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Coherent microwave transmission using a nonrefrigerated waveguide at room temperature

Montasir Qasymeh, Hichem Eleuch

Published Jul 26, 2021• 0 citations
OpenAlex

Abstract

Coherent quantum microwave transmission is key to realizing modular superconducting quantum computers and distributed quantum networks. However, a large number of incoherent photons are thermally generated in the microwave frequency spectrum. Hence, coherent transmission of microwave fields has long been believed to be infeasible without refrigeration. In this work, we propose a novel method for coherent microwave transmission using a typical microwave waveguide at room temperature. The proposed scheme considers two cryogenic nodes (i.e., a transmitter and a receiver) connected by a room-temperature microwave waveguide. At the receiver side, we implement a cryogenic loop antenna coupled to an LC harmonic oscillator inside the output port of the waveguide, while the LC harmonic oscillator is located outside the waveguide. The loop antenna converts the quantum microwave fields (which contain both signal and thermal noise photons) to a quantum voltage across the coupled LC harmonic oscillator. We show that by properly designing the loop antenna, the number of detected noise photons can be significantly less than one. Simultaneously, the detected signal photons can be maintained at a sufficient number greater than one by transmitting a proper number of photons at the input port of the waveguide. For example, we show that for a 10 GHz microwave signal, when using a room-temperature transmission waveguide of 5m length, 35 coherent photons are detected across the LC circuit by transmitting 32x10^4 signal photons at the input port of the waveguide. Interestingly, the number of detected noise photons is maintained as small as 6.3x10^-3. The microwave transmission scheme proposed in this work paves the way towards realizing practical modular quantum computers with a simple architecture.

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Coherent quantum microwave transmission is key to realizing modular superconducting quantum computers and distributed quantum networks. However, a large number of incoherent photons are thermally generated in the microwave frequency spectrum. Hence, coherent transmission of microwave fields has long been believed to be infeasible without refrigeration. In this work, we propose a novel method for coherent microwave transmission using a typical microwave waveguide at room temperature. The proposed scheme considers two cryogenic nodes (i.e., a transmitter and a receiver) connected by a room-temperature microwave waveguide. At the receiver side, we implement a cryogenic loop antenna coupled to an LC harmonic oscillator inside the output port of the waveguide, while the LC harmonic oscillator is located outside the waveguide. The loop antenna converts the quantum microwave fields (which contain both signal and thermal noise photons) to a quantum voltage across the coupled LC harmonic oscillator. We show that by properly designing the loop antenna, the number of detected noise photons can be significantly less than one. Simultaneously, the detected signal photons can be maintained at a sufficient number greater than one by transmitting a proper number of photons at the input port of the waveguide. For example, we show that for a 10 GHz microwave signal, when using a room-temperature transmission waveguide of 5m length, 35 coherent photons are detected across the LC circuit by transmitting 32x10^4 signal photons at the input port of the waveguide. Interestingly, the number of detected noise photons is maintained as small as 6.3x10^-3. The microwave transmission scheme proposed in this work paves the way towards realizing practical modular quantum computers with a simple architecture.
Quantum microwave transmission is key to realizing modular superconducting quantum computers and distributed quantum networks. A large number of incoherent photons are thermally generated within the microwave frequency spectrum. The closeness of the transmitted quantum state to the source-generated quantum state at the input of the transmission link (measured by the transmission fidelity) degrades due to the presence of the incoherent photons. Hence, high-fidelity quantum microwave transmission has long been considered to be infeasible without refrigeration. In this study, we propose a novel method for high-fidelity quantum microwave transmission using a room-temperature lossy waveguide. The proposed scheme consists of connecting two cryogenic nodes (i.e., a transmitter and a receiver) by the room-temperature lossy microwave waveguide. First, cryogenic preamplification is implemented prior to transmission. Second, at the receiver side, a cryogenic loop antenna is placed inside the output port of the waveguide and coupled to an LC harmonic oscillator located outside the waveguide. The loop antenna converts quantum microwave fields to a quantum voltage across the coupled LC harmonic oscillator. Noise photons are induced across the LC oscillator including the source generated noise, the preamplification noise, the thermal occupation of the waveguide, and the fluctuation-dissipation noise. The loop antenna detector at the receiver is designed to extensively suppress the induced photons across the LC oscillator. The signal transmittance is maintained intact by providing significant preamplification gain. Our calculations show that high-fidelity quantum transmission (i.e., more than [Formula: see text]) is realized based on the proposed scheme for transmission distances reaching 100 m.
Since superconducting quantum computers use microwave signals to control and readout their qubits, microwave transmission lines and circuits that can be used under cryogenic conditions are required. For microwave transmission lines, thermal insulation is essential to enhance wiring density and to prevent parasitic heating that can lead to increased computational errors. Thus, here we design and characterize a scalable, thermally insulated microwave transmission line. The transmission line designed for superconducting qubit readout at 5 GHz band achieves both thermal insulation and low loss, by employing resonator-based impedance matching in a microstripline structure. A flexible cable including eight planar transmission lines was designed, fabricated and characterized using S-parameter measurement, demonstrating low crosstalk below 40dB and signal attenuation of -23.5±0.07dB; this coincides with electromagnetic simulations. These microwave properties remained stable across a wide temperature range from room temperature down to 4 K, including steep gradients. The integration of chip attenuators further reduces thermal noise and protects sensitive cryogenic components. Our approach offers a promising solution for compact, high-density microwave wiring using flat cables under thermal gradients, necessary for large scale quantum computing.
With the rapid development of superconducting quantum computing and the implementation of surface code, large-scale quantum computing is emerging as an urgent demand. In a superconducting computing system, the qubit is maintained in a cryogenic environment to avoid thermal excitation. Thus, the transmission of control signals, which are generated at room temperature, is needed. Typically, the transmission of these signals to the qubit relies on a coaxial cable wiring approach. However, in a large-scale computing system with hundreds or even thousands of qubits, the coaxial cables will pose great space and heat load to the dilution refrigerator. Here, to tackle this problem, we propose and demonstrate a direct-modulation-based optical transmission line. In our experiment, the average single-qubit XEB error and control error are measured as 0.139% and 0.014% separately, demonstrating the feasibility of the optical wiring approach and paving the way for large-scale superconducting quantum computing.
Quantum illumination is a sensing technique that employs entangled signal-idler beams to improve the detection efficiency of low-reflectivity objects in environments with large thermal noise. The advantage over classical strategies is evident at low signal brightness, a feature which could make the protocol an ideal prototype for non-invasive scanning or low-power short-range radar. Here we experimentally investigate the concept of quantum illumination at microwave frequencies, by generating entangled fields using a Josephson parametric converter which are then amplified to illuminate a room-temperature object at a distance of 1 meter. Starting from experimental data, we simulate the case of perfect idler photon number detection, which results in a quantum advantage compared to the relative classical benchmark. Our results highlight the opportunities and challenges on the way towards a first room-temperature application of microwave quantum circuits.
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.
Quantum technologies promise a radically new way to solve classically intractable computing problems. Superconducting circuits as a platform are at the forefront of this field. The cryogenic operation temperatures of superconducting circuits however impose challenges for the further scaling to many connected quantum information processing units into a local area or global network. In this work, we present a hardware solution for connecting quantum devices operating at microwave frequencies into local area networks, which enable the exchange of quantum information between spatially separated parties. Specifically, we demonstrate a modular system spanning distances of 5, 10 and 30 meters operated at cryogenic temperatures and connecting two superconducting circuit systems, located in individual dilution refrigerators, through a quantum communication channel. We develop a thermal model to evaluate the heat transfer processes in the setup, optimize the design and select appropriate materials for its construction. The assembled 30-meter-long system achieves operating temperatures of below 50 mK after a cooldown time of about six and a half days. This link enables the execution of distributed quantum computing and communication algorithms. It also adds the resource of non-locality, certified by a loophole-free Bell test, to the field of quantum science and technology with superconducting circuits.