ARTICLE NATURE COMMUNICATIONS DOl:10.1038/ncomms1263 n many discussions on the foundations of physics,single-particle diffraction at a double slit!-or gratings-12 is regarded as a para- digmatic example for a highly non-classical feature of quantum mechanics,which has never been observed for objects of our mac- roscopic world.The quantum superposition principle has become of paramount importance also for the growing field of quantum information science'3.Correspondingly,research in many labora- tories around the world is focusing on our understanding of the role of decoherence for increasingly complex quantum systems and possible practical or truly fundamental limits to the observation of quantum dynamics 4s Here we report on a new leap in quantum interference with large organic molecules.In contrast to earlier successful experi- ments with internal molecular wave packets,our study focuses on the wave evolution in the centre of mass motion of the molecule as a whole,that is,pure de Broglie interference.We do this with compounds that have been customized to provide useful molecu- lar beams at moderate temperatures78.Figure 1 compares the size of two perfluoroalkylated nanospheres,PFNS8 and PFNS10,with a single Co fullerene and it relates a single tetraphenylporphyrin Figure 1|Gallery of molecules used in our interference study.(a)The molecule (TPP)to its complex derivatives TPPF84 and TPPF152. fullerene Cao(m=720 AMU,60 atoms)serves as a size reference and We demonstrate the wave nature of all these molecules in a three- for calibration purposes;(b)The perfluoroalkylated nanosphere PFNS8 grating near-field interferometer of the Kapitza-Dirac-Talbot- (Cao[CF2s]a,m=5.672 AMU,356 atoms)is a carbon cage with eight Lau type22,as shown in Figure 2. perfluoroalkyl chains.(c)PFNS10 (Cao[CFsm=6,910 AMU,430 atoms)has ten side chains and is the most massive particle in the set Results (d)A single tetraphenylporphyrin TPP(CHaoN,m=614 AMU,78 Experimental setup.The particles are evaporated in a thermal atoms)is the basis for the two derivatives(e)TPPF84(CHFNS source.Their velocity is selected using the gravitational free-fall m=2,814AMU,202 atoms)and (f)TPPF152 (CesHa4F s2O NS4. through a sequence of three slits.The interferometer itself consists m=5,310 AMU,430 atoms).In its unfolded configuration,the latter is the of three gratings G,G,and G:in a vacuum chamber at a pressure largest molecule in the set.Measured by the number of atoms,TPPF152 of p<10-mbar.The first grating is a SiN,membrane with 90-nm and PFNS10 are equally complex.All molecules are displayed to scale.The wide slits arranged with a periodicity ofd=266nm.Each slit of G scale bar corresponds to 10 A. imposes a constraint onto the transverse molecular position that, following Heisenberg's uncertainty relation,leads to a momentum Detector uncertainty.The latter turns into a growing delocalization and transverse coherence of the matter wave with increasing distance from G.The second grating,G,is a standing laser light wave with a wavelength of A=532 nm.The interaction between the electric laser light field and the molecular optical polarizability creates asinusoidal G potential,which phase-modulates the incident matter waves.The distance between the first two gratings is chosen such that quantum interference leads to the formation of a periodic molecular density pattern 105 mm behind G2.This molecular nanostructure is sampled by scanning a second SiN,grating(G3,identical to G,)across the Lens molecular beam while counting the number of the transmitted particles in a quadrupole mass spectrometer(QMS). In extension to earlier experiments,we have added various tech- nological refinements:the oven was adapted to liquid samples,a liquid-nitrogen-cooled chamber became essential to maintain the Oven source pressure low,a new mass analyser allowed us to increase the detected molecular flux by a factor of four and many optimi- zation cycles in the interferometer alignment were needed to meet Figure 2 Layout of the Kapitza-Dirac-Talbot-Lau (KDTL)interference all requirements for high-contrast experiments with very massive experiment.The effusive source emits molecules that are velocity-selected particles. by the three delimiters S,,S,and S.The KDTL interferometer is composed of two SiN,gratings G,and G,as well as the standing light wave G.The Observed interferograms.We recorded quantum interferograms optical dipole force grating imprints a phase modulation (x)P/(vw) for all molecules of Figure 1,as shown in Figure 3.In all cases the onto the matter wave.Hereis the optical polarizability,Pthe laser measured fringe visibility V,that is,the amplitude of the sinusoidal power,v the molecular velocity and w,the laser beam waist perpendicular modulation normalized to the mean of the signal,exceeds the maxi- to the molecular beam.The molecules are detected using electron impact mally expected classical moire fringe contrast by a significant multi- ionization and quadrupole mass spectrometry. ple of the experimental uncertainty.This is best shown for TPPF84 and PFNS8,which reached the highest observed interference con- for TPPF152 (see Figure 3),in which our classical model predicts trast in our high-mass experiments so far,with individual scans V=1%.This supports our claim of true quantum interference for up to V=33%for TPPF84 (m=2,814AMU)and Vh=49%for all these complex molecules. PFNS8 at a mass of m=5,672 AMU.In addition,we have observed The most massive molecules are also the slowest and therefore a maximum contrast of V=174%for PFNS10 and V=16+2%the most sensitive ones to external perturbations.In our particle NATURE COMMUNICATIONS 2:263 DOl:10.1038/ncomms1263 www.nature.com/naturecommunications 2011 Macmillan Publishers Limited.All rights reserved.ARTICLE  nature communications | DOI: 10.1038/ncomms1263 nature communications | 2:263 | DOI: 10.1038/ncomms1263 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved. I n many discussions on the foundations of physics, single-particle diffraction at a double slit1–4 or grating5–12 is regarded as a para￾digmatic example for a highly non-classical feature of quantum mechanics, which has never been observed for objects of our mac￾roscopic world. The quantum superposition principle has become of paramount importance also for the growing field of quantum information science13 . Correspondingly, research in many labora￾tories around the world is focusing on our understanding of the role of decoherence for increasingly complex quantum systems and possible practical or truly fundamental limits to the observation of quantum dynamics14,15. Here we report on a new leap in quantum interference with large organic molecules. In contrast to earlier successful experi￾ments with internal molecular wave packets,16 our study focuses on the wave evolution in the centre of mass motion of the molecule as a whole, that is, pure de Broglie interference. We do this with compounds that have been customized to provide useful molecu￾lar beams at moderate temperatures17,18. Figure 1 compares the size of two perfluoroalkylated nanospheres, PFNS8 and PFNS10, with a single C60 fullerene19 and it relates a single tetraphenylporphyrin molecule (TPP) to its complex derivatives TPPF84 and TPPF152. We demonstrate the wave nature of all these molecules in a three￾grating near-field interferometer20,21 of the Kapitza-Dirac-Talbot￾Lau type22,23, as shown in Figure 2. Results Experimental setup. The particles are evaporated in a thermal source. Their velocity is selected using the gravitational free-fall through a sequence of three slits. The interferometer itself consists of three gratings G1, G2 and G3 in a vacuum chamber at a pressure of p<10−8mbar. The first grating is a SiNx membrane with 90-nm wide slits arranged with a periodicity of d=266nm. Each slit of G1 imposes a constraint onto the transverse molecular position that, following Heisenberg’s uncertainty relation, leads to a momentum uncertainty. The latter turns into a growing delocalization and transverse coherence of the matter wave with increasing distance from G1. The second grating, G2, is a standing laser light wave with a wavelength of λ=532nm. The interaction between the electric laser light field and the molecular optical polarizability creates a sinusoidal potential, which phase-modulates the incident matter waves. The distance between the first two gratings is chosen such that quantum interference leads to the formation of a periodic molecular density pattern 105mm behind G2. This molecular nanostructure is sampled by scanning a second SiNx grating (G3, identical to G1) across the molecular beam while counting the number of the transmitted particles in a quadrupole mass spectrometer (QMS). In extension to earlier experiments, we have added various tech￾nological refinements: the oven was adapted to liquid samples, a liquid-nitrogen-cooled chamber became essential to maintain the source pressure low, a new mass analyser allowed us to increase the detected molecular flux by a factor of four and many optimi￾zation cycles in the interferometer alignment were needed to meet all requirements for high-contrast experiments with very massive particles. Observed interferograms. We recorded quantum interferograms for all molecules of Figure 1, as shown in Figure 3. In all cases the measured fringe visibility V, that is, the amplitude of the sinusoidal modulation normalized to the mean of the signal, exceeds the maxi￾mally expected classical moiré fringe contrast by a significant multi￾ple of the experimental uncertainty. This is best shown for TPPF84 and PFNS8, which reached the highest observed interference con￾trast in our high-mass experiments so far, with individual scans up to Vobs=33% for TPPF84 (m=2,814AMU) and Vobs=49% for PFNS8 at a mass of m=5,672AMU. In addition, we have observed a maximum contrast of Vobs=17±4% for PFNS10 and Vobs=16±2% for TPPF152 (see Figure 3), in which our classical model predicts Vclass=1%. This supports our claim of true quantum interference for all these complex molecules. The most massive molecules are also the slowest and therefore the most sensitive ones to external perturbations. In our particle Figure 1 | Gallery of molecules used in our interference study. (a) The fullerene C60 (m=720AMU, 60 atoms) serves as a size reference and for calibration purposes; (b) The perfluoroalkylated nanosphere PFNS8 (C60[C12F25]8, m=5,672AMU, 356 atoms) is a carbon cage with eight perfluoroalkyl chains. (c) PFNS10 (C60[C12F25]10, m=6,910AMU, 430 atoms) has ten side chains and is the most massive particle in the set. (d) A single tetraphenylporphyrin TPP (C44H30N4, m=614AMU, 78 atoms) is the basis for the two derivatives (e) TPPF84 (C84H26F84N4S4, m=2,814AMU, 202 atoms) and (f) TPPF152 (C168H94F152O8N4S4, m=5,310AMU, 430 atoms). In its unfolded configuration, the latter is the largest molecule in the set. Measured by the number of atoms, TPPF152 and PFNS10 are equally complex. All molecules are displayed to scale. The scale bar corresponds to 10Å. y X Detector G1 G2 G3 S3 S2 S1 Oven Lens Laser Z Figure 2 | Layout of the Kapitza-Dirac-Talbot-Lau (KDTL) interference experiment. The effusive source emits molecules that are velocity-selected by the three delimiters S1 , S2 and S3. The KDTL interferometer is composed of two SiNx gratings G1 and G3, as well as the standing light wave G2. The optical dipole force grating imprints a phase modulation ϕ(x)∝αopt·P/(v·wy) onto the matter wave. Here αopt is the optical polarizability, P the laser power, v the molecular velocity and wy the laser beam waist perpendicular to the molecular beam. The molecules are detected using electron impact ionization and quadrupole mass spectrometry