34.1 THz Cryo-CMOS Backscatter Transceiver: A Contactless 4 Kelvin-300 Kelvin Data Interface

Modern low-temperature large-scale systems, such as high-sensitivity IR/THz imaging arrays and quantum computers, require massive signal connections between the cooled system core and external room-temperature (RT) components. An error-protected quantum computer needs thousands or even millions of qubits operating at cryogenic temperature. Albeit the rapid development of highly-integrated quantum processors [1], future scalability may still be largely limited by the cables connecting the processors and peripheral control/processing units [2]; because even using low-thermal-conductivity metal materials, those cables still transport non-negligible amount of heat due to the large temperature gradient. In a dilution refrigerator, a stainless-steel UT-085-SS-SS cable could pose close to one mW of heat load to the 4K stage, and tens of mW to the 50K stage [2]. To drive such a non-copper cable in the cryogenic-to-RT (i.e. uplink) data path, standard I/O circuits in cooled sensors/processors typically consume pJ/b-level energy, further raising the overall heat load. Recently, optical fibers along with cryogenic InGaAs photodiode [3] and VCSEL lasers [4] are used for <20mK coherent qubit control/readout mode and 4K-to-300K uplink data mode, respectively. The energy consumption in [4] reduces to 406.2fJ/b at 10Gbps and
$< 10^{-12}$
bit-error-rate (BER). In this paper, a terahertz (THz) transceiver chip operating at 4K is presented as a CMOS-based, fully-integrable alternative to the interconnect solutions above. The carrier frequency of 260GHz, which is determined based on the trade-off between the antenna dimension and the efficiency of the transistors, is sufficiently high to minimize the link footprint and to avoid potential disturbance of qubits, such as the superconducting or nitrogen-vacancy ones, that typically operate at gigahertz; on the other hand, it also leads to much lower quantum noise
$(\sim \hbar\omega)$
compared to that in photonic links. The transceiver avoids the power-hungry terahertz (THz) generation by adopting a passive backscatter communication scheme (Fig. 34.1.1), and a 4Gbps uplink is demonstrated with only 176fJ/b added heat load. In the downlink, the heat load is further reduced to 34fJ/b at 4.4Gbps. This fully contactless 4K-RT interface can be used to deliver digitized control/readout data, and even some analog/RF signals such as low phase noise clocks.