1 examined the role of NE and 5-HT depletion on latent inhibition: rats with dorsal bundle or raphe lesions or vehicle control rats were trained to one of two pre-exposure conditions (single vs multiple stimuli) or to a non-pre-exposure control prior to conditioning. The results of this experiment showed that latent inhibition was attenuated under either condition of pre-exposure following depletion of 5-HT but that attenuation was present in NE depleted rats only when pre-exposed to multiple stimuli. Experiment 2 assessed the rate at which a conditioned inhibitor was established following NE or 5-HT depletion. The results of Experiment 2 indicated that the formation of a conditioned inhibitor was unimpaired following depletion of either NE or 5-HT. The results of Experiment 1 demonstrate that 5-HT and NE have different effects on associative processes: 5-HT apparently affects a habituation-like process, whereas NE is an important influence on more complex mechanisms involving the comparison of new with previously acquired information. The findings of Experiment 2 are consistent with behavioral investigations which show that conditioned inhibition affects output processes rather than the salience of stimuli.

Aortic and other arterial injuries.

Three hundred sixty arterial injuries in 353 patients are reviewed. They covered a wide spectrum of injuries and included 36 aortic injuries and 19 cases of carotid truama. The mortality rate of 12% was in large part due to aortic injuries. Shock was the predominant cause of death. Infection was the most frequent non-fatal complication. Pulmonary complications were surprisingly uncommon. With methods and techniques discussed in the paper, 90% satisfactory end results were achieved. The amputation rate was 6% where extremity injuries were involved.