Quantum State’s Phase Revealed in Emitted Light

A. Sultanov and collaborators at the Leibniz Institute of Photonic Technology investigate how the phase of light emitted from a qubit reveals information about its quantum coherence. Measuring the phase distribution of photons released from a superconducting transmon qubit allows tracking of its quantum state evolution over time. The findings establish a direct connection between a qubit’s loss of coherence and the statistical properties of the light it emits, offering a new method for quantitatively assessing qubit performance even in noisy environments. The team’s results show that superposition states leave a measurable imprint on the emitted light’s phase, providing a key set of tools for characterising open quantum systems.

Phase statistics reveal qubit coherence and dynamics beyond previous timescales

The mean phase of emitted radiation decayed with a characteristic decoherence time of approximately 40ns, representing a substantial improvement over previous limitations. Previously, observation was restricted to timescales shorter than the qubit’s coherence. This advance allows full tracking of the emitter’s coherence dynamics, which was previously obscured by rapid signal degradation. Before this work, discerning subtle changes in the quantum state was impossible due to the speed of decoherence, but a direct link between a qubit’s decoherence and the statistical properties of its radiated field now opens avenues for remote, non-destructive qubit characterisation.

Quantitative probing of qubit coherence is now possible using phase statistics, revealing information about superposition states even amidst high levels of noise. This provides a valuable tool for assessing quantum bit performance. Analysis of 5 × 10⁵ single-shot realizations revealed that the mean resultant length, R, a measure of phase concentration, increases with the number of averaged measurements, improving signal clarity. Fitting experimental data using a phenomenological model yielded values of approximately 0.038 and 0.066 for the product of emission probability and signal-to-noise ratio at integration times of 4ns and 36ns respectively, indicating a partial loss of signal coherence. Distributions exhibited pronounced peaks separated by π for preparation angles of π/2 and 3π/2, confirming faithful encoding of the qubit’s state. However, these findings currently rely on a specific waveguide geometry and do not yet demonstrate scalability to more complex qubit architectures or the ability to compensate for environmental factors impacting coherence in real-world devices.

Reconstructing qubit state via single-shot heterodyne phase measurements

Single-shot heterodyne detection, a highly sensitive measurement technique, proved central to this work, functioning akin to capturing a full photograph of a light wave rather than just its brightness. This method allowed recording the complete complex field emitted by the superconducting transmon qubit, including both its amplitude and phase, with each individual photon detected. By repeating this measurement many times, a statistical picture of the emitted light’s phase distribution could be built, revealing subtle patterns obscured by noise through averaging over numerous single-shot measurements.

This statistical approach bypassed the limitations of directly measuring the qubit itself, offering a remote and non-destructive way to probe its quantum state and track its evolution over time. The technique was employed to measure the phase of light emitted from a superconducting transmon qubit connected to an open waveguide. Capturing both amplitude and phase enabled statistical analysis of the emitted light’s phase distribution over 5x 10⁵ repetitions, allowing for non-destructive observation of its evolution over a timescale of approximately 40 nanoseconds.

Microwave statistics reveal qubit coherence without direct measurement

Increasingly, researchers are focused on remotely assessing the health of quantum bits, or qubits, without directly probing their fragile states. This approach sidesteps a key limitation of traditional qubit characterisation, which can disrupt the very quantum properties being measured. Current methods relying on statistical analysis of emitted photons, however, require careful calibration and are sensitive to the specific geometry of the experimental setup.

Acknowledging that precise calibration and waveguide geometry influence photon-based measurements, this technique offers a valuable complementary approach to qubit analysis. It provides a means of tracking qubit coherence, the duration a qubit maintains its quantum state, by examining the statistical properties of emitted microwave radiation. Understanding decoherence, the loss of this quantum information, is vital for building stable and reliable quantum computers. This work demonstrates a method for remotely assessing qubit coherence by analysing the phase of emitted microwave photons. Detecting subtle changes in this phase distribution provides a quantitative measure of how long a qubit maintains its quantum state despite environmental noise, bypassing the need for direct measurement and offering a valuable tool for characterising open quantum systems.

The research demonstrated that the phase distribution of microwave photons emitted from a superconducting transmon qubit encodes information about its quantum state. This is significant because it allows scientists to assess qubit coherence, how long a qubit maintains its quantum state, without directly measuring the qubit itself, which can cause disruption. By analysing the phase statistics of over 5x 10⁵ emitted photons, researchers tracked the qubit’s evolution over approximately 40 nanoseconds and quantified decoherence. The authors suggest this method provides a valuable way to probe open quantum systems and understand the loss of quantum information.