MIT’s Quarton Coupler Revolutionizes Qubit Readout, Boosting Quantum Computing Progress

Researchers from MIT’s Department of Electrical Engineering and Computer Science and the Research Laboratory of Electronics have developed the Quarton Coupler, a new readout scheme designed to improve the readout of superconducting qubits, a key component in quantum information processing. The Quarton Coupler facilitates a large cross-Kerr interaction between a transmon qubit and its readout resonator. It achieves a readout time of 5 ns with a readout and quantum non-demolition (QND) fidelity greater than 99%. This advancement could significantly improve superconducting qubit readout and play a crucial role in the future of quantum information processing.

What is the Quarton Coupler and How Does it Improve Qubit Readout?

The Quarton Coupler is a new readout scheme developed by researchers from the Department of Electrical Engineering and Computer Science and the Research Laboratory of Electronics at the Massachusetts Institute of Technology. This scheme is designed to improve the readout of superconducting qubits, which are a key component in quantum information processing. The Quarton Coupler facilitates a large cross-Kerr interaction between a transmon qubit and its readout resonator, resulting in a readout time of 5 ns with a readout and quantum non-demolition (QND) fidelity greater than 99%.

The Quarton Coupler differs from the state-of-the-art dispersive readout scheme in that it relies on a transmon with linearized transitions as the readout resonator. This operational point is determined through a detailed theoretical treatment and parameter study of the coupled system. The Quarton Coupler is also experimentally feasible and preserves the coherence properties of the qubit. This new readout scheme opens up a new path for significant improvements in superconducting qubit readout by engineering nonlinear light-matter couplings in parameter regimes unreachable by existing designs.

The Quarton Coupler is a significant advancement in the field of quantum information processing. Fast and high-fidelity qubit readout is essential for quantum error correction and other feedback schemes in quantum computing and communication, including teleportation and state-initialization. Superconducting qubits are a leading material platform for quantum information processing, in part due to their reliably fast, high-fidelity, and quantum non-demolition (QND) readout.

How Does the Quarton Coupler Improve the Signal-to-Noise Ratio?

High readout fidelity requires a high measurement signal-to-noise ratio (SNR). The SNR for cross-Kerr based qubit measurement conveniently scales with the readout time required to reach a desired SNR. This can be reduced by increasing either the effective rate at which measurement photons are collected or the amount of phase information each photon carries. Over the past decades, there have been significant advances in designing to maximize the amount of phase information each photon carries and engineering devices that improve quantum efficiency towards the theoretical maximum.

However, non-idealities in dispersive coupling tend to limit the average readout resonator photon number to low values when the qubit is a state-of-the-art transmon. A simple way to improve readout SNR is by increasing the coupling rate of the readout resonator to the environment. While larger coupling rates are easily achievable by increasing coupling capacitance, designs with high coupling rates are practically difficult due to a number of reasons, many of which are ultimately caused by the perturbative nature of dispersive coupling.

What are the Limitations of Dispersive Readout and How Does the Quarton Coupler Overcome Them?

Dispersive readout has certain limitations. The first is the limit on the cross-Kerr interaction. Larger cross-Kerr interactions are needed to accompany larger coupling rates, but dispersive cross-Kerr interactions for state-of-the-art transmon qubits with low anharmonicity are limited. Secondly, the underlying linear coupling in dispersive coupling causes eigenstates of the qubit-resonator system to be combinations of qubit and readout resonator bare states, so a stronger resonator-environment coupling invariably increases many qubit eigenstate decoherence rates, such as Purcell decay.

The Quarton Coupler overcomes these limitations by providing a non-perturbative source of cross-Kerr interaction between superconducting qubits and resonators that allows for a much larger cross-Kerr interaction and therefore much larger coupling rate, which can lead to proportionally larger readout SNR. By designing high cross-Kerr interaction, a cross-Kerr based readout scheme can result in about an order of magnitude faster readout time, just 5 ns to reach 99% readout fidelity and QND fidelity.

How Does the Quarton Coupler Work?

The Quarton Coupler works by leveraging the quarton coupler, which is capable of ultra-strong cross-Kerr coupling between a transmon qubit and resonator. This resonator is not the typical standing-wave waveguide mode but rather a linearized transmon with its intrinsic negative nonlinearity canceled by the quarton coupler’s induced positive nonlinearity. The circuit for the proposed quartonic readout scheme includes the quarton, which couples the linearized readout resonator to the qubit. A Purcell filter prevents qubit state leakage through the resonator.

The readout drive is between the resonator frequencies for qubit state 0 and 1. The Purcell filter is centered at the resonator frequency. The qubit state-dependent evolution of the resonator coherent state is then measured. This method of readout is faster and more efficient than previous methods, making it a significant advancement in the field of quantum information processing.

What is the Future of the Quarton Coupler?

The Quarton Coupler represents a significant advancement in the field of quantum information processing. Its ability to facilitate a large cross-Kerr interaction between a transmon qubit and its readout resonator, resulting in a readout time of 5 ns with a readout and QND fidelity greater than 99%, makes it a promising tool for future developments in quantum computing and communication.

The Quarton Coupler is also experimentally feasible and preserves the coherence properties of the qubit, making it a practical solution for improving qubit readout. As research in this field continues, it is likely that the Quarton Coupler and similar technologies will play a crucial role in the advancement of quantum information processing.