Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

arXiv:2603.28433

Phase statistics of a single qubit emission as a direct probe of its coherence

A. Sultanov, E. Mutsenik, L. Kaczmarek, M. Schmelz, G. Oelsner, R. Stolz, E. Il'ichev

quant-phSubmitted Mar 30, 2026• Announced Mar 31, 2026• 0 citations
arXivHTMLPDF

Abstract

The emission of photon from an individual atom encodes the phase of its initialized quantum state. Using single-shot heterodyne detection, we measure the phase distribution of the emission from a superconducting transmon qubit in an open waveguide configuration and track its evolution over time. We demonstrate that the presence of a quantum superposition is encoded in the phase statistics of the emission and remains resolvable despite a high noise level. These phase statistics serve as a quantitative probe of the qubit coherence. The decay of the emission envelope with increasing integration time reveals the energy relaxation rate of the emitted wavepacket, while phase distribution broadening tracks pure dephasing processes. We thereby establish a direct link between the decoherence dynamics of an open quantum system and the statistical properties of its radiated field.

Related papers

Phase statistics of a single qubit emission as a direct probe of its coherence

Aidar Sultanov, E. Mutsenik, Leonie Kaczmarek +4 more

Abstract The emission of photon from an individual atom encodes the phase of its initialized quantum state. Using single-shot heterodyne detection, we measure the phase distribution of the emission from a superconducting transmon qubit in an open waveguide configuration and track its evolution over time. We demonstrate that the presence of a quantum superposition is encoded in the phase statistics of the emission and remains resolvable despite a high noise level. These phase statistics serve as a quantitative probe of the qubit coherence. The decay of the emission envelope with increasing integration time reveals the energy relaxation rate of the emitted wavepacket, while phase distribution broadening tracks pure dephasing processes. Thereby we establish the phase statistics of the emitted field as a direct, minimal and robust probe of coherence dynamics in an open quantum system.

•Jun 2, 2026•0 citations

Observing Quantum State Diffusion by Heterodyne Detection of Fluorescence

Philippe Campagne-Ibarcq, Pierre Six, Landry Bretheau +4 more

A qubit can relax by fluorescence, which prompts the release of a photon into its electromagnetic environment.By counting the emitted photons, discrete quantum jumps of the qubit state can be observed.The succession of states occupied by the qubit in a single experiment, its quantum trajectory, depends in fact on the kind of detector.How are the quantum trajectories modified if one measures continuously the amplitude of the fluorescence field instead?Using a superconducting parametric amplifier, we have performed heterodyne detection of the fluorescence of a superconducting qubit.For each realization of the measurement record, we can reconstruct a different quantum trajectory for the qubit.The observed evolution obeys quantum state diffusion, which is characteristic of quantum measurements subject to zero point fluctuations.Independent projective measurements of the qubit at various times provide a quantitative validation of the reconstructed trajectories.By exploring the statistics of quantum trajectories, we demonstrate that the qubit states span a deterministic surface in the Bloch sphere at each time in the evolution.Additionally, we show that when monitoring fluorescence, coherent superpositions are generated during the decay from excited to ground state.Counterintuitively, measuring light emitted during relaxation can give rise to trajectories with increased excitation probability.

arXiv:1511.01415•Physical Review X•Jan 5, 2016•107 citations

Measuring coherent dynamics of a superconducting qubit in an open waveguide

A. Sh. Sultanov, E. Mutsenik, Matthias Schmelz +5 more

We measured the relaxation and decoherence rates of a superconducting transmon qubit in a resonator-free setting. In our experiments, the qubit is coupled to an open coplanar waveguide such that the transmission of microwaves through this line depends on the qubit's state. To determine the occupation of the first excited qubit energy level, we introduced a two-pulse technique. The first applied pulse, at a frequency close to the eigenfrequency of the qubit, serves to excite the qubit. A second pulse is then used for probing the transition between the first and second excited energy levels. Utilizing this measurement technique allowed for the reconstruction of the relaxation dynamics and Rabi oscillations. Furthermore, we demonstrate the consistency between the extracted parameters and the corresponding estimations from frequency-domain measurements.

arXiv:2504.11028•Applied Physics Letters•Jul 28, 2025•3 citations

Characterizing decoherence rates of a superconducting qubit by direct microwave scattering

Yong Lu, Andreas Bengtsson, Jonathan Burnett +8 more

Abstract We experimentally investigate a superconducting qubit coupled to the end of an open transmission line, in a regime where the qubit decay rates to the transmission line and to its own environment are comparable. We perform measurements of coherent and incoherent scattering, on- and off-resonant fluorescence, and time-resolved dynamics to determine the decay and decoherence rates of the qubit. In particular, these measurements let us discriminate between non-radiative decay and pure dephasing. We combine and contrast results across all methods and find consistent values for the extracted rates. The results show that the pure dephasing rate is one order of magnitude smaller than the non-radiative decay rate for our qubit. Our results indicate a pathway to benchmark decoherence rates of superconducting qubits in a resonator-free setting.

arXiv:1912.02124•npj Quantum Information•Feb 16, 2021•48 citations

Decoherence benchmarking of superconducting qubits

Jonathan Burnett, Andreas Bengtsson, Marco Scigliuzzo +4 more

Abstract We benchmark the decoherence of superconducting transmon qubits to examine the temporal stability of energy relaxation, dephasing, and qubit transition frequency. By collecting statistics during measurements spanning multiple days, we find the mean parameters $$\overline {T_1}$$ T 1 ¯ = 49 μs and $$\overline {T_2^ \ast }$$ T 2 * ¯ = 95 μs; however, both of these quantities fluctuate, explaining the need for frequent re-calibration in qubit setups. Our main finding is that fluctuations in qubit relaxation are local to the qubit and are caused by instabilities of near-resonant two-level-systems (TLS). Through statistical analysis, we determine sub-millihertz switching rates of these TLS and observe the coherent coupling between an individual TLS and a transmon qubit. Finally, we find evidence that the qubit’s frequency stability produces a 0.8 ms limit on the pure dephasing which we also observe. These findings raise the need for performing qubit metrology to examine the reproducibility of qubit parameters, where these fluctuations could affect qubit gate fidelity.

arXiv:1901.04417•npj Quantum Information•Jun 26, 2019•381 citations

Photon shot noise dephasing in the strong-dispersive limit of circuit QED

Adam Sears, Andrei Petrenko, Gianluigi Catelani +7 more

We study the photon shot noise dephasing of a superconducting transmon qubit in the strong-dispersive limit, due to the coupling of the qubit to its readout cavity. As each random arrival or departure of a photon is expected to completely dephase the qubit, we can control the rate at which the qubit experiences dephasing events by varying in situ the cavity mode population and decay rate. This allows us to verify a pure dephasing mechanism that matches theoretical predictions, and in fact explains the increased dephasing seen in recent transmon experiments as a function of cryostat temperature. We observe large increases in coherence times as the cavity is decoupled from the environment, and after implementing filtering find that the intrinsic coherence of small Josephson junctions when corrected with a single Hahn echo is greater than several hundred microseconds. Similar filtering and thermalization may be important for other qubit designs in order to prevent photon shot noise from becoming the dominant source of dephasing.

arXiv:1206.1265•Physical Review B•Nov 12, 2012•155 citations
The emission of photon from an individual atom encodes the phase of its initialized quantum state. Using single-shot heterodyne detection, we measure the phase distribution of the emission from a superconducting transmon qubit in an open waveguide configuration and track its evolution over time. We demonstrate that the presence of a quantum superposition is encoded in the phase statistics of the emission and remains resolvable despite a high noise level. These phase statistics serve as a quantitative probe of the qubit coherence. The decay of the emission envelope with increasing integration time reveals the energy relaxation rate of the emitted wavepacket, while phase distribution broadening tracks pure dephasing processes. We thereby establish a direct link between the decoherence dynamics of an open quantum system and the statistical properties of its radiated field.
Abstract The emission of photon from an individual atom encodes the phase of its initialized quantum state. Using single-shot heterodyne detection, we measure the phase distribution of the emission from a superconducting transmon qubit in an open waveguide configuration and track its evolution over time. We demonstrate that the presence of a quantum superposition is encoded in the phase statistics of the emission and remains resolvable despite a high noise level. These phase statistics serve as a quantitative probe of the qubit coherence. The decay of the emission envelope with increasing integration time reveals the energy relaxation rate of the emitted wavepacket, while phase distribution broadening tracks pure dephasing processes. Thereby we establish the phase statistics of the emitted field as a direct, minimal and robust probe of coherence dynamics in an open quantum system.
A qubit can relax by fluorescence, which prompts the release of a photon into its electromagnetic environment.By counting the emitted photons, discrete quantum jumps of the qubit state can be observed.The succession of states occupied by the qubit in a single experiment, its quantum trajectory, depends in fact on the kind of detector.How are the quantum trajectories modified if one measures continuously the amplitude of the fluorescence field instead?Using a superconducting parametric amplifier, we have performed heterodyne detection of the fluorescence of a superconducting qubit.For each realization of the measurement record, we can reconstruct a different quantum trajectory for the qubit.The observed evolution obeys quantum state diffusion, which is characteristic of quantum measurements subject to zero point fluctuations.Independent projective measurements of the qubit at various times provide a quantitative validation of the reconstructed trajectories.By exploring the statistics of quantum trajectories, we demonstrate that the qubit states span a deterministic surface in the Bloch sphere at each time in the evolution.Additionally, we show that when monitoring fluorescence, coherent superpositions are generated during the decay from excited to ground state.Counterintuitively, measuring light emitted during relaxation can give rise to trajectories with increased excitation probability.
We measured the relaxation and decoherence rates of a superconducting transmon qubit in a resonator-free setting. In our experiments, the qubit is coupled to an open coplanar waveguide such that the transmission of microwaves through this line depends on the qubit's state. To determine the occupation of the first excited qubit energy level, we introduced a two-pulse technique. The first applied pulse, at a frequency close to the eigenfrequency of the qubit, serves to excite the qubit. A second pulse is then used for probing the transition between the first and second excited energy levels. Utilizing this measurement technique allowed for the reconstruction of the relaxation dynamics and Rabi oscillations. Furthermore, we demonstrate the consistency between the extracted parameters and the corresponding estimations from frequency-domain measurements.
Abstract We experimentally investigate a superconducting qubit coupled to the end of an open transmission line, in a regime where the qubit decay rates to the transmission line and to its own environment are comparable. We perform measurements of coherent and incoherent scattering, on- and off-resonant fluorescence, and time-resolved dynamics to determine the decay and decoherence rates of the qubit. In particular, these measurements let us discriminate between non-radiative decay and pure dephasing. We combine and contrast results across all methods and find consistent values for the extracted rates. The results show that the pure dephasing rate is one order of magnitude smaller than the non-radiative decay rate for our qubit. Our results indicate a pathway to benchmark decoherence rates of superconducting qubits in a resonator-free setting.
Abstract We benchmark the decoherence of superconducting transmon qubits to examine the temporal stability of energy relaxation, dephasing, and qubit transition frequency. By collecting statistics during measurements spanning multiple days, we find the mean parameters T1‾\overline {T_1}T1​​ T 1 ¯ = 49 μs and T2∗‾\overline {T_2^ \ast }T2∗​​ T 2 * ¯ = 95 μs; however, both of these quantities fluctuate, explaining the need for frequent re-calibration in qubit setups. Our main finding is that fluctuations in qubit relaxation are local to the qubit and are caused by instabilities of near-resonant two-level-systems (TLS). Through statistical analysis, we determine sub-millihertz switching rates of these TLS and observe the coherent coupling between an individual TLS and a transmon qubit. Finally, we find evidence that the qubit’s frequency stability produces a 0.8 ms limit on the pure dephasing which we also observe. These findings raise the need for performing qubit metrology to examine the reproducibility of qubit parameters, where these fluctuations could affect qubit gate fidelity.
We study the photon shot noise dephasing of a superconducting transmon qubit in the strong-dispersive limit, due to the coupling of the qubit to its readout cavity. As each random arrival or departure of a photon is expected to completely dephase the qubit, we can control the rate at which the qubit experiences dephasing events by varying in situ the cavity mode population and decay rate. This allows us to verify a pure dephasing mechanism that matches theoretical predictions, and in fact explains the increased dephasing seen in recent transmon experiments as a function of cryostat temperature. We observe large increases in coherence times as the cavity is decoupled from the environment, and after implementing filtering find that the intrinsic coherence of small Josephson junctions when corrected with a single Hahn echo is greater than several hundred microseconds. Similar filtering and thermalization may be important for other qubit designs in order to prevent photon shot noise from becoming the dominant source of dephasing.