Akash V. Dixit and colleagues from the National Institute of Standards and Technology and the University of Colorado Boulder have demonstrated a qubit readout with over 99% accuracy without relying on a quantum limited amplifier. The team achieved this performance by using millimeter waves to interact with a transmon qubit within a circuit quantum electrodynamic system, coupled at a rate of 230 MHz.

A key to their success was suppressing unwanted state transitions by maintaining a large frequency separation, where $ω_{r} / ω_{q}$ > 10, and using up to 1,000 drive photons. This approach offers a path toward simpler and potentially less expensive quantum computing infrastructure.

Millimeter Waves Enable Hybrid Quantum System Connectivity

This level of accuracy demonstrates a potential simplification of quantum computing infrastructure, reducing the need for complex and costly amplification systems. Researchers envision this technology as a key component in future hybrid quantum systems, integrating the strengths of diverse platforms like superconducting qubits, circuit optomechanics, and Rydberg atoms. Millimeter waves, they suggest, could facilitate state preparation and measurement using superconducting qubits, information storage via circuit optomechanics, and long-range communication through optical transitions in Rydberg atoms.

This work extends the capabilities of circuit QED into the millimeter wave range, allowing for the study of interactions between transmon qubits and highly detuned photons. Such interactions could prove valuable for developing quantum memories, error correction protocols and millimeter wave frequency metrology.

Circuit QED System: Transmon-Millimeter Wave Cavity Coupling

A qubit’s state was read with 99% fidelity without a quantum limited amplifier, a result demonstrating potential for simplified quantum computing designs. This suppression of intra-qubit resonances allows for stronger drive signals during readout, improving the signal-to-noise ratio and ultimately boosting measurement fidelity. The researchers demonstrated no resonant state transitions even with the high photon count, indicating a high degree of control over the qubit’s behavior within the millimeter-wave circuit.

The potential extends beyond readout improvements, offering a pathway to build quantum memories and implement error correction protocols at millimeter wave frequencies. Such systems could significantly advance the development of robust quantum memories and efficient error correction schemes. The ability to operate cQED experiments with qubits and photons of disparate frequencies represents a step toward more versatile and interconnected quantum technologies.

Large Detuning Suppresses Unwanted Qubit Transitions

Maintaining a substantial frequency difference between a qubit and the photons used to read its state is important for achieving high-fidelity measurements, as demonstrated by recent work on millimeter-wave readout systems. The researchers established a detuning where $ω_{r} / ω_{q}$ exceeds 10, effectively suppressing unwanted transitions during the readout process and enabling the use of stronger drive signals.

This suppression is vital because it allows for more photons to be used in the readout, improving the signal strength without introducing errors from unintended qubit state changes. The ability to use over 100 photons for readout, while maintaining a measurement fidelity exceeding 99% without a quantum limited amplifier, represents an advancement in qubit control.

This level of suppression is achieved by operating with a readout resonator in the millimeter-wave range, above 30 GHz, while keeping the transmon qubit in the conventional 3-6 GHz range. This is particularly important as superconducting qubit processors scale to larger numbers of qubits, as the number of drives needed for control and readout will also increase. The ability to reliably address and measure each qubit without introducing errors is essential for building practical and scalable quantum computers.

99% Qubit Readout Fidelity Achieved with Millimeter Waves

Measurements revealed no resonant state transitions even with up to 1,000 drive photons applied to the qubit, demonstrating a level of control previously unseen in millimeter wave readout systems. The fidelity was calculated by preparing the qubit in both the |0⟩ and |1⟩ states 50,000 times, then measuring the readout signal for a fixed duration. Increasing the average number of readout photons, $n_{r}$ , directly improved measurement accuracy, progressing from 0.535 with $n_{r}$ = 1 to 0.790 with $n_{r}$ = 10.

Further optimization, reaching an average of 109 photons, yielded a final fidelity of 0.992 for both qubit states, a result that highlights the potential of millimeter waves for high-precision quantum measurements. The system’s coupling rate, g/2π = 230 MHz, defines the strength of the interaction between the qubit and the millimeter-wave resonator, and is ten times larger than those typically found in circuit quantum electrodynamic experiments.

This strong coupling is a key factor in achieving the observed suppression of unwanted state transitions. The measured cross-Kerr shift, χ1 = −2π × 1, The researchers note that improvements to qubit decay time and readout time could further enhance measurement fidelity, reducing both decay and heating-induced errors. Millimeter waves are emerging as an enabling technology to connect disparate quantum platforms such as circuit optomechanics, Rydberg atoms, and superconducting qubits, highlighting the broader implications of this work for hybrid quantum systems.

Transmon Frequency Range and Millimeter Wave Readout

Simulations demonstrate that resonant state transitions are highly suppressed when readout drives are in the millimeter wave frequency range, a result enabled by a frequency ratio where $ω_{r} / ω_{q}$ exceeds 10. This approach allows for operation with minimal residual thermal population, potentially reducing qubit dephasing and improving overall performance. The experimental setup uses a transmon qubit with an anharmonicity of -2π × 141 MHz coupled to a 3D aluminum cavity operating at millimeter wave frequencies.

By increasing the readout frequency to above 30 GHz, researchers were able to achieve the necessary large detuning while preserving the transmon’s anharmonicity, a property essential for fast gate operations. This design choice also offers the potential for reduced thermal occupancy of the readout mode at typical cryostat temperatures, further enhancing qubit coherence.

The simulations show a significant reduction in hybridization of the initial qubit state with higher energy levels, indicating a minimized risk of unwanted transitions. This is a significant advancement, as quantum limited amplifiers add complexity and cost to quantum computing infrastructure. The potential applications extend beyond improved qubit readout, with the platform offering opportunities for quantum memories and error correction.

Drive Strength Impact on Qubit State Transitions

Maintaining a stable qubit state during readout is critical for accurate quantum computation, and recent work demonstrates a method for suppressing unwanted transitions even with strong drive signals. Experiments revealed no measurable resonant state transitions when probing the transmon qubit with varying drive strengths, a result achieved by detuning the readout mode from the transmon by a factor greater than 10 ( $ω_{r} / ω_{q}$ > 10). This large detuning effectively isolates the qubit from disruptive energy exchanges during the measurement process, allowing for a significant increase in the number of photons used for readout.

The absence of observable resonant transitions, even with a substantial drive, confirms the effectiveness of the detuning strategy in preventing spurious state flips. This approach relies on a specific mathematical correction, expressed as a ratio of Lorentzian responses, to account for the interaction between the drive and the transmon.

This suppression of unwanted transitions is not merely a theoretical exercise. It has direct implications for scaling up quantum processors. As the number of qubits in a system increases, so too does the demand for control and readout infrastructure. The ability to perform high-fidelity readout with a simpler setup, one that avoids the need for quantum limited amplification, represents a step toward more practical and cost-effective quantum computing. The researchers report, highlighting the stability of the system.

Millimeter Wave Photons as a Superconducting Quantum Resource

This substantial detuning suppresses unwanted state transitions during readout, allowing for stronger drive signals and improved measurement accuracy. While the current measurement chain exhibits 1.74% efficiency and introduces a system-added noise of 28 photons, the researchers note that integrating a quantum limited amplifier at millimeter wave frequencies would significantly enhance performance. Further investigation into state leakage from inelastic scattering or quasiparticle generation at these frequencies is also planned, aiming to refine the system’s robustness.

The ability to prepare, measure, and store quantum information across these platforms using millimeter wave photons could unlock new applications in quantum memories, error correction and millimeter wave metrology. The system’s performance establishes millimeter wave photons as a valuable resource for superconducting quantum systems, offering a pathway toward more scalable and efficient quantum information processing. The researchers suggest that optimization of state preparation and reset will further improve the probability of initializing the qubit in the intended state, contributing to overall system reliability.

👉 More information
🗞 Millimeter Wave Readout of a Superconducting Qubit
✍️ Akash V. Dixit, Zachary L. Parrott, Dennis Chunikhin, Trevyn F. Q. Larson, Bradley D. Hauer and John D. Teufel
🧠 DOI: http://link.aps.org/doi/10.1103/c84y-ggqs

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