A frequency metrology system for the molecular lattice clock

State-of-the-art quantum metrology expands the technological and scientific capabilities of theDOD, and provides unparalleled platforms for educating the next generation of scientificworkforce. Most modern quantum clocks are based on electronic transitions in atoms that are inthe microwave or optical frequency range. Recently, we demonstrated a fundamentally new anddistinct type of clock that is based on molecular vibrations. This clock connects the terahertzvibrational frequencies to both the optical and the radio frequency domains via the clock gears,or the femtosecond frequency comb. The comb is stabilized to an ultralow-noise laser source,and this optical coherence is transferred to all other lasers that are phase-locked to the comb,including two that act as probes for the molecular clock. At the heart of the molecular clock arediatomic strontium molecules, created from ultracold strontium vapor at a millionth of a degreekelvin. The molecules are trapped in a standing wave of light, or an optical lattice. The tighttrapping permits optical spectroscopy without motional shifts or broadening of the clock spectralline. To avoid decoherence of the clock state superpositions, the lattice trap depths must beequal for the two states. This condition is called magic-wavelength or state-insensitive trapping.We recently demonstrated that magic trapping is possible for vibrational molecular statesuperpositions. With this technique, molecule-light coherence times have increased by overthree orders of magnitude. To achieve another two orders of magnitude of enhancement and thusexceed 1 second for coherence times, a modern optical metrology system must be put in place.The terahertz clock transition is currently probed by two optical sources in a lambdaconfiguration where the two transition strengths are strongly unbalanced. These sources must bereplaced by stable lasers in the 780 nm frequency range, both for a more efficient two-photontransition and to facilitate the use of an already-available ultrastable 780 nm laser for combstabilization. Furthermore, the frequency comb, sponsored by earlier DOD DURIP funds, is anolder model where excess noise is written onto the light when the overall offset of the combmodes is stabilized. The upgraded state-of-the-art comb laser and the associated electronics willenable us to achieve unprecedented coherence times for the molecular clock quantum-statesuperpositions, and extend the lifetime of the instrument by over a decade. Long molecule-lightcoherence times, pushed to the cutting edge by the molecular clock, are crucial not only forprecision metrology, but also for experiments in quantum information and quantum simulations.Molecules with electric dipole moments constitute qubits for quantum computing that can storeinformation over long times and at the same time enable gate manipulations via interactionsbetween the qubits. To achieve these goals, superpositions of qubit states must remain coherentwithout dephasing over long durations on the scale of 1 second.