Simulating and Optimising Quantum Thermometry Using Single Photons.

A classical thermometer typically works by exchanging energy with the system being measured until it comes to equilibrium, at which point the readout is related to the final energy state of the thermometer. A recent paper noted that with a quantum thermometer consisting of a single spin/qubit, temperature discrimination is better achieved at finite times rather than once equilibration is essentially complete. Furthermore, preparing a qubit thermometer in a state with quantum coherence instead of an incoherent one improves its sensitivity to temperature differences. Implementing a recent proposal for efficiently emulating an arbitrary quantum channel, we use the quantum polarisation state of individual photons as models of "single-qubit thermometers" which evolve for a certain time in contact with a thermal bath. We investigate the optimal thermometer states for temperature discrimination, and the optimal interaction times, confirming that there is a broad regime where quantum coherence provides a significant improvement. We also discuss the more practical question of thermometers composed of a finite number of spins/qubits (greater than one), and characterize the performance of an adaptive protocol for making optimal use of all the qubits.

Ultrafast Stark-induced polaritonic switches.

A laser pulse, several meV red detuned from the excitonic line of a quantum well, has been shown to induce an almost instantaneous and rigid shift of the lower and upper polariton branches. Here we demonstrate that through this shift ultrafast all-optical control of the polariton population in a semiconductor microcavity should be achievable. In the proposed setup, a Stark field is used to bring the lower polariton branch in or out of resonance with a quasiresonant continuous-wave laser, thereby favoring or inhibiting the injection of polaritons into the cavity. Moreover, we show that this technique allows for the implementation of optical switches with extremely high repetition rates.

Adaptive quantum state tomography improves accuracy quadratically.

We introduce a simple protocol for adaptive quantum state tomography, which reduces the worst-case infidelity [1-F(ρ,ρ)] between the estimate and the true state from O(1/sqrt[N]) to O(1/N). It uses a single adaptation step and just one extra measurement setting. In a linear optical qubit experiment, we demonstrate a full order of magnitude reduction in infidelity (from 0.1% to 0.01%) for a modest number of samples (N ≈ 3 × 10(4)).

Improving quantum state estimation with mutually unbiased bases.

When used in quantum state estimation, projections onto mutually unbiased bases have the ability to maximize information extraction per measurement and to minimize redundancy. We present the first experimental demonstration of quantum state tomography of two-qubit polarization states to take advantage of mutually unbiased bases. We demonstrate improved state estimation as compared to standard measurement strategies and discuss how this can be understood from the structure of the measurements we use. We experimentally compared our method to the standard state estimation method for three different states and observe that the infidelity was up to 1.84 ± 0.06 times lower by using our technique than it was by using standard state estimation methods.

Experimental joint weak measurement on a photon pair as a probe of Hardy's paradox.

It has been proposed that the ability to perform joint weak measurements on postselected systems would allow us to study quantum paradoxes. These measurements can investigate the history of those particles that contribute to the paradoxical outcome. Here we experimentally perform weak measurements of joint (i.e., nonlocal) observables. In an implementation of Hardy's paradox, we weakly measure the locations of two photons, the subject of the conflicting statements behind the paradox. Remarkably, the resulting weak probabilities verify all of these statements but, at the same time, resolve the paradox.

Squeezing and over-squeezing of triphotons.

Quantum mechanics places a fundamental limit on the accuracy of measurements. In most circumstances, the measurement uncertainty is distributed equally between pairs of complementary properties; this leads to the 'standard quantum limit' for measurement resolution. Using a technique known as 'squeezing', it is possible to reduce the uncertainty of one desired property below the standard quantum limit at the expense of increasing that of the complementary one. Squeezing is already being used to enhance the sensitivity of gravity-wave detectors and may play a critical role in other high precision applications, such as atomic clocks and optical communications. Spin squeezing (the squeezing of angular momentum variables) is a powerful tool, particularly in the context of quantum light-matter interfaces. Although impressive gains in squeezing have been made, optical spin-squeezed systems are still many orders of magnitude away from the maximum possible squeezing, known as the Heisenberg uncertainty limit. Here we demonstrate how an optical system can be squeezed essentially all the way to this fundamental bound. We construct spin-squeezed states by overlapping three indistinguishable photons in an optical fibre and manipulating their polarization (spin), resulting in the formation of a squeezed composite particle known as a 'triphoton'. The symmetry properties of polarization imply that the measured triphoton states can be most naturally represented by quasi-probability distributions on the surface of a sphere. In this work we show that the spherical topology of polarization imposes a limit on how much squeezing can occur, leading to the quasi-probability distributions wrapping around the sphere-a phenomenon we term 'over-squeezing'. Our observations of spin-squeezing in the few-photon regime could lead to new quantum resources for enhanced measurement, lithography and information processing that can be precisely engineered photon-by-photon.

Bright filter-free source of indistinguishable photon pairs.

We demonstrate a high-brightness source of pairs of indistinguishable photons based on a type-II phase-matched doubly-resonant optical parametric oscillator operated far below threshold. The cavityenhanced down-conversion output of a PPKTP crystal is coupled into two single-mode fibers with a mode coupling efficiency of 58%. The high degree of indistinguishability between the photons of a pair is demonstrated by a Hong-Ou-Mandel interference visibility of higher than 90% without any filtering at an instantaneous coincidence rate of 450,000 pairs/s per mW of pump power per nm of down-conversion bandwidth. For the degenerate spectral mode with a linewidth of 7 MHz at 795 nm a rate of 70 pairs/(s mW MHz) is estimated, increasing the spectral brightness for indistinguishable photons by two orders of magnitude compared to similar previous sources.

Multiparticle state tomography: hidden differences.

We address the problem of completely characterizing multiparticle states including loss of information to unobserved degrees of freedom. In systems where nonclassical interference plays a role, such as linear-optics quantum gates, such information can degrade interference in two ways, by decoherence and by distinguishing the particles. Distinguishing information, often the limiting factor for quantum optical devices, is not correctly described by previous state-reconstruction techniques, which account only for decoherence. We extend these techniques and find that a single modified density matrix can completely describe partially coherent, partially distinguishable states. We use this observation to experimentally characterize two-photon polarization states in single-mode optical fiber.

Observation of high-order quantum resonances in the kicked rotor.

Quantum resonances in the kicked rotor are characterized by a dramatically increased energy absorption rate, in stark contrast to the momentum localization generally observed. These resonances occur when the scaled Planck's constant Planck's [over ]=r/s 4pi, for any integers r and s. However, only the variant Planck's [over ]=r2pi resonances are easily observable. We have observed high-order quantum resonances (s>2) utilizing a sample of low energy, noncondensed atoms and a pulsed optical standing wave. Resonances are observed for variant Planck's [over ]=r/16 4pi for integers r=2-6. Quantum numerical simulations suggest that our observation of high-order resonances indicate a larger coherence length (i.e., coherence between different wells) than expected from an initially thermal atomic sample.

Quantum nonlocality obtained from local states by entanglement purification.

We have applied an entanglement purification protocol to produce a single entangled pair of photons capable of violating a Clauser-Horne-Shimony-Holt Bell inequality from two pairs that individually could not. The initial poorly entangled photons were created by a controllable decoherence that introduced complex errors. All of the states were reconstructed using quantum state tomography which allowed for a quantitative description of the improvement of the state after purification.

Super-resolving phase measurements with a multiphoton entangled state.

Interference phenomena are ubiquitous in physics, often forming the basis of demanding measurements. Examples include Ramsey interferometry in atomic spectroscopy, X-ray diffraction in crystallography and optical interferometry in gravitational-wave studies. It has been known for some time that the quantum property of entanglement can be exploited to perform super-sensitive measurements, for example in optical interferometry or atomic spectroscopy. The idea has been demonstrated for an entangled state of two photons, but for larger numbers of particles it is difficult to create the necessary multiparticle entangled states. Here we demonstrate experimentally a technique for producing a maximally entangled three-photon state from initially non-entangled photons. The method can in principle be applied to generate states of arbitrary photon number, giving arbitrarily large improvement in measurement resolution. The method of state construction requires non-unitary operations, which we perform using post-selected linear-optics techniques similar to those used for linear-optics quantum computing.

Extracting joint weak values with local, single-particle measurements.

Weak measurement is a new technique which allows one to describe the evolution of postselected quantum systems. It appears to be useful for resolving a variety of thorny quantum paradoxes, particularly when used to study properties of pairs of particles. Unfortunately, such nonlocal or joint observables often prove difficult to measure directly in practice (for instance, in optics-a common testing ground for this technique-strong photon-photon interactions would be needed to implement an appropriate von Neumann interaction). Here we derive a general, experimentally feasible, method for extracting these joint weak values from correlations between single-particle observables.

Experimental application of decoherence-free subspaces in an optical quantum-computing algorithm.

For a practical quantum computer to operate, it is essential to properly manage decoherence. One important technique for doing this is the use of "decoherence-free subspaces" (DFSs), which have recently been demonstrated. Here we present the first use of DFSs to improve the performance of a quantum algorithm. An optical implementation of the Deutsch-Jozsa algorithm can be made insensitive to a particular class of phase noise by encoding information in the appropriate subspaces; we observe a reduction of the error rate from 35% to 7%, essentially its value in the absence of noise.

Diagnosis, prescription, and prognosis of a bell-state filter by quantum process tomography.

We apply the techniques of quantum process tomography to characterize errors and decoherence in a prototypical two-photon operation, a singlet-state filter. The quantum process tomography results indicate a large asymmetry in the process and also the required operation to correct for this asymmetry. We quantify residual errors and decoherence of the filtering operation after this modification.

Quantum state preparation and conditional coherence.

It is well known that spontaneous parametric down-conversion can be used to probabilistically prepare single-photon states. We have performed an experiment in which arbitrary superpositions of zero- and one-photon states can be prepared by appropriate postselection. The optical phase, which is meaningful only for superpositions of photon number, is related to the relative phase between the zero- and one-photon states. Whereas the light from spontaneous parametric down-conversion has an undefined phase, we show that this technique collapses one beam to a state of well-defined optical phase when a measurement succeeds on the other beam.

Nonlinear optics with less than one photon.

We demonstrate suppression and enhancement of spontaneous parametric down-conversion via quantum interference with two weak fields from a local oscillator (LO). Effectively, pairs of LO photons up-convert with high efficiency for appropriate phase settings, exhibiting an effective nonlinearity enhanced by at least 10 orders of magnitude. This constitutes a two-photon switch and promises to be applicable to a wide variety of quantum nonlinear optical phenomena.

Posttraumatic stress and depressive reactions among Nicaraguan adolescents after hurricane Mitch.

OBJECTIVE: This study determined the severity of posttraumatic stress and depressive reactions among Nicaraguan adolescents after Hurricane Mitch and the relationship of these reactions to objective and subjective features of hurricane exposure, death of a family member, forced relocation, and thoughts of revenge. METHOD: Six months after the hurricane, 158 adolescents from three differentially exposed cities were evaluated by using a hurricane exposure questionnaire, the Child Posttraumatic Stress Disorder Reaction Index, and the Depression Self-Rating SCALE: RESULTS: Severe levels of posttraumatic stress and depressive reactions were found among adolescents in the two most heavily affected cities. Severity of posttraumatic stress and depressive reactions and features of objective hurricane-related experiences followed a "dose-of-exposure" pattern that was congruent with the rates of death and destruction across cities. Level of impact (city), objective and subjective features, and thoughts of revenge accounted for 68% of the variance in severity of posttraumatic stress reaction. Severity of posttraumatic stress reaction, death of a family member, and sex accounted for 59% of the variance in severity of depression. CONCLUSIONS: After a category 5 hurricane, adolescents in heavily affected areas with extreme objective and subjective hurricane-related traumatic features of exposure experience severe and chronic posttraumatic stress and comorbid depressive reactions. The recovery of the severely affected Nicaraguan adolescents is vital to the social and economic recovery of a country ravaged by years of political violence and poverty. These findings strongly indicate the need to incorporate public mental health approaches, including systematic screening and trauma/grief-focused interventions, within a comprehensive disaster recovery program.

Prospective study of posttraumatic stress, anxiety, and depressive reactions after earthquake and political violence.

OBJECTIVE: The authors sought to assess the severity and longitudinal course of posttraumatic stress, anxiety, and depressive reactions among two groups of adults differentially exposed to severe and mild earthquake trauma and a third group exposed to severe violence. They also examined interrelationships among these reactions and predictors of outcome and compared posttraumatic stress disorder (PTSD) symptom category profile and course between those exposed to earthquake and those exposed to violence. METHOD: Seventy-eight non-treatment-seeking subjects were assessed with self-report instruments approximately 1.5 and 4.5 years after the 1988 Spitak earthquake in Armenia and the 1988 pogroms against Armenians in Azerbaijan. RESULTS: The two groups that had been exposed to severe trauma (earthquake or violence) had high initial and follow-up PTSD scores that did not remit over the 3-year interval. Overall, depressive symptoms subsided. Posttraumatic stress, anxiety, and depressive reactions were highly intercorrelated within and across both time intervals. No significant differences in PTSD severity, profile, or course were seen between subjects exposed to severe earthquake trauma versus those exposed to severe violence. CONCLUSIONS: After exposure to severe trauma, either an earthquake or violence, adults are at high risk of developing severe and chronic posttraumatic stress reactions that are associated with chronic anxiety and depressive reactions. Clinical evaluation and therapeutic intervention should include specific attention to these reactions. Early mental health intervention is recommended to prevent their chronicity.

A developmental psychopathology model of childhood traumatic stress and intersection with anxiety disorders.

Empirical findings regarding childhood traumatic stress are placed within a developmental life-trajectory model that incorporates a tripartite etiology of posttrauma distress. This approach recognizes an intricate matrix of child-intrinsic factors, developmental maturation and experience, life events, and evolving family and social ecologies. Of central developmental importance in the field of traumatic stress is the ontogenesis of appraisal, emotional response, emotional and physiological regulation, and consideration of protective action with regard to danger. The complexity of traumatic situations and their aftermath suggests the relevance of multiple stress diatheses in understanding individual variability in proximal and distal effects. Neurobiological systems that subserve danger mature over childhood and adolescence. Neurophysiological and neurohormonal studies among traumatized children and adolescents suggest potential neurodevelopmental stage-related vulnerabilities within these systems. Advances in child development and traumatic stress provide tools for investigating proximal and distal interplay of psychopathology, disturbances in the acquisition and maintenance of developmental competencies, and life-trajectory outcomes. A developmental psychopathology model suggests different avenues by which dangerous circumstances, childhood traumatic experiences, and posttraumatic stress disorder (PTSD) can intersect with other anxiety disorders over the life span.