Continuous phase stabilization and active interferometer control using two modes

We present a computer-based active interferometer stabilization method that can be set to an arbitrary phase difference and does not rely on modulation of the interfering beams. The scheme utilizes two orthogonal modes propagating through the interferometer with a constant phase difference between them to extract a common phase and generate a linear feedback signal. Switching times of 50ms over a range of 0 to 6 pi radians at 632.8nm are experimentally demonstrated. The phase can be stabilized up to several days to within 3 degrees.Comment: 3 pages, 2 figure

Somatomedin C in dairy cows related to energy and protein supply and to milk production

Somatomedin C and other hormones, as well as blood metabolites, were measured during the dry period and during lactation in dairy cows, given different amounts of energy and protein, to study metabolic and endocrine adaptations. Somatomedin C, specifically measured by radioimmunoassay after separation from its binding protein, did not exhibit typical diurnal variations, in contrast to somatotropin and insulin, which increased particularly after concentrate intake. Somatomedin C markedly decreased at parturition and reached lowest values around the peak of lactation, while levels of somatotropin, nonesterified fatty acids and ketone bodies were high and those of glucose, insulin, thyroxine and triiodothyronine were low. Thereafter somatomedin C values slowly increased up to the 12th week of lactation and remained elevated. Low energy and protein balances were characterized by particularly low somatomedin C concentrations. An additional protein deficit at peak lactation, when cows were already provided with low amounts of energy, did not further decrease somatomedin C levels. However, when high amounts of energy were given in the form of starch or crystalline fat, somatomedin C increased. Overall, there was a positive correlation of somatomedin C primarily with energy, but also with protein balances and a negative correlation with milk yield. Conversely, somatotropin increased markedly after parturition and was positively correlated with milk production and negatively with protein and energy balances. Thus, somatomedin C levels were paradoxically low in the presence of high circulating somatotropin. Insulin most closely paralleled somatomedin C levels. Therefore the anabolic state of metabolism at the end of pregnancy was characterized by high somatomedin C and insulin and relatively low somatotropin, whereas the catabolic state of early lactation was characterized by high somatotropin, low somatomedin C, insulin and thyroid hormone

A proposed testbed for detector tomography

Measurement is the only part of a general quantum system that has yet to be characterized experimentally in a complete manner. Detector tomography provides a procedure for doing just this; an arbitrary measurement device can be fully characterized, and thus calibrated, in a systematic way without access to its components or its design. The result is a reconstructed POVM containing the measurement operators associated with each measurement outcome. We consider two detectors, a single-photon detector and a photon-number counter, and propose an easily realized experimental apparatus to perform detector tomography on them. We also present a method of visualizing the resulting measurement operators.Comment: 9 pages, 4 figure

Measuring Measurement: Theory and Practice

Recent efforts have applied quantum tomography techniques to the calibration and characterization of complex quantum detectors using minimal assumptions. In this work we provide detail and insight concerning the formalism, the experimental and theoretical challenges and the scope of these tomographical tools. Our focus is on the detection of photons with avalanche photodiodes and photon number resolving detectors and our approach is to fully characterize the quantum operators describing these detectors with a minimal set of well specified assumptions. The formalism is completely general and can be applied to a wide range of detectorsComment: 22 pages, 27 figure

Mapping coherence in measurement via full quantum tomography of a hybrid optical detector

Quantum states and measurements exhibit wave-like --- continuous, or particle-like --- discrete, character. Hybrid discrete-continuous photonic systems are key to investigating fundamental quantum phenomena, generating superpositions of macroscopic states, and form essential resources for quantum-enhanced applications, e.g. entanglement distillation and quantum computation, as well as highly efficient optical telecommunications. Realizing the full potential of these hybrid systems requires quantum-optical measurements sensitive to complementary observables such as field quadrature amplitude and photon number. However, a thorough understanding of the practical performance of an optical detector interpolating between these two regions is absent. Here, we report the implementation of full quantum detector tomography, enabling the characterization of the simultaneous wave and photon-number sensitivities of quantum-optical detectors. This yields the largest parametrization to-date in quantum tomography experiments, requiring the development of novel theoretical tools. Our results reveal the role of coherence in quantum measurements and demonstrate the tunability of hybrid quantum-optical detectors.Comment: 7 pages, 3 figure

Absolute efficiency estimation of photon-number-resolving detectors using twin beams

A nonclassical light source is used to demonstrate experimentally the absolute efficiency calibration of a photon-number-resolving detector. The photon-pair detector calibration method developed by Klyshko for single-photon detectors is generalized to take advantage of the higher dynamic range and additional information provided by photon-number-resolving detectors. This enables the use of brighter twin-beam sources including amplified pulse pumped sources, which increases the relevant signal and provides measurement redundancy, making the calibration more robust

Avalanche Photo-Detection for High Data Rate Applications

Avalanche photo detection is commonly used in applications which require single photon sensitivity. We examine the limits of using avalanche photo diodes (APD) for characterising photon statistics at high data rates. To identify the regime of linear APD operation we employ a ps-pulsed diode laser with variable repetition rates between 0.5MHz and 80MHz. We modify the mean optical power of the coherent pulses by applying different levels of well-calibrated attenuation. The linearity at high repetition rates is limited by the APD dead time and a non-linear response arises at higher photon-numbers due to multiphoton events. Assuming Poissonian input light statistics we ascertain the effective mean photon-number of the incident light with high accuracy. Time multiplexed detectors (TMD) allow to accomplish photon- number resolution by photon chopping. This detection setup extends the linear response function to higher photon-numbers and statistical methods may be used to compensate for non-linearity. We investigated this effect, compare it to the single APD case and show the validity of the convolution treatment in the TMD data analysis.Comment: 16 pages, 5 figure

Integrated Photonic Sensing

Loss is a critical roadblock to achieving photonic quantum-enhanced technologies. We explore a modular platform for implementing integrated photonics experiments and consider the effects of loss at different stages of these experiments, including state preparation, manipulation and measurement. We frame our discussion mainly in the context of quantum sensing and focus particularly on the use of loss-tolerant Holland-Burnett states for optical phase estimation. In particular, we discuss spontaneous four-wave mixing in standard birefringent fibre as a source of pure, heralded single photons and present methods of optimising such sources. We also outline a route to programmable circuits which allow the control of photonic interactions even in the presence of fabrication imperfections and describe a ratiometric characterisation method for beam splitters which allows the characterisation of complex circuits without the need for full process tomography. Finally, we present a framework for performing state tomography on heralded states using lossy measurement devices. This is motivated by a calculation of the effects of fabrication imperfections on precision measurement using Holland-Burnett states.Comment: 19 pages, 7 figure

Manipulating the quantum information of the radial modes of trapped ions: Linear phononics, entanglement generation, quantum state transmission and non-locality tests

We present a detailed study on the possibility of manipulating quantum information encoded in the "radial" modes of arrays of trapped ions (i.e., in the ions' oscillations orthogonal to the trap's main axis). In such systems, because of the tightness of transverse confinement, the radial modes pertaining to different ions can be addressed individually. In the first part of the paper we show that, if local control of the radial trapping frequencies is available, any linear optical and squeezing operation on the locally defined modes - on single as well as on many modes - can be reproduced by manipulating the frequencies. Then, we proceed to describe schemes apt to generate unprecedented degrees of bipartite and multipartite continuous variable entanglement under realistic noisy working conditions, and even restricting only to a global control of the trapping frequencies. Furthermore, we consider the transmission of the quantum information encoded in the radial modes along the array of ions, and show it to be possible to a remarkable degree of accuracy, for both finite-dimensional and continuous variable quantum states. Finally, as an application, we show that the states which can be generated in this setting allow for the violation of multipartite non-locality tests, by feasible displaced parity measurements. Such a demonstration would be a first test of quantum non-locality for "massive" degrees of freedom (i.e., for degrees of freedom describing the motion of massive particles).Comment: 21 pages; this paper, presenting a far more extensive and detailed analysis, completely supersedes arXiv:0708.085