While nonclassical light sources are fundamental to quantum communication and computing, solid-state platforms like color centers and quantum dots require cryogenic temperatures to reach the performance levels necessary for practical applications. Over the past decade, low-temperature engineering has transitioned from manual handling of liquid cryogens to automated closed-cycle cryostats. This review details the principles behind modern cooling hardware ranging from flow cryostats to mechanical cryocoolers and dilution refrigerators, with a specific focus on the requirements of optical quantum devices. Aimed at the practicing scientist, this overview provides the technical insights and historical context needed to navigate the current cryogenic landscape and evaluate its role in the future of quantum technology deployment.
While nonclassical light sources are fundamental to quantum communication and computing, solid-state platforms like color centers and quantum dots require cryogenic temperatures to reach the performance levels necessary for practical applications. Over the past decade, low-temperature engineering has transitioned from manual handling of liquid cryogens to automated closed-cycle cryostats. This review details the principles behind modern cooling hardware ranging from flow cryostats to mechanical cryocoolers and dilution refrigerators, with a specific focus on the requirements of optical quantum devices. Aimed at the practicing scientist, this overview provides the technical insights and historical context needed to navigate the current cryogenic landscape and evaluate its role in the future of quantum technology deployment.
The terahertz (THz) regime of electromagnetic radiation, spanning frequencies from 0.1 to tens of THz, has found widespread use in communication, sensing, quality control, and biomedical applications. Quantum realm of THz domain is, however, still at an earlier development stage and mostly referenced in the context of applied technologies for single-photon generation and detection, with rarer examples of exploration of quantum phenomena. Recent advances in cryogenic hybrid quantum systems suggest that THz radiation may play a pivotal role in emerging quantum technologies. These systems, which require operation at milliKelvin temperatures, offer new opportunities for quantum memory, frequency transduction, and precision measurements, with THz radiation enabling access to molecular vibrational, rotational, and librational transitions. Despite progress in THz components at low temperatures, there remains a critical gap in the systematic integration of THz technologies within sub-Kelvin cryogenic platforms such as dilution refrigerators and adiabatic demagnetization systems. This review presents a comprehensive roadmap for the development of THz photonics in extreme cryogenic environments, addressing implementation strategies, material considerations, waveguide and fiber technologies, and quantum-compatible detection schemes. We further highlight prospective applications in quantum optics, superconductivity, and fundamental physics, aiming to establish THz photonics as a versatile tool for controlling and probing quantum states in sub-Kelvin regimes.
Quantum light and in particular single photons have become essential resources for a growing number of quantum applications including quantum computing, quantum key distribution and quantum metrology. Solid-state atomlike systems such as semiconductor quantum dots and color defects in crystals have become the hallmark of highly pure single photon emitters in the past two decades. A particular interest has been developed in nanocrystal quantum dots (NQDs) and color centers in diamond as potential compact room-temperature emitters. There are however several challenges that inhibit the use of such sources in current technologies including low photon extraction efficiency, low emission rates and relatively low single photon purities. In this work we will review our efforts in overcoming these technical difficulties using several complementary methods including designing several nanoantenna devices that enhance the directionality and emission rate of the nanoemitter. In addition, we developed several temporal heralding techniques to overcome the hurdle of low single photon purity in NQDs in an effort to produce a highly pure, bright and efficient single photon source on-chip.
The terahertz (THz) regime of electromagnetic radiation, spanning frequencies from 0.1 to tens of THz, has found widespread use in communication, sensing, quality control, and biomedical applications. Quantum realm of THz domain is, however, still at an earlier development stage and mostly referenced in the context of applied technologies for single-photon generation and detection, with rarer examples of exploration of quantum phenomena. Recent advances in cryogenic hybrid quantum systems suggest that THz radiation may play a pivotal role in emerging quantum technologies. These systems, which require operation at milliKelvin temperatures, offer new opportunities for quantum memory, frequency transduction, and precision measurements, with THz radiation enabling access to molecular vibrational, rotational, and librational transitions. Despite progress in THz components at low temperatures, there remains a critical gap in the systematic integration of THz technologies within sub-Kelvin cryogenic platforms such as dilution refrigerators and adiabatic demagnetization systems. This review presents a comprehensive roadmap for the development of THz photonics in extreme cryogenic environments, addressing implementation strategies, material considerations, waveguide and fiber technologies, and quantum-compatible detection schemes. We further highlight prospective applications in quantum optics, superconductivity, and fundamental physics, aiming to establish THz photonics as a versatile tool for controlling and probing quantum states in sub-Kelvin regimes.
Quantum computing promises an exponential speed-up of computation compared to what is nowadays achievable with classical computers. In this way, it enables the evaluation of more complex models and the breaching of current security algorithms. For the operation of a quantum system, many questions remain to be answered. Currently, there are several quantum technologies that promise to be both reliable and scalable, two features required for large scale quantum operations. Common to all technologies is the operating temperature that needs to be close to absolute zero, i.e. below 100 mK, to suppress environment noise and allow the quantum properties to become 'visible'. In order for any quantum processor to be operated, a so-called quantum-classical interface is required for the quantum bit (qubit) read-out and control. This interface consists of various electronic blocks, such as analog-to-digital converters, digital-to-analog converters, mixers, amplifiers and a digital controller. Especially the analog blocks require effort to meet the noise and stability constraints as not to disturb the very sensitive qubits and allow reading of the tiny signals. As the qubits live in extremely deep-cryogenic temperatures, long wires interface the cryogenic with the room temperature environment, where most of the electronics is situated. However, for a scalable system, heat injection becomes a serious problem, with many wires between 300 K and sub-Kelvin. Furthermore, such amount of interconnects is challenging to mechanically place in a dilution refrigerator. Therefore, in this work, we propose to implement the electronics not at room temperature, but at a temperature much closer to the qubits, for example at 4 K. This not only reduces significantly the wiring between room temperature and the qubits, but we can also benefit from lower electronic noise at such a temperature. The operation of conventional electronics almost 200°C below its normal temperature range is not trivial as device properties alter significantly and most circuits no longer operate as intended. In CMOS processes, the main technology in the integrated electronics world, the behaviour of the transistors, required for the implementation of any circuit, deviates considerably at low temperatures. The transistor's threshold voltage goes up, the mobility increases and the subthreshold slope becomes steeper, to name just a few of these deviations. Although there are improvements in performance, there are also some counter effects, and characterization of the transistors is needed to observe the changes. Once devices are characterized at such low temperatures, new models can be built and circuits can be simulated and adapted to operate properly at cryogenic temperatures. Luckily, there are various commercially available devices that can already withstand the chills of cold. We demonstrated various commercially available devices, such as a field-programmable gate array (FPGA) implemented in a 28 nm CMOS process, to be operating without major concerns at 4~K. Its properties alter only slightly, within 5 to 10%, and all tested circuit implementations, working at 300~K, also worked at 4 K. We combined both commercially available devices, that operate 200 K below their specified temperature range, with custom designed CMOS circuits to implement a cryogenic read-out platform for spin qubits. This system comprises amplifiers, an ADC, an FPGA, voltage regulators and a clock generator. It allows to amplify the tiny signal from the qubits and digitize it directly in the FPGA in order to process the data locally at 4~K. This system is one of the first systematic attempts at operating a part of the quantum-classical interface at cryogenic temperatures and forms the basis for future systems comprising the complete electronic interface for qubits to operate at such low temperatures. One of the main problems to tackle for cryogenic electronic systems is their power consumption. Power budgets are simply limited to roughly 1 or 2 Watts at 4 K and exponentially lower at deeper cryogenic temperatures, thus limiting the size of large-scale electronic systems. Our approach of combined commercial and custom circuits will have to be steadily replaced by a single (custom) technology that meets both power and scalability constraints. One of the best candidates is CMOS, a technology that the industry has relied upon for several decades and benefits from many optimizations thanks to Moore's law.
Quantum computing holds the potential to solve problems beyond the capabilities of classical computers. However, realizing practical large-scale quantum computers faces critical challenges, including the key bottleneck in wiring quantum processors operating at cryogenic temperatures to their room temperature control electronics. This article explores cryo-CMOS ICs as an enabling technology to circumvent such a wiring bottleneck. After reviewing the basics of quantum computing and the behavior of CMOS devices at cryogenic temperatures, the requirements for the interface electronics for superconducting and semiconductor qubits are discussed, and state-of-the-art cryo-CMOS circuit solutions are briefly described, including a few hints to other qubit platforms. Finally, the article concludes with an overview of future challenges to be addressed to realize the vision of cryo-CMOS technology as an enabler of large-scale quantum computers with practical applications.
Quantum communication technologies are starting to develop into a rising industry branch of their own. The associated light sources and detectors/sensorics mostly require cryogenic cooling. In order to scale up this new industry and make it accessible for commercial use, new compact, autonomous, and low power cryogenics are a vital key condition. We have developed a revolutionary compressor technology together with high-performance cryogenic cooling systems, that operate on low power consumption, without the use of cooling water, and in a quantum industry compatible 19” rack format. We will show how this enables critical building blocks of photonic based quantum computing and communication technologies such as single photon sources & single photon detectors to be made truly scaleable outside of laboratories. First use cases together with detailed performance data will be reviewed.