Quantum interference metrology at deep-UV wavelengths using phase-controlled ultrashort laser pulses

High-resolution metrology at wavelengths shorter than ultraviolet is in general hampered by a limited availability of appropriate laser sources. It is demonstrated that this limitation can be overcome by quantum-interference metrology with frequency up-converted ultrafast laser pulses. The required phase controlled pulses are obtained by amplifying the output of a frequency comb laser. Efficient harmonic up-conversion in crystals and gases is then possible because of the high peak power of the pulses, paving the way for extension of frequency comb metrology in atoms and ions to the extreme ultraviolet and soft-x-ray spectral regions. A proof-of-principle experiment was performed in krypton on a two-photon transition that was excited with 2 × 212.55 nm light. The accuracy of the absolute transition frequency and isotope shifts were improved by more than an order of magnitude compared to previous experiments which used nanosecond timescale pulsed lasers