Comparing optical-microwave conversion and all-microwave control schemes for a transmon qubit
Volodymyr Monarkha, Massimo Borrelli, Reza Hajitashakkori Kenari, Mohammad Kobba, Eugenio Cataldo, Beer de Zoeten, Mahnaz Zarrinfar, Kamal Pandey, Abhinand Pusuluri, Filippo D. Michelacci, Eliot Jouan, Bennett Sprague, Simon Groeblacher, Thierry C. van Thiel, Robert Stockill, Russell E. Lake
quant-phphysics.app-phSubmitted Mar 19, 2026• Announced Mar 20, 2026• 0 citations
We report a comparative study on transmon qubit control using (i) conventional attenuated coaxial microwave line and (ii) an optical control system using modulated laser light delivered over telecommunications optical fiber to a photodiode located at the 1K stage of a dilution cryostat. During each experiment, we performed repeated measurements of the energy relaxation and coherence times of a transmon qubit using one of the control signal delivery methods. Each measurement run spanned 20 hours of measurement time and from these datasets we observe no measurable effect on coherence of the qubit compared to random coherence fluctuations. Our results open up the possibility of large scale integration of the optical qubit control system.
We report a comparative study on transmon qubit control using (i) conventional attenuated coaxial microwave line and (ii) an optical control system using modulated laser light delivered over telecommunications optical fiber to a photodiode located at the 1K stage of a dilution cryostat. During each experiment, we performed repeated measurements of the energy relaxation and coherence times of a transmon qubit using one of the control signal delivery methods. Each measurement run spanned 20 hours of measurement time and from these datasets we observe no measurable effect on coherence of the qubit compared to random coherence fluctuations. Our results open up the possibility of large scale integration of the optical qubit control system.
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% ± 0.005% and 99.676% ± 0.041%, respectively, which have reached the requirement of the surface code.
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.
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 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.
Compared to traditional semiconductor control electronics (TSCE) located at room temperature, cryogenic single flux quantum (SFQ) electronics can provide qubit measurement and control alternatives that address critical issues related to scalability of cryogenic quantum processors. Single-qubit control and readout have been demonstrated recently using SFQ circuits coupled to superconducting qubits. Experiments where the SFQ electronics are co-located with the qubit have suffered from excess decoherence and loss due to quasiparticle poisoning of the qubit. A previous experiment by our group showed that moving the control electronics to the 3 K stage of the dilution refrigerator avoided this source of decoherence in a high-coherence 3D transmon geometry. In this paper, we also generate the pulses at the 3 K stage but have optimized the qubit design and control lines for scalable 2D transmon devices. We directly compare the qubit lifetime T1, coherence time T2∗ and gate fidelity when the qubit is controlled by the Josephson pulse generator (JPG) circuit versus the TSCE setup. We find agreement to within the daily fluctuations for T1 and T2∗, and agreement to within 10% for randomized benchmarking. We also performed interleaved randomized benchmarking on individual JPG gates demonstrating an average error per gate of 0.46% showing good agreement with what is expected based on the qubit coherence and higher-state leakage. These results are an order of magnitude improvement in gate fidelity over our previous work and demonstrate that a Josephson microwave source operated at 3 K is a promising component for scalable qubit control.
This paper investigates the effect of a resonator-coupled microwave control line on transmon qubit operations. It is shown that introducing an intermediate resonator and appropriately positioning the qubit along the length of the microwave control line increases the relaxation-time at the qubit resonant frequency, which is highly desirable for executing high-fidelity quantum operations reliably. It is further highlighted that even though an intermediate resonator increases the qubit relaxation-time, it causes pulse-broadening of the microwave control signal leading to non-ideal quantum gate operations.