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  • 2017 European Conference on Lasers and Electro-Optics and European Quantum Electronics Conference
  • (Optica Publishing Group, 2017),
  • paper PD_1_1

Direct comb spectroscopy by quantum-Zeno-effect assisted detection

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The time evolution of a quantum system can be entirely prohibited by applying frequent quantum measurements which repeatedly introduce wavefunction collapse. The effect is typically termed Quantum Zeno Effect (QZE) which was originally proposed to inhibit the decay of an unstable particle through repeated measurements [1], and experimentally demonstrated later with trapped ions [2]. The QZE is not only interesting as a clear manifestation of the counterintuitive behaviour of quantum mechanics, but may also have practical applications: we show QZE-assisted detection is useful for high precision spectroscopy. Fig.1(a) shows the level scheme of an atomic (ionic) system under consideration. The levels | ↑ ⟩and | ↓ ⟩ possess long lifetimes and form a qubit which can be coherently controlled via microwave pulses. After initial state preparation into ground state | ↑ ⟩, the system is exposed to a π microwave-pulse on the | ↑ ⟩ → | ↓ ⟩ transition. In the scheme of QZE-assisted spectroscopy, at the same time as the system is exposed to the microwave-pulses, a spectroscopy laser which probes optical transitions of | ↓ ⟩ ↔ |e1⟩ and/or | ↑ ⟩ ↔ |e2⟩ is also present. If the spectroscopy laser introduce spontaneous decay from |e1⟩ or |e2⟩ states, the system experiences "quantum measurement" and the wave function spanned by | ↑ ⟩ and | ↓ ⟩ states collapses into one of the two bases of | ↑ ⟩ or | ↓ ⟩ . In its strongest form when the "quantum measurement" is performed frequent enough, it completely prevents the transfer of population from | ↑ ⟩ to | ↓ ⟩. The effect is found to be strongly dependent on the detuning of the spectroscopy laser and offers a sensitive spectroscopy signal which allows for high precision spectroscopy of transitions with small excitation rate. An example is direct frequency comb spectroscopy of a single photon transition where only the minute power contained in a single comb mode contributes to the excitation. In this work, we demonstrate QZE-assisted spectroscopy of single 25Mg+ ions trapped in a Paul trap. The spectroscopy laser is a deep-ultraviolet (DUV) frequency comb at 280 nm and generated by frequency tripling a Ti:sapphire mode-locked ring laser. We choose to use a DUV frequency comb as a spectroscopy laser because it can readily demonstrate the main advantages of the technique, which are higher sensitivity and smaller background noise. Typical spectroscopy signal for the QZE-assisted spectroscopy is shown in Fig.1(b). The remaining population of the initial state | ↑ ⟩ after the spectroscopy period is plotted for different detunings of the spectroscopy laser, which is a measure for the strength of the QZE introduced by the spectroscopy laser. The repeating structure with a period that corresponds to comb mode spacing reflects the mode structure of the frequency comb. Pairs of lines are observed which correspond to | ↑ ⟩ ↔ |e2⟩ and | ↓ ⟩ ↔ |e1⟩ transitions. The latter transition corresponds to the "null measurement" case where the observation of no photon scattering events from the spectroscopy laser confirms the existence of no population in the state and thus serves as a quantum measurement. Under our experimental conditions, we found that even 50 μw of average power of frequency comb, which corresponds to approximately 1.5 nw per comb mode, was sufficient to introduce significant QZE and detect the spectroscopy signal with high signal to noise ratio. The QZE-assisted detection is expected to be useful for high precision spectroscopy experiments in extreme-ultraviolet regions where even smaller comb power is available.

© 2017 IEEE

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