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Nature Communications

Manipulations of a transmon qubit with a null-biased electro-optic fiber link

Wenqu Xu, Tingting Guo, Kaixuan Zhang, Zishuo Li, Tianshi Zhou, Quan Zuo, Yifan Sheng, Lingxiao Jing, Huashi Ma, Mingyuan Yu, Sining Zhou, Binglin Li, Shiyao Yang, Yongyang Yu, Junzhou Zhang, Jiyuan Zhu, Chunhai Cao, Guanghao Zhu, Guozhu Sun, Peiheng Wu

Published Mar 17, 2025• 4 citations
Publisher

Abstract

In recent years, significant progress has been made in the field of superconducting quantum circuits, particularly in improving the complexity of quantum processors for large-scale quantum computing and quantum simulation tasks. To enable the execution of quantum information processing tasks on large-scale quantum circuits containing millions of qubits, it is essential to minimize thermal effects on control and measurement lines, ensuring that circuit components are superconducting and that qubits are not significantly thermally excited. Recent studies have shown that a quadrature-biased electro-optic fiber link can operate qubits with a much reduced thermal load, thereby facilitating the simultaneous operation of a large number of qubits. Expanding on this, here we propose and demonstrate that coherent manipulations of superconducting qubits can also be achieved by setting the bias point of the electro-optic modulator at the null point instead of the quadrature point. Major advantages of our null-point bias method include further reduction of the thermal load and improvement of the signal-to-noise ratio, and relaxed requirement for experimental implementations. Simultaneous control of two qubits is also demonstrated using the proposed null-biased fiber-optic link, which is the first time to the best of our knowledge. Recent work demonstrated control and readout of a superconducting qubit using a quadrature-biased electro-optic modulator. Here the authors report an alternative approach by biasing the modulator at the null point, which offers design simplifications and performance advantages.

DOI
10.1038/s41467-025-57820-8

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In recent years, significant progress has been made in the field of superconducting quantum circuits, particularly in improving the complexity of quantum processors for large-scale quantum computing and quantum simulation tasks. To enable the execution of quantum information processing tasks on large-scale quantum circuits containing millions of qubits, it is essential to minimize thermal effects on control and measurement lines, ensuring that circuit components are superconducting and that qubits are not significantly thermally excited. Recent studies have shown that a quadrature-biased electro-optic fiber link can operate qubits with a much reduced thermal load, thereby facilitating the simultaneous operation of a large number of qubits. Expanding on this, here we propose and demonstrate that coherent manipulations of superconducting qubits can also be achieved by setting the bias point of the electro-optic modulator at the null point instead of the quadrature point. Major advantages of our null-point bias method include further reduction of the thermal load and improvement of the signal-to-noise ratio, and relaxed requirement for experimental implementations. Simultaneous control of two qubits is also demonstrated using the proposed null-biased fiber-optic link, which is the first time to the best of our knowledge. Recent work demonstrated control and readout of a superconducting qubit using a quadrature-biased electro-optic modulator. Here the authors report an alternative approach by biasing the modulator at the null point, which offers design simplifications and performance advantages.
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.
Entangling superconducting quantum processors via light would enable new means of secure communication and distributed quantum computing. However, transducing quantum signals between these disparate regimes of the electromagnetic spectrum remains an outstanding goal, and interfacing superconducting qubits with electro-optic transducers presents significant challenges due to the deleterious effects of optical photons on superconductors. Moreover, many remote entanglement protocols require multiple qubit gates both preceding and following the upconversion of the quantum state, and thus an ideal transducer should leave the state of the qubit unchanged: more precisely, the backaction from the transducer on the qubit should be minimal. Here we demonstrate non-destructive optical readout of a superconducting transmon qubit via a continuously operated electro-optic transducer. The modular nature of the transducer and circuit QED system used in this work enable complete isolation of the qubit from optical photons, and the backaction on the qubit from the transducer is less than that imparted by thermal radiation from the environment. Moderate improvements in transducer bandwidth and added noise will enable us to leverage the full suite of tools available in circuit QED to demonstrate transduction of non-classical signals from a superconducting qubit to the optical domain.
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.
Superconducting quantum circuits operate at millikelvin temperatures, typically requiring independent microwave cables for each qubit for connecting room-temperature control and readout electronics. However, scaling to large-scale processors hosting hundreds of qubits faces a severe input/output (I/O) bottleneck, as the dense cable arrays impose prohibitive constraints on physical footprint, thermal load, wiring complexity, and cost. Here we demonstrate a complete optical I/O architecture for superconducting quantum circuits, in which all control and readout signals are transmitted exclusively via optical photons. Employing a broadband traveling-wave Brillouin microwave-to-optical transducer, we achieve simultaneous frequency-multiplexed optical readout of two qubits. Combined with fiber-integrated photodiode arrays for control signal delivery, this closed-loop optical I/O introduces no measurable degradation to qubit coherence times, with an optically driven single-qubit gate fidelity showing only a 0.19% reduction relative to standard microwave operation. These results establish optical interconnects as a viable path toward large-scale superconducting quantum processors, and open the possibility of networking multiple superconducting quantum computers housed in separate dilution refrigerators through a centralized room-temperature control infrastructure.
Today's hundred-qubit quantum computers require a dramatic scale up to millions of qubits to become practical for solving real-world problems. Although a variety of qubit technologies have been demonstrated, scalability remains a major hurdle. Superconducting (SC) qubits are one of the most mature and promising technologies to overcome this challenge. However, these qubits reside in a millikelvin cryogenic dilution fridge, isolating them from thermal and electrical noise. They are controlled by a rack-full of external electronics through extremely complex wiring and cables. Although thousands of qubits can be fabricated on a single chip and cooled down to millikelvin temperatures, scaling up the control and readout electronics remains an elusive goal. This is mainly due to the limited available cooling power in cryogenic systems constraining the wiring capacity and cabling heat load management. In this article, we focus on scaling up the number of XY-control lines by using cryogenic RF-photonic links. This is one of the major roadblocks to build a thousand qubit superconducting QC. We will first review and study the challenges of state-of-the-art proposed approaches, including cryogenic CMOS and deep-cryogenic photonic methods, to scale up the control interface for SC qubit systems. We will discuss their limitations due to the active power dissipation and passive heat leakage in detail. By analytically modeling the noise sources and thermal budget limits, we will show that our solution can achieve a scale up to a thousand of qubits. Our proposed method can be seamlessly implemented using advanced silicon photonic processes, and the number of required optical fibers can be further reduced by using wavelength division multiplexing (WDM).
Superconducting circuits exhibit remarkable potential for constructing large-scale quantum simulation and computation systems, featuring numerous qubits, extended coherence time, and precise control. Nevertheless, the growing number of signal cables poses a challenge in dilution refrigerators due to space and heat load constraints. To overcome this issue, we experimentally implemented an optically-assisted transmission line as an alternative to coaxial cables. By modulating microwave signals on laser intensities at room temperature and regenerating the signals at a cryogenic plate within the dilution refrigerator, we demonstrated full control of superconducting qubits using photocurrent. We demonstrate and benchmark both single-qubit and two-qubit gates on frequency tunable transmon qubits, achieving fidelities of 99.915% ±\pm± 0.005% and 99.676% ±\pm± 0.041%, respectively, which have reached the requirement of the surface code.