The quantum bits (qubits) on which superconducting quantum computers are based have energy scales corresponding to photons with GHz frequencies. The energy of photons in the gigahertz domain is too low to allow transmission through the noisy room-temperature environment, where the signal would be lost in thermal noise. Optical photons, on the other hand, have much higher energies, and signals can be detected using highly efficient single-photon detectors. Transduction from microwave to optical frequencies is therefore a potential enabling technology for quantum devices. High conversion efficiency and high fidelity (the similarity of input microwave state to the output optical state.) are important characteristics of these devices. In such devices, the efficiency of the up-conversion process is typically improved by increasing the power in the pump. In addition to this, the optical pump is dissipated as thermal photons via absorption-induced heating into the system. Since the frequency of thermal photons and qubits are in the same range, this can affect the quality of the transferred quantum state which is characterized by fidelity. Thus, the optical pump can be a source of thermal noise and degrade the fidelity. Realizing an optimum pump power level while investigating its effect on fidelity via temperature are important tasks in the evolution of these upconversion devices. In this work, we explore two kinds of hybrid quantum devices that can be used as frequency upconverts; the first one is a resonant electro-optic modulator consisting of a microwave cavity and a WGM resonator (made of LiNbO3) and the next one is a magneto-optic modulator consisting of a microwave cavity and the rare earth crystal gadolinium vanadate (GdVO4) in the cuboid shape. We model the sub-Kelvin thermal behavior of an electro-optic transducer based on a lithium niobate whispering gallery mode resonator. We compute thermal photon numbers and find that there is an optimum power level for a continuous pump, whilst pulsed operation of the pump increases the fidelity of the conversion. In the magneto-optic modulator via photon-magnon coupling, we investigate the effect of GdVO4 temperature on the linewidth of magnons and the strength of photon-magnon coupling inside a dilution refrigerator. In the next step, we investigate the effect of absorption-induced heating of GdVO4 on the linewidth of magnons and the strength of photon-magnon coupling inside a dilution refrigerator, and the temperature of GdVO4 is estimated via comparison of two methods. The effect of heating of the GdVO4 via the mentioned methods on the transmission spectrum and the resonance frequency of the microwave cavity is studied. The transmission spectrum is used to comprehend the time evolution of the thermal equilibrium of the sample and estimate its temperature.
The quantum bits (qubits) on which superconducting quantum computers are based have energy scales corresponding to photons with GHz frequencies. The energy of photons in the gigahertz domain is too low to allow transmission through the noisy room-temperature environment, where the signal would be lost in thermal noise. Optical photons, on the other hand, have much higher energies, and signals can be detected using highly efficient single-photon detectors. Transduction from microwave to optical frequencies is therefore a potential enabling technology for quantum devices. High conversion efficiency and high fidelity (the similarity of input microwave state to the output optical state.) are important characteristics of these devices. In such devices, the efficiency of the up-conversion process is typically improved by increasing the power in the pump. In addition to this, the optical pump is dissipated as thermal photons via absorption-induced heating into the system. Since the frequency of thermal photons and qubits are in the same range, this can affect the quality of the transferred quantum state which is characterized by fidelity. Thus, the optical pump can be a source of thermal noise and degrade the fidelity. Realizing an optimum pump power level while investigating its effect on fidelity via temperature are important tasks in the evolution of these upconversion devices. In this work, we explore two kinds of hybrid quantum devices that can be used as frequency upconverts; the first one is a resonant electro-optic modulator consisting of a microwave cavity and a WGM resonator (made of LiNbO3) and the next one is a magneto-optic modulator consisting of a microwave cavity and the rare earth crystal gadolinium vanadate (GdVO4) in the cuboid shape. We model the sub-Kelvin thermal behavior of an electro-optic transducer based on a lithium niobate whispering gallery mode resonator. We compute thermal photon numbers and find that there is an optimum power level for a continuous pump, whilst pulsed operation of the pump increases the fidelity of the conversion. In the magneto-optic modulator via photon-magnon coupling, we investigate the effect of GdVO4 temperature on the linewidth of magnons and the strength of photon-magnon coupling inside a dilution refrigerator. In the next step, we investigate the effect of absorption-induced heating of GdVO4 on the linewidth of magnons and the strength of photon-magnon coupling inside a dilution refrigerator, and the temperature of GdVO4 is estimated via comparison of two methods. The effect of heating of the GdVO4 via the mentioned methods on the transmission spectrum and the resonance frequency of the microwave cavity is studied. The transmission spectrum is used to comprehend the time evolution of the thermal equilibrium of the sample and estimate its temperature.
Abstract The quantum bits (qubits) on which superconducting quantum computers are based have energy scales corresponding to photons with GHz frequencies. The energy of photons in the gigahertz domain is too low to allow transmission through the noisy room-temperature environment, where the signal would be lost in thermal noise. Optical photons, on the other hand, have much higher energies, and signals can be detected using highly efficient single-photon detectors. Transduction from microwave to optical frequencies is therefore a potential enabling technology for quantum devices. However, in such a device the optical pump can be a source of thermal noise and thus degrade the fidelity; the similarity of input microwave state to the output optical state. In order to investigate the magnitude of this effect we model the sub-Kelvin thermal behavior of an electro-optic transducer based on a lithium niobate whispering gallery mode resonator. We find that there is an optimum power level for a continuous pump, whilst pulsed operation of the pump increases the fidelity of the conversion.
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.
The development of scalable quantum networks requires coherent interfaces capable of converting microwave photons used in superconducting quantum processors into optical photons suitable for long-distance fiber transmission. This review surveys recent progress in microwave-to-optical quantum transduction across optomechanical, electro-optic, and magneto-optic platforms, with emphasis on conversion efficiency, bandwidth, added noise, and operating temperature. In addition to standard metrics, we propose the internal efficiency eta_in and the magnon decay rate kappa_m/2pi as normalized parameters that enable fairer comparison across heterogeneous implementations. Optomechanical systems achieve internal phonon-to-photon efficiencies of 93% with sub-quantum added noise of 0.25 quanta at millikelvin temperatures. Electro-optic devices based on LiNbO3 and AlN have advanced from room-temperature efficiencies below 1% to millikelvin systems with internal efficiencies approaching 99.5%, added noise as low as 0.16 quanta at 60 mK, and bandwidths extending to several tens of megahertz. Magneto-optic (optomagnonic) platforms exhibit the lowest efficiencies (typically 10−10 to 10−8), but offer intrinsic non-reciprocity and broadband magnonic operation, with emerging approaches based on topological heterostructures and magnon squeezing predicting enhancements up to 10−4. Optomechanical systems appear promising for high-fidelity quantum state transfer, electro-optic transducers for high-bandwidth coherent links, and magneto-optic devices for non-reciprocal network components. We discuss the fundamental trade-off between efficiency and added noise across all three platforms, and argue that heterogeneous microwave-optical transduction is emerging as a key enabling technology for distributed quantum computing and large-scale quantum networks.
Superconducting qubits are currently one of the leading qubit designs, showing great success in some of the world's most powerful quantum computers. Superconducting qubits, however, communicate at microwave frequencies, which is problematic when it comes to the task of long distance communication as unless the communication channel is cryogenically cooled the microwave photons are swamped by thermal noise. Optical photons, on the other hand, are ideal for long distance communication, particularly at frequencies that lie within the telecommunications band. One solution to the superconducting qubits communication problem is to create a device that can convert an input microwave photon into an optical photon with unit quantum efficiency. A device of such could then be used alongside existing optical fibre networks to connect distant superconducting processors. Furthermore, an upconversion device would allow superconducting qubit systems to take advantage of quantum technologies in the optical region, such as optical quantum memories.\n\nSeveral different approaches have been taken in attempts to experimentally implement such a device. Recently a theoretical proposal showed that the strong non-linearities that occur near the collective resonances in fully concentrated rare-earth crystals can be used to efficiently upconvert microwave photons. In this thesis we begin experimental investigations of two fully concentrated rare-earth crystals, GdVO4 and DyPO4, for their application to microwave upconversion.\n\nAbsorption spectra of the two crystals in the optical region are measured using a Fourier transform infrared spectrometer, in order to characterise and explore the possible optical states to be used in the upconversion process. Experiments are then carried out at microwave frequencies to explore the coupling between the collective spin resonances (magnons) that occur within the crystals and a microwave cavity. It is found that not only can the ultrastrong coupling regime be reached in these systems but that the linewidth of the magnon mode is narrow, measuring 35MHz at T = 25mK. This is the first experimental evidence of ultrastrong coupling between a microwave cavity and antiferromagnetic magnon modes in a rare-earth crystal. Furthermore, narrow transition linewidths are a crucial requirement in the proposed upconversion method, therefore the narrow linewidth experimentally measured here is an important validation of the proposals feasibility. \n\nThe results presented in this thesis show that microwave upconversion mediated by fully concentrated rare-earth crystals has an exciting future. Perhaps one day rare-earth crystals will be used to overcome the major communication problem superconducting qubits currently face.
Transduction of quantum signals between the microwave and the optical ranges will unlock powerful hybrid quantum systems enabling information processing with superconducting qubits and low-noise quantum networking through optical photons. Most microwave-to-optical quantum transducers suffer from thermal noise due to pump absorption. We analyze the coupled thermal and wave dynamics in electro-optic transducers that use a two-step scheme based on an intermediate frequency state in the THz range. Our analysis, supported by numerical simulations, shows that the two-step scheme operating with a continuous pump offers near-unity external efficiency with a multiorder noise suppression compared to single-step transduction. As a result, two-step electro-optic transducers may enable quantum-noise-limited interfacing of superconducting quantum processors with optical channels at MHz-scale bit rates.
Achieving efficient microwave–optical transduction is a critical challenge for quantum networks where information is shared optically across disparate microwave systems and for coherent optical detection and manipulation of microwave states. Resonant electro-optic systems are being pursued as a means of efficiently bridging these frequencies. Here, we present a platform that employs a bulk lithium niobate crystal whose large dielectric constant creates wavelength-scale confinement of microwave photons. By incorporating this crystal within a high-finesse Fabry–Pérot optical cavity, microwave photons couple to optical photons through the electro-optic effect. We demonstrate the ability to tune our system into triply resonant operation, where microwave photons, optical pump photons, and upconverted optical photons are simultaneously resonant with high quality factor electromagnetic modes of the system. The device achieves photon-number conversion efficiency at the percent level, comparable to that of state-of-the-art devices at room temperature—sufficient to resolve the thermal occupation of the microwave mode—by taking advantage of the high-power handling, low loss, and low noise of all-dielectric devices. These results establish our all-dielectric devices as a promising platform for high-precision sensing of optically detected microwave fields and as a viable route toward single-photon-level microwave–optical quantum transduction.