Terahertz Quantum Cascade Lasers as Enabling Quantum Technology

Abstract

Quantum cascade lasers (QCLs) represent the most fascinating achievement of quantum engineering, showing how artificial materials can be generated through quantum design, with tailor-made properties. Their inherent quantum nature deeply affects their core physical parameters. QCLs indeed display intrinsic linewidths approaching the quantum limit, and show spontaneous phase-locking of their emitted modes via intracavity four-wave-mixing, meaning that they can naturally operate as miniaturized metrological frequency rulers, also in frontier frequency domains, as the far-infrared, yet unexplored in quantum science. Here, the authors discuss the fundamental quantum properties of QCLs operating at terahertz frequencies and their key technological performances, highlighting future perspectives of this frontier research field in disruptive areas of quantum technologies such as quantum sensing, quantum metrology, quantum imaging, and photonic-based quantum computation.

All in One‐Chip, Electrolyte‐Gated Graphene Amplitude Modulator, Saturable Absorber Mirror and Metrological Frequency‐Tuner in the 2–5 THz Range

“…Such a demand is actually extending also to the far-IR or terahertz (THz) frequency range of the electromagnetic spectrum (0.1-10 THz, 3000 -30 µm), an emerging frontier research field in quantum science.In this technologically appealing frequency domain, high-resolution spectroscopic systems for molecular sensing and metrology applications [13,14] might also highly benefit from the development of efficient, tunable modulators capable of amplitude, [1,2] frequency, [6] and phase stabilization [4,5] of miniaturized metrological sources, as the recently emerged quantum cascade laser (QCL) frequency combs (FCs). [15,16] QCLs can indeed support very high modulation rates (up to tens of GHz), [17,18] through direct modulation of their operating current, [19] although at the price of current instabilities, [20,21] or spurious amplitude and frequency self-modulation, [20,21] detrimental for quantum applications requiring a tight control of the optical phase and frequency jitter. Electro-optical modulators, possibly integrated on-chip, are therefore highly desirable in this context.In the last decade, different approaches, based on III-V semiconductors as silicon and gallium arsenide, or employing two-dimensional (2D) electron gas in AlGaAs/InGaAs heterostructures, have been adopted to devise THz-frequency modulators, with modulation speeds up to 14 GHz.…”

“…In this technologically appealing frequency domain, high-resolution spectroscopic systems for molecular sensing and metrology applications [13,14] might also highly benefit from the development of efficient, tunable modulators capable of amplitude, [1,2] frequency, [6] and phase stabilization [4,5] of miniaturized metrological sources, as the recently emerged quantum cascade laser (QCL) frequency combs (FCs). [15,16] QCLs can indeed support very high modulation rates (up to tens of GHz), [17,18] through direct modulation of their operating current, [19] although at the price of current instabilities, [20,21] or spurious amplitude and frequency self-modulation, [20,21] detrimental for quantum applications requiring a tight control of the optical phase and frequency jitter. Electro-optical modulators, possibly integrated on-chip, are therefore highly desirable in this context.…”

Terahertz Sources Based on Metrological‐Grade Frequency Combs

“…[1] Being inherently narrow linewidth, these light sources allow for absolute frequency measurements throughout the electromagnetic spectrum and have proved to be an appealing technology in quantum science. [2] This is especially true in quantum sensing and metrology, [3,4] thanks to the resonance with rotational molecular energy levels, and in quantum communication, in view of wireless implementations of quantum key distribution. [5] Widely developed in the visible, [4] nearinfrared, [6] and mid-infrared [7] regions of the electromagnetic spectrum, FCs conventionally rely on a chain of optical components, which limits their usability and potential for on-chip integration.…”

“…The recent observation of spontaneous FC generation in electrically pumped quantum cascade lasers (QCLs) [8,9] has stimulated broad research efforts in the development of chip-scale FCs operating in the still mostly unused terahertz (THz) frequency range, which is strategic for future quantum applications. [2] Unlike conventional FCs that rely on the emission of short pulses from mode locked sources, QCL combs produce a generally frequency-modulated (FM) output with an almost constant intensity and a linear frequency chirp, [10] due to the large nonlinear susceptibility of the semiconductor heterostructure gain medium. At THz frequencies (2-5 THz), frequency and amplitude modulation act simultaneously [11][12][13] as an effect of the four wave mixing process generated by either fast saturable gain, [10] which controls the frequency modulation, or by loss, [9,13] which governs amplitude modulation.…”

Design and Performance of Extraordinary Low-Cost Compact Terahertz Imaging System Based on Electronic Components and Paraffin Wax Optics

“…Electronic multipliers are more compact and less power consuming and, thus, can be more convenient in THz imaging experiments, particularly in the sub-THz range [ 5 , 6 ]. The next step in THz emitter miniaturization can be attributed to the handling of quantum cascade lasers [ 7 ] in imaging systems; however, due to the requirements of effective operation at room temperature, only intracavity mixing schemes in quantum cascade structures [ 8 , 9 ] can be a rational way to proceed in the direct implementation as the highest operating temperature using conventional cascade generation schemes can reach a maximum of 250 K currently [ 10 ]. A compact and low-priced solution in passive optics can be realized by replacing the bulky parabolic or spherical mirrors with relevant diffractive optic components [ 11 ] or metamaterial-based optical elements [ 12 ].…”