2 Source & selection Interference & lithography Si(111) 7x7 Transfer Analysis Transfer rod Wobble stick s Load lock Bellow Motor Velocity selector p ~ 10 mbar -7 77K baffle G$_{1}$ G$_{2}$ L STM d STM control & display p ~ 10 mbar -10 p < 10 mbar -10 z x y D here. Its advantage lies in the fact that all transmitted particles arrive with the same speed and the nanostruc- ture has the same contrast across the entire surface that is illuminated by the molecular beam. Being indepen- dent of gravitation the selection scheme can be used in any orientation of the experiment. About 110 cm behind the source, the molecules en- counter the first diffraction grating G$_{1}$ of a near-field interferometer [22, 24, 25]. Diffraction at each of the in- dividual slits within G$_{1}$ expands the molecular coherence function to an extent that it covers several slits of the second grating G$_{2}$ . Diffraction at G$_{2}$ , coherent evolution and interference subsequently generate a molecular den- sity pattern, which we accumulate on the detector screen D . The SiN x gratings were fabricated by Dr. Savas at MIT, Cambridge [26] with a highly accurate period of d = 257 . 40(1) nm and with open slit windows as small as 75 nm in G$_{1}$ and 150 nm in G$_{2}$. In our symmetri- cal setup, the separation L between the two gratings is equal to the distance between G$_{2}$ and D . The molecu- lar distribution on the detector is then an approximate self-image of the transmission function of G$_{2}$ , if the Tal- bot condition is met, i.e. if L = d $^{2}$/λ$_{dB}$ in the absence of any external potentials. Although the nanomechani- cal gratings are fabricated with a rectangular transmis- sion profile, interference and phase shifts in the gratings lead to a near-sinusoidal molecular density pattern on the screen. The fringe visibility can then be extracted as V = ( S$_{max}$ − S$_{min}$ ) / ( S$_{max}$ + S$_{min}$ ), where S ( x ) is the local molecular surface density at position x . of high-contrast patterns in this kind of lithography. The total experimental sequence is as follows: A silicon surface is thermally reconstructed, characterized in the STM and then transferred into the interferometer. The angular frequency of the helical velocity selector is set to transmit the velocity class v = 115 ± 5 m/s. The molec- ular exposure lasts about 30 min and we deposit about 0.001 molecular monolayers (ML) on the target. This low coverage is required to maintain the single-particle character in the quantum demonstration. The exposed sample is then transferred into the detector chamber. The detector screen is a thermally reconstructed Si (111) 7 × 7 surface. Silicon is known to capture and bind fullerenes exceptionally well [27] and it is also the natural choice for interfacing molecular deposits to future electronic readout or control. The atomically clean and flat surface immobilizes the incident molecules and it al- lows us to operate the experiment at room temperature. It requires, however, a proper surface cleaning, preheat- ing and flash-heating in the read-out chamber which also contains the variable temperature scanning tunneling mi- croscope(RHK STM UHV 700). The STM surface scan of Figure 2 clearly reveals the individual silicon substrate atoms as well as several im- mobilized single C$_{60}$ molecules. The surface binding of the fullerenes is so strong that we could not observe any clustering, even over two weeks. High-resolution tunnel- ing microscopy at low temperatures even allows us to get a glimpse on the internal structure of the deposited fullerenes [28], as shown in the inset of Figure 2. For a grating separation of L = 13 . 2 mm we expect the maximum interference contrast of the first Talbot order for a de Broglie wavelength of 5 pm, i.e. for C$_{60}$ with a mean velocity of 111 m/s. The van der Waals interaction between the molecule and the grating wall reduces the effective grating opening and shifts the the- oretical fringe visibility. At a mean molecular velocity of v = 115 m/s we expect a maximum contrast of about V = 60 %. If the molecules were classical billiard balls, i.e. following trajectories of Newtonian physics, theory would predict a nearly flat molecular distribution at the detector, i.e. a vanishing fringe visibility of only V = 1 %. This comparison of visibilities underlines the importance of the molecular quantum wave nature for the emergence In order to see the interference pattern we probe an area of 2 µ m 2 at a resolution that still allows to identify the individual molecules. This scan takes about thirty minutes and returns 10 6 data points. We identify the molecules by their height: if a recorded pixel (i,j) is 0.5- 0.9 nm higher than the average of the neighboring pixels ( i ± 3 , j ± 3) it is identified as a fullerene molecule. The analog images are thus converted into a binary matrix, where the presence of a molecule is represented by the FIG. 1: Molecular int erference lithography combines coherent molecule wave propag ation with surface deposition and scan- ning probe microscop y. The furnace emits a thermal fullerene beam, velocity selecte d to Δ v/v = 5%, which passes two SiN gratings ( d = 257 . 40 nm) separated by L = 12 . 5 mm. A prepared silicon sam ple is placed in distance L behind G 2 . The molecular deposi t is imaged with single molecule resolu- tion using scanning t unneling microscopy in a separate UHV chamber.