Quantum System Achieves 99% Sensitivity Near Exceptional Point

The pursuit of highly sensitive measurement tools drives innovation in quantum technologies, and recent research explores the potential of non-Hermitian systems operating near so-called exceptional points. Aziza Almanakly, Reouven Assouly, and Harry Hanlim Kang, alongside colleagues from the Massachusetts Institute of Technology and MIT Lincoln Laboratory, investigate these points, where energy levels dramatically change, potentially offering enhanced sensitivity. Their work emulates a specific non-Hermitian system using superconducting qubits coupled to a waveguide, allowing them to precisely characterise the system’s behaviour as it approaches an exceptional point. The team’s findings, however, challenge the prevailing theory, demonstrating that sensitivity does not automatically increase near these points, and instead clarifies the limitations of this approach for building ultra-sensitive devices.

Tunable Qubit Coupling For Sensing Applications

Researchers modeled various sensing approaches utilizing coupled qubits, focusing on how tunable coupling between two qubits, Q1 and Q2, responds to external parameters. They explored sensing methods based on Q1’s population, Q2’s coherence, and the emission from Q2 into a waveguide, meticulously explaining the modeling of each approach and validating the simulations against experimental data. This work provides a comprehensive understanding of the underlying physics and instills confidence in the accuracy and reliability of the models. Scientists developed a continuous-wave model to simulate a probe signal passing through a waveguide coupled to the qubits, determining sensitivity by measuring changes in the transmission spectrum with varying qubit coupling.

Further modeling focused on Q1’s population and Q2’s coherence, employing analytical expressions and complex simulations to calculate sensitivity. These approaches successfully matched simulation results with experimental data, validating the models and detailing sensitivity calculations. The research details how sensor sensitivity is calculated and optimized, crucial for achieving peak performance. The Q2 sensing approach requires a more complex simulation, suggesting potential challenges in implementation but also offering the possibility of higher sensitivity. This detailed analysis of the experimental setup, data analysis, and theoretical models ensures reproducibility and advances the field of quantum sensing, demonstrating a sound theoretical basis and potential for high sensitivity.

Emulating Parity-Time Symmetry with Superconducting Qubits

This work pioneers a superconducting qubit system designed to emulate a passive parity-time (\PT) symmetric dimer, a configuration useful for exploring the dynamics of open quantum systems. The team constructed a device comprising two transmon qubits, Q1 and Q2, introducing significant asymmetry in their loss rates; Q1 exhibits a low decay rate, while Q2 is strongly coupled to a coplanar waveguide, creating a larger decay rate and establishing the necessary imbalance for \PT-symmetry emulation. The qubits are coupled via a tunable transmon coupler, allowing precise control over the interaction strength. By manipulating the coupler’s frequency, scientists dynamically adjust the effective coupling between the qubits, enabling measurements across the exceptional point where the coupling strength reaches a critical value.

The system’s behavior is modeled using a non-Hermitian Hamiltonian, incorporating the dissipation from Q2 into the environment, and simplified to represent a passive \PT dimer within the single-excitation subspace. Experiments involved preparing Q1 in either the excited state or an equal superposition state using resonant microwave pulses, then activating the interaction between the qubits by rapidly changing the coupler’s frequency. By varying the amplitude and duration of these pulses, scientists systematically swept the coupling strength and interaction time, mapping the system’s response near the exceptional point. Population measurements of Q1, obtained using dispersive readout techniques, provided a direct probe of the system’s evolution, allowing the team to extract the complex eigenenergies and investigate the system’s sensitivity to variations in the coupling strength.

Dissipation and Exceptional Points in Superconducting Qubits

Scientists constructed and characterized a non-Hermitian system using superconducting qubits to emulate a passive parity-time (\PT) symmetric dimer. The research team coupled a high-coherence qubit to a deliberately lossy qubit via a tunable coupler, introducing dissipation into the system and creating conditions for observing exceptional points where energy levels and corresponding eigenvectors converge. The lossy qubit was strongly coupled to a waveguide, facilitating the controlled introduction of dissipation and enabling direct measurement of its dynamics. Experiments involved both pulsed and continuous-wave measurements to probe the system’s sensitivity to variations in coupling strength near the exceptional point.

By preparing the initial qubit in either an excited state or an equal superposition, scientists tracked the population dynamics of the first qubit and the coherence of the second qubit as a function of time and coupling strength. The team observed a transition from exponential decay to oscillatory behavior in the qubit population as the coupling strength approached the exceptional point, confirming the system’s response to the changing energy landscape. Analysis of the time-domain measurements allowed the team to extract the complex eigenenergies associated with the two qubit modes. Results demonstrate that below a relative coupling of 1, the eigenenergies are purely imaginary, while above 1, they become complex-valued with a consistent imaginary offset corresponding to the average loss rate.

The extracted eigenenergies closely align with analytical models, validating the system’s behavior as a passive \PT dimer. Furthermore, direct measurement of the second qubit’s emission into the waveguide environment provided a complementary method for characterizing the system’s coherence dynamics and confirming the extracted eigenenergies. The research reveals that exceptional points do not naturally enable quantum-enhanced sensing.

Quantum Dimer Dynamics Near Exceptional Points

This research successfully emulates a passive parity-time (\PT) symmetric dimer using superconducting qubits coupled to a waveguide, allowing for detailed characterization of its dynamics near an exceptional point. By carefully controlling the system and performing both pulsed and continuous-wave measurements, the team extracted the complex eigenenergies associated with the qubit modes as they traversed the exceptional point, confirming theoretical predictions regarding the system’s spectral response. This study demonstrates a clear understanding of how these non-Hermitian systems behave in a controlled quantum environment. Despite the anticipated abrupt spectral changes near the exceptional point, the team observed no enhancement in sensing capabilities within their experimental setup. This finding clarifies the limitations of relying on exceptional points for improved precision measurements, particularly in scenarios already approaching the standard quantum limit. The research highlights that while exceptional point physics is a valuable area of study, it does not automatically translate to practical improvements in sensing.