Quantum metrology with THz QCLs

1. THz quantum cascade laser frequency combs

We engineered broadband quantum cascade lasers (QCLs) emitting at Terahertz (THz) frequencies, which exploit a heterogeneous active region scheme and have a current density dynamic range (Jdr) of 3.2, significantly larger than the state of the art, over a 1.3 THz bandwidth. We demonstrate that the devised broadband lasers operate as THz optical frequency comb synthesizers in continuous-wave, with a maximum optical output power of 4 mW (0.73µW in the comb regime). Measurement of the intermode beatnote map reveals a clear dispersion-compensated frequency comb regime extending over a continuous 106 mA current range (current density dynamic range of 1.24), significantly larger than the state of the art reported under similar geometries, with a corresponding emission bandwidth of ≈ 1.05 THz and a stable and narrow (4.15 KHz) beatnote detected with a signal-to-noise ratio of 34 dB.

Quantum cascade laser emitting a discrete set of equally spaced modes, each corresponding to the tooth of an optical frequency comb. The semiconductor heterostructure embedded in the QCL active core is schematically shown as a sequence of striped lines.

1. K. Garrasi, F.P. Mezzapesa, L. Salemi, L. Li, L. Consolino, S. Bartalini, P. De Natale, A.G. Davies, E.H. Linfield & M. S. Vitiello, “High Dynamic Range, Heterogeneous, Terahertz Quantum Cascade Lasers Featuring Thermally Tunable Frequency Comb Operation over a Broad Current Range”, ACS Photonics 6, 73-78, (2019).

2. Free running emission linewidth of THz quantum emitters

By exploiting a metrological grade system comprising a terahertz frequency comb synthesizer, we measure, for the first time, the free-running emission linewidth (LW), the tuning characteristics, and the absolute center frequency of individual emission lines of different frequency generation (DFG) THz QCLs with an uncertainty of 4 × 10−10. The unveiled emission LW (400 kHz at 1-ms integration time) indicates that DFG-QCLs are well suited to operate as local oscillators and to be used for a variety of metrological, spectroscopic, communication, and imaging applications that require narrow-LW THz sources.

(a) Schematic diagrams of the experimental setup for measuring the beatnote signal between a free running THz comb and a THz DFG QCL. (b) A typical beat note spectrum observed on a spectrum analyzer for 2 ms integration time.

1. L. Consolino, S. Jung, A. Campa, M. De Regis, S. Pal, J. H. Kim, K. Fujita, A. Ito, M. Hitaka, S. Bartalini, P. De Natale, M. A Belkin & M.S. Vitiello, “Spectral purity and tunability of terahertz quantum cascade laser sources based on intracavity difference-frequency generation”, Sci. Adv. 3, e1603317 (2017).

3. Quantum limited linewidth of a THz laser

We report experimental evidence of linewidth values approaching the quantum limit in far-infrared quantum cascade lasers. By performing noise measurements with unprecedented sensitivity levels, we demonstrate that properly designed semiconductor-heterostructure lasers can unveil the mechanisms underlying the laser-intrinsic phase noise, revealing the link between device properties and the quantum-limited linewidth.

Frequency noise power spectral density. Comparison of the experimental FNPSD of the terahertz QCL (orange trace) and the contribution of the frequency noise of the CNPSD for the current driver (blue trace). The dashed green line indicates the white noise level.

1. M.S. Vitiello, L. Consolino, S. Bartalini, A. Taschin, A. Tredicucci, M. Inguscio & P. De Natale, “Quantum-limited frequency fluctuations in a terahertz laser”, Nat. Photonics 6, 525 (2012).

4. Quantum metrology: Phase locking and absolute frequency measurements in the far-infrared

We demonstrate phase-locking of a 2.5 THz quantum cascade laser to a free-space comb, generated in a LiNbOwaveguide and covering the 0.1–6 THz frequency range. We show that even a small fraction (< 100 nW) of the radiation emitted from the quantum cascade laser is sufficient to generate a beat note suitable for phase-locking to the comb, paving the way to novel metrological-grade terahertz applications, including high-resolution spectroscopy, manipulation of cold molecules, astronomy and telecommunications.

As a proof of concept application we developed a metrological-grade THz spectroscopic system based on the combination of a THz frequency-comb synthesizer (FCS) and a THz quantum cascade laser (QCL). The achieved absolute frequency scale provides an uncertainty of a few parts in 10−11 on the laser frequency and 10−9 on the linecenter determination, ranking this technique among the most precise ever developed in the THz range.

The scheme of the experimental setup is shown, with the diagram describing how the traceability of the primary Cs frequency standard is transferred to the THz QCL-based spectroscopy via the stabilization of the repetition rate (frep) of the pump laser and the THz FCS. The mechanism for tuning the QCL frequency (νQCL) is also sketched.

1. L. Consolino, A. Taschin, P. Bartolini, S. Bartalini, P. Cancio, A. Tredicucci, H.E. Beere, D.A. Ritchie, R. Torre, M.S. Vitiello & P. De Natale, “Phase-locking to a free-space terahertz comb for metrological-grade terahertz lasers” Nat. Commun. 3, 1040 (2012).2. S. Bartalini, M.S. Vitiello & P. De Natale, “Quantum cascade lasers: a versatile source for precise measurements in the mid/far-infrared range”Meas. Sci. Technol. 25, 012001 (2014).3. S. Bartalini, L. Consolino, P. Cancio, P. De Natale, P. Bartolini, A. Taschin, M. De Pas, H. Beere, D.A. Ritchie, M.S. Vitiello & R. Torre, “Frequency-Comb-Assisted Terahertz Quantum Cascade Laser Spectroscopy”Phys. Rev. X 4, 021006 (2014).