Single-atom-resolved detection in optical lattices using quantum-gas microscopes has enabled a new generation of experiments in the field of quantum simulation. While such devices have been realised with bosonic species, a fermionic quantum-gas microscope has remained elusive. Here we demonstrate single-site-and single-atom-resolved fluorescence imaging of fermionic potassium-40 atoms in a quantum-gas microscope setup, using electromagnetically-induced-transparency cooling. We detected on average 1000 fluorescence photons from a single atom within 1.5 s, while keeping it close to the vibrational ground state of the optical lattice. A quantum simulator for fermions with single-particle access will be an excellent test bed to investigate phenomena and properties of strongly correlated fermionic quantum systems, allowing for direct measurement of ordered quantum phases and out-of-equilibrium dynamics, with access to quantities ranging from spin-spin correlation functions to many-particle entanglement.The ability to observe and control quantum systems at the single-particle level has revolutionised the field of quantum optics over the last decades. At the same time, the possibility to study quantum many-body systems in well-controlled engineered environments using ultracold atoms in optical lattices has proven to be a powerful tool for the investigation of strongly-correlated quantum systems [1]. It was only recently that the great challenge to have full single-site resolution and single-atom control in optical lattices was overcome by the development of quantum-gas microscopes [2,3] Some of the most interesting effects in many-body quantum systems are due to the properties of strongly interacting Fermi gases. Within solid state physics, the fermionic nature of the electron is vital to understand a range of phenomena, such as electron pairing in superconductivity, and quantum magnetism (including colossal magneto-resistance). However, some of the properties of many-body fermionic quantum systems are very challenging to compute, even with the most advanced numerical methods, due to the antisymmetric nature of the wave function, and the resulting sign problem for quantum Monte-Carlo methods [7]. A quantum simulator for fermions with single-particle resolution would be an excellent test bed to investigate many of the phenomena and properties of strongly-correlated fermionic quantum systems. Such a fermionic quantum-gas microscope will provide the possibility to probe quantities that are difficult to access directly, such as spin-spin-correlation functions or string-order [8]. It would allow the study of out-of-equilibrium dynamics, the spreading of corre- * present address: University of St. Andrews, School of Physics and Astronomy, United Kingdom † Electronic address: stefan.kuhr@strath.ac.uk lations [9] and the build-up of entanglement in manyparticle fermionic quantum systems [10]. It could perform quantum simulation of the Fermi-Hubbard model, which is conjectured to capture the key mechanism behind high-T c superc...

The accurate determination and control of the wavelength of light is fundamental to many fields of science. Speckle patterns resulting from the interference of multiple reflections in disordered media are well-known to scramble the information content of light by complex but linear processes. However, these patterns are, in fact, exceptionally rich in information about the illuminating source. We use a fibre-coupled integrating sphere to generate wavelength-dependent speckle patterns, in combination with algorithms based on the transmission matrix method and principal component analysis, to realize a broadband and sensitive wavemeter. We demonstrate sub-femtometre wavelength resolution at a centre wavelength of 780 nm, and a broad calibrated measurement range from 488 to 1,064 nm. This compares favourably to the performance of conventional wavemeters. Using this speckle wavemeter as part of a feedback loop, we stabilize a 780 nm diode laser to achieve a linewidth better than 1 MHz.

The measurement of the wavelength of light using speckle is a promising tool for the realization of compact and precise wavemeters and spectrometers. However, the resolution of these devices is limited by strong correlations between the speckle patterns produced by closely-spaced wavelengths. Here, we show how principal component analysis of speckle images provides a route to overcome this limit. Using this, we demonstrate a compact wavemeter which measures wavelength changes of a stabilized diode laser of 5.3 am, eight orders of magnitude below the speckle correlation limit.