Spatial correlations in parametric down-conversion

The transverse spatial effects observed in photon pairs produced by parametric down-conversion provide a robust and fertile testing ground for studies of quantum mechanics, non-classical states of light, correlated imaging and quantum information. Over the last 20 years there has been much progress in this area, ranging from technical advances and applications such as quantum imaging to investigations of fundamental aspects of quantum physics such as complementarity relations, Bell's inequality violation and entanglement. The field has grown immensely: a quick search shows that there are hundreds of papers published in this field. The objective of this article is to review the building blocks and major theoretical and experimental advances in the field, along with some possible technical applications and connections to other research areas.Comment: 116 pages, 35 figures. To appear in Physics Report

Image and Coherence Transfer in the Stimulated Down-conversion Process

The intensity transverse profile of the light produced in the process of stimulated down-conversion is derived. A quantum-mechanical treatment is used. We show that the angular spectrum of the pump laser can be transferred to the stimulated down-converted beam, so that images can also be transferred from the pump to the down-converted beam. We also show that the transfer can occur from the stimulating beam to the down-converted one. Finally, we study the process of diffraction through an arbitrarily shaped screen. For the special case of a double-slit, the interference pattern is explicitly obtained. The visibility for the spontaneous emitted light is in accordance with the van Cittert - Zernike theorem for incoherent light, while the visibility for the stimulated emitted light is unity. The overall visibility is in accordance with previous experimental results

Experimental determination of multipartite entanglement with incomplete information

Multipartite entanglement is very poorly understood despite all the theoretical and experimental advances of the last decades. Preparation, manipulation and identification of this resource is crucial for both practical and fundamental reasons. However, the difficulty in the practical manipulation and the complexity of the data generated by measurements on these systems increase rapidly with the number of parties. Therefore, we would like to experimentally address the problem of how much information about multipartite entanglement we can access with incomplete measurements. In particular, it was shown that some types of pure multipartite entangled states can be witnessed without measuring the correlations [M. Walter et al., Science 340, 1205 (2013)] between parties, which is strongly demanding experimentally. We explore this method using an optical setup that permits the preparation and the complete tomographic reconstruction of many inequivalent classes of three- and four-partite entangled states, and compare complete versus incomplete information. We show that the method is useful in practice, even for non-pure states or non ideal measurement conditions.Comment: 12 pages, 7 figures. Close to published versio

Control of conditional pattern with polarization entanglement

Conditional interference patterns can be obtained with twin photons from spontaneous parametric down-conversion and the phase of the pattern can be controlled by the relative transverse position of the signal and idler detectors. Using a configuration that produces entangled photons in both polarization and transverse momentum we report on the control of the conditional patterns by acting on the polarization degree of freedom.Comment: Submitted for publication in Optics Communication

Experimental investigation of quantum key distribution with position and momentum of photon pairs

We investigate the utility of Einstein-Podolsky-Rosen correlations of the position and momentum of photon pairs from parametric down-conversion in the implementation of a secure quantum key distribution protocol. We show that security is guaranteed by the entanglement between downconverted pairs, and can be checked by either direct comparison of Alice and Bob's measurement results or evaluation of an inequality of the sort proposed by Mancini et al. (Phys. Rev. Lett. 88, 120401 (2002)).Comment: 6 pages, 6 figures, subimitted for publicatio

Quantum key distribution with higher-order alphabets using spatially-encoded qudits

We propose and demonstrate a quantum key distribution scheme in higher-order $d$-dimensional alphabets using spatial degrees of freedom of photons. Our implementation allows for the transmission of 4.56 bits per sifted photon, while providing improved security: an intercept-resend attack on all photons would induce an error rate of 0.47. Using our system, it should be possible to send more than a byte of information per sifted photon.Comment: 4 pages, 5 figures. Replaced with published versio

Transverse coloplasty pouch and colonic J-pouch for rectal cancer: a comparative study

OBJECTIVES: The introduction of the colonic J-pouch has markedly improved the functional outcome of restorative rectal cancer surgery. However colonic J-pouch surgery can be problematic and may present some late evacuatory problems. To overcome these limitations a novel pouch has been proposed: the transverse coloplasty pouch. The purpose of our study was to compare the functional outcomes of these two different types of pouches--the transverse coloplasty pouch (TCP) and the colonic J-pouch (CJP)--during the first 12 months postoperatively. PATIENTS AND METHODS: A prospective randomized trial was conducted in which a total of 30 patients with mid and low rectal cancer were submitted either to a transverse coloplasty pouch or a colonic J-pouch. Clinical defaecatory function was assessed and anorectal physiological assessment was carried out, pre-operatively and at 3, 6 and 12 months postoperatively, by means of a standard clinical questionnaire and by anorectal manometry. RESULTS: No statistically significant differences were found between the two groups regarding bowel function. The postoperative frequency of daily bowel movements was lower in the TCP group in all the phases of the study (3.9 vs. 4.1 at 3 months; 3.1 vs. 3.4 at 6 months; 2.1 vs. 2.8 at 12 months), the same occurring with fragmentation (33% vs. 40% at 3 months; 26.6%vs. 33.3% at 6 months; 7.1%vs. 14.3% at 12 months). Less urgency was also seen in the TCP group during the first 6 months (20%vs. 26.7%), with identical values at 12 months (14.3% vs. 14.3%). No significant differences were also found concerning incontinence grading and scoring, with TCP patients having less nocturnal leaks. At one year two CJP patients (14.3%) needed the use of enemas to evacuate the pouch and provoke defaecation, a problem never seen in TCP patients. The anorectal manometry data was similar in both types of pouches. The local complication rates were also identical in the two groups (20%); more anastomotic leaks were seen in TCP patients (13.2% vs. 6.6%), without reaching a statistical significance. CONCLUSION: The transverse coloplasty pouch has similar functional results but fewer evacuation problems than the J-Pouch, making it a safe and reliable alternative to the colonic J-pouch