Abstract Encoding information onto optical fields using electro-optical modulation is the backbone of modern telecommunication networks, offering vast bandwidth and low-loss transport via optical fibers. For these reasons, optical fibers are also replacing electrical cables for short range communications within data centers. Compared to electrical coaxial cables, optical fibers also introduce two orders of magnitude smaller heat load from room to milli-Kelvin temperatures, making optical interconnects based on electro-optical modulation an attractive candidate for interfacing superconducting quantum circuits and hybrid superconducting devices. Yet, little is known about optical modulation at cryogenic temperatures. Here we demonstrate a proof-of-principle cryogenic electro-optical interconnect, showing that currently employed Ti-doped lithium niobate phase modulators are compatible with operation down to 800mK ---below the typical operation temperature of conventional microwave amplifiers based on high electron mobility transistors (HEMTs)---and maintain their room temperature Pockels coefficient. We utilize cryogenic electro-optical modulation to perform spectroscopy of a superconducting circuit optomechanical system, measuring optomechanically induced transparency (OMIT). In addition, we encode thermomechanical sidebands from the microwave domain onto an optical signal processed at room temperature. Although the currently achieved noise figure is significantly higher than that of a typical HEMT, substantial noise reduction should be attainable by harnessing recent advances in integrated modulators, by increasing the modulator length, or by using materials with a higher electro-optic coefficient, leading to noise levels on par with HEMTs. Our work highlights the potential of electro-optical modulators for massively parallel readout for emerging quantum computing or cryogenic classical computing platforms.
Abstract Encoding information onto optical fields using electro-optical modulation is the backbone of modern telecommunication networks, offering vast bandwidth and low-loss transport via optical fibers. For these reasons, optical fibers are also replacing electrical cables for short range communications within data centers. Compared to electrical coaxial cables, optical fibers also introduce two orders of magnitude smaller heat load from room to milli-Kelvin temperatures, making optical interconnects based on electro-optical modulation an attractive candidate for interfacing superconducting quantum circuits and hybrid superconducting devices. Yet, little is known about optical modulation at cryogenic temperatures. Here we demonstrate a proof-of-principle cryogenic electro-optical interconnect, showing that currently employed Ti-doped lithium niobate phase modulators are compatible with operation down to 800mK ---below the typical operation temperature of conventional microwave amplifiers based on high electron mobility transistors (HEMTs)---and maintain their room temperature Pockels coefficient. We utilize cryogenic electro-optical modulation to perform spectroscopy of a superconducting circuit optomechanical system, measuring optomechanically induced transparency (OMIT). In addition, we encode thermomechanical sidebands from the microwave domain onto an optical signal processed at room temperature. Although the currently achieved noise figure is significantly higher than that of a typical HEMT, substantial noise reduction should be attainable by harnessing recent advances in integrated modulators, by increasing the modulator length, or by using materials with a higher electro-optic coefficient, leading to noise levels on par with HEMTs. Our work highlights the potential of electro-optical modulators for massively parallel readout for emerging quantum computing or cryogenic classical computing platforms.
Quantum computing is a rapidly evolving field with potential for solving complex problems beyond the reach of classical computers. Scalability in qubit design and signal processing architecture within cryostats is necessary for taking advantage of quantum computing's potential. Current state of the art qubit technology requires both a feed and readout line for each qubit within a quantum processor, with these lines generally being metallic coaxial cables. For stainless steel cables this represents a significant heat load into the cryostat. An alternative approach involves using optical fibers for signal transmission, as optical fibers have a thermal conductivity two orders of magnitude lower than stainless steel coaxial cables. The proposed method for optical signal transmission relies on electro-optic modulation, where a voltage applied to an electro-optic material changes its refractive index, allowing for a controllable phase change to be applied to any light passing through the material. This concept can be combined with on-chip photonic waveguide technology to build fiber-coupled phase modulators as well as intensity modulators using on chip interferometers. The proposed architecture uses a room temperature intensity modulator to encode microwave signals onto a laser signal, which is recovered at cryogenic temperatures using a photodiode and used as input for a device under test. The output of the test device is encoded onto light using a phase modulator and recovered at room temperature through interferometry. Proof-of-concept experiments have been carried out at room temperature, including noise characterization and preliminary tests of components at cryogenic temperatures.
Radio frequency (RF) radiation is a powerful tool in computing, communication, and sensing due to the ability to engineer strong electrical interactions. Within cryogenic systems, in particular, microwave (MW) frequency superconducting qubits have demonstrated promise as a platform for quantum computing, and MW and millimeter-wave (mmW) systems have been valuable tools in sensing. However, owing to their low energy, the weak, single-photon level signals employed by these systems suffer high sensitivity to thermal noise when they exit the thermal bath, as well as short attenuation lengths due to absorption and scattering in cables. Telecommunication frequency optical photons, meanwhile, have long propagation lengths and can be supported by low-loss optical fibers. The high photon energy makes signals insensitive to thermal noise, and the high frequency can allow many RF signals to be multiplexed onto a single optical carrier. Interfacing RF and optical radiation to leverage both strong interactions with macroscopic systems characteristic of electrical interactions and low-loss propagation characteristic of optical photons has been critical to realizing our global communication system. Leveraging the infrastructure developed to support this industry, such as single photon detectors, is attractive for scaling cryogenic RF systems. Coherent conversion processes, especially, can benefit a wide range of applications, especially in computing and sensing, by preserving quantum states, and retaining phase information necessary for quantum gates, and enable the use of measurement techniques that can increase detection sensitivity. This dissertation focuses on using the electro-optic effect in thin-film lithium niobate to form an efficient, coherent interface between radio- and telecommunication-frequency photons. Lithium niobate is an attractive platform due to its high electro-optic coefficient (approx. 30 pm/V), wide transparency window (DC to 400 THz), and ability to support low-loss optical waveguides (.2 dB/m), all of which deviate little from room temperature down to near-zero Kelvin environments. First, I explore the development of low-loss lithium niobate optical cavities and operation of lithium niobate waveguides at low temperatures. While lithium niobate is an electro-optic material with a DC response, using the electro-optic response at room temperature for stable biasing and low-frequency operation has remained a challenge. A temperature- and frequency-dependent study is conducted to identify lithium charge migration as a potential screening mechanism suppressed at low temperatures. Next, I explore the use of RF-cavity enhanced, triply resonant photonic molecules (created from two coupled low-loss optical cavities) for forming efficient electro-optic interfaces with microwave (3-10 GHz) and millimeter wave (100 GHz) radiation. In the microwave regime, I explore microwave-optical transduction for forming optical interfaces with microwave qubits, specifically focusing on superconducting qubits. The characterization of one such transducer is demonstrated, and the device is used to optically synthesize microwave signals at low temperatures, bypassing the need for input microwave control lines. Outlook for other interfaces, including optical photon mediated quantum information transfer, is discussed. At mmW frequencies, the triply resonant photonic molecule is explored for low-power ( mW) optical-frequency shifting.
In recent years, standard CMOS microprocessors have approached their maximum power dissipation per unit area, effectively placing a limit on computational power. This highlights the urgent need to explore alternative technologies. One promising avenue is the use of superconductors, which demonstrate zero resistivity below a critical temperature. However, circuits based on superconductors necessitate the use of cryostats to maintain low temperatures, presenting challenges in data transfer with the room temperature environment. While coaxial cables are often employed for this purpose, they suffer from limited data transfer rates and contribute significantly to heat load. On the contrary, photonics integrated circuits (PICs) coupled with optical fibers present a viable solution. They enable scalable, cost-effective, and power-efficient optical interconnections capable of supporting high data transfer rates while minimizing heat transfer. In this presentation, We will discuss the latest advancements in cryogenic PICs, focusing on their application in interfacing with cryogenic computing systems such as single-flux-quantum logic circuits and superconducting qubits.
Electro-optic modulation is central to classical optical communications and emerging quantum technologies. High-confinement optomechanical crystal modulators enable microwave-optical transduction through strong optomechanical interactions and offer a promising interface between superconducting qubits and optical fibers. However, their performance is limited by thermal noise from optical absorption. Release-free optomechanical crystals provide improved thermal anchoring but have not yet been integrated into a microwave-optical transducer. Here, we demonstrate a release-free electro-optomechanical transducer combining strong optomechanical interactions in silicon with the efficient piezoelectricity of lithium niobate via micro-transfer printing. We observe electro- and optomechanical coupling rates compatible with quantum-level operation when co-integrated with a superconducting microwave circuit. This advance moves release-free electro-optomechanical devices toward practical microwave-optical interfaces.
Scalable, high speed data transfer between cryogenic (0.1-4 K) and room temperature environments is instrumental in a broad range of fields including quantum computing, superconducting electronics, single photon imaging and space-based communications. A promising approach to overcome the limitations of conventional wire-based readout is the use of optical fiber communication. Optical fiber presents a 100-1,000x lower heat load than conventional electrical wiring, relaxing the requirements for thermal anchoring, and is also immune to electromagnetic interference, which allows routing of sensitive signals with improved robustness to noise and crosstalk. Most importantly, optical fibers allow for very high bandwidth densities (in the Tbps/mm2 range) by carrying multiple signals through the same physical fiber (Wavelength Division Multiplexing, WDM). Here, we demonstrate for the first time optical readout of a superconducting nanowire single-photon detector (SNSPD) directly coupled to a CMOS photonic modulator, without the need for an interfacing device. By operating the modulator in the forward bias regime at a temperature of 3.6 K, we achieve very high modulation efficiency (1000-10,000 pm/V) and a low input impedance of 500 Ω with a low power dissipation of 40 μW. This allows us to obtain optical modulation with the low, millivolt-level signal generated by the SNSPD. Optical communication is becoming the preferred I/O solution in modern, room temperature high performance systems, and this work demonstrates its suitability for scalable cryogenic readout, which could help achieve the full potential of superconducting technologies.
Superconducting nanowire single-photon detectors have become indispensable for experiments in quantum optics and photonic quantum information processing. Nevertheless, these detectors require temperatures below 4 K to operate. These devices typically require coaxial cables as electrical connections for biasing and readout. Replacing these electrical connections with optical fibers both isolates the SNSPD from noise in room-temperature control electronics and simultaneously achieves a connection with a low thermal load. Nevertheless, the optoelectronic interfaces required for this purpose must be fully compatible with the cryogenic operating conditions of the detector. Recently, optical readout of an SNSPD using an electro-optic modulator at cryogenic temperatures has been demonstrated [1]–[3]. In addition, we have shown an opto-electronic bias of an SNSPD by generating the bias current using a photodiode co-located on the cold stage of the cryostat [4]. Both methods replace elements of electrical connection for the SNSPD in the cryostat by an optical fiber. Here, we show the combination of the opto-electronic bias and optical read readout in a single device, which enables us to operate an SNSPD fully isolated from room temperature electronics.