Using a qubit to measure photon number statistics of a driven, thermal oscillator (original) (raw)
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Qubit-photon interactions in a cavity: Measurement-induced dephasing and number splitting
Physical Review A, 2006
We theoretically study measurement induced-dephasing of a superconducting qubit in the circuit QED architecture and compare the results to those obtained experimentally by Schuster et al., [Phys. Rev. Lett. 94, 123602 (2005)]. Strong coupling of the qubit to the resonator leads to a significant ac-Stark shift of the qubit transition frequency. As a result, quantum fluctuations in the photon number populating the resonator cause dephasing of the qubit. We find good agreement between the predicted line shape of the qubit spectrum and the experimental results. Furthermore, in the strong dispersive limit, where the Stark shift per photon is large compared to the cavity decay rate and the qubit linewidth, we predict that the qubit spectrum will be split into multiple peaks, with each peak corresponding to a different number of photons in the cavity.
Quantum thermometry by single-qubit dephasing
The European Physical Journal Plus, 2019
We address the dephasing dynamics of a qubit as an effective process to estimate the temperature of its environment. Our scheme is inherently quantum, since it exploits the sensitivity of the qubit to decoherence, and does not require thermalization with the system under investigation. We optimize the quantum Fisher information with respect to the interaction time and the temperature in the case of Ohmic-like environments. We also find explicitly the qubit measurement achieving the quantum Cramér-Rao bound to precision. Our results show that the conditions for optimal estimation originate from a non-trivial interplay between the dephasing dynamics and the Ohmic structure of the environment. In general, optimal estimation is achieved neither when the qubit approaches the stationary state, nor for full dephasing.
Distinguishing Coherent and Thermal Photon Noise in a Circuit Quantum Electrodynamical System
Physical review letters, 2018
In the cavity-QED architecture, photon number fluctuations from residual cavity photons cause qubit dephasing due to the ac Stark effect. These unwanted photons originate from a variety of sources, such as thermal radiation, leftover measurement photons, and cross talk. Using a capacitively shunted flux qubit coupled to a transmission line cavity, we demonstrate a method that identifies and distinguishes coherent and thermal photons based on noise-spectral reconstruction from time-domain spin-locking relaxometry. Using these measurements, we attribute the limiting dephasing source in our system to thermal photons rather than coherent photons. By improving the cryogenic attenuation on lines leading to the cavity, we successfully suppress residual thermal photons and achieve T_{1}-limited spin-echo decay time. The spin-locking noise-spectroscopy technique allows broad frequency access and readily applies to other qubit modalities for identifying general asymmetric nonclassical noise s...
Dissipation in circuit quantum electrodynamics: lasing and cooling of a low-frequency oscillator
New Journal of Physics, 2008
Superconducting qubits coupled to electric or nanomechanical resonators display effects previously studied in quantum electrodynamics (QED) and extensions thereof. Here we study a driven qubit coupled to a low-frequency tank circuit with particular emphasis on the role of dissipation. When the qubit is driven to perform Rabi oscillations, with Rabi frequency in resonance with the oscillator, the latter can be driven far from equilibrium. Blue detuned driving leads to a population inversion in the qubit and lasing behavior of the oscillator ("single-atom laser"). For red detuning the qubit cools the oscillator. This behavior persists at the symmetry point where the qubit-oscillator coupling is quadratic and decoherence effects are minimized. Here the system realizes a "single-atom-twophoton laser". Φ Φ x (t) ac J J J M M C L U
Measurement-Induced Qubit State Mixing in Circuit QED from Up-Converted Dephasing Noise
Physical Review Letters, 2012
We observe measurement-induced qubit state mixing in a transmon qubit dispersively coupled to a planar readout cavity. Our results indicate that dephasing noise at the qubit-readout detuning frequency is up-converted by readout photons to cause spurious qubit state transitions, thus limiting the nondemolition character of the readout. Furthermore, we use the qubit transition rate as a tool to extract an equivalent flux noise spectral density at f ∼ 1 GHz and find agreement with values extrapolated from a 1/f α fit to the measured flux noise spectral density below 1 Hz. PACS numbers: 42.50.Lc, 42.50.Pq, 03.67.Lx, High-fidelity measurement is a crucial tool in quantum information science. For superconducting qubits [1, 2], one widely used framework for performing quantum nondemolition (QND) [3] measurement is the circuit quantum electrodynamics (cQED) architecture . In cQED, a qubit is coupled to a microwave-frequency resonant cavity through a Jaynes-Cummings-type interaction, in analogy to an atom in an optical Fabry-Perot cavity. In the dispersive limit, probing the qubit-statedependent resonant frequency of the cavity implements, to first order, a QND measurement of the qubit state.
Vacuum-Fluctuation-Induced Dephasing of a Qubit in Circuit Quantum Electrodynamics
Journal of the Physical Society of Japan, 2014
We investigate the measurement-induced dephasing of a qubit coupled with a single-mode cavity in the vacuum limit. Dephasing of the qubit state takes place through the entanglement of the qubit and the single probe photon sent to the cavity, while the cavity mode never occupies a photon. We find that the qubit state is dephased even if the cavity is always in the vacuum state. This dephasing is caused purely by the interaction between the qubit and the vacuum field. We also show that this vacuum-fluctuation-induced dephasing takes place much faster than the spontaneous decay of the qubit excited state, and therefore our prediction is observable in a real experiment.
2017
An analytical description of a qubit interacting non-linearly and non-resonantly with a lossy cavity via intensity-dependent coupling has been obtained. With the amplitude cavity damping as a particular type of the thermal amplitude reservoir damping, Wehrl entropy and Wehrl density are used to investigate the dynamics of the loss of both qubit coherence and information. We show that the Q-function Wehrl entropy and its density are very sensitive not only to the amplitude cavity damping and the intensity of the coherent state but also to the frequency detuning. The information of the phase space and the coherence are quickly lost due to the coupling to the environment. When the qubit interacting non-linearly with the lossy cavity, we observe: (1) The mixedness of the atomic state can be decreased by increasing the coupling to the environment. (2) For the off-resonance case, if the cavity damping is increased, the information of the mixed evolved state can be protected.
Quantum State Reduction and Conditional Time Evolution of Wave-Particle Correlations in Cavity QED
Physical Review Letters, 2000
We report measurements in cavity QED of a wave-particle correlation function which records the conditional time evolution of the field of a fraction of a photon. Detection of a photon prepares a state of well-defined phase that evolves back to equilibrium via a damped vacuum Rabi oscillation. We record the regression of the field amplitude. The recorded correlation function is nonclassical and provides an efficiency independent path to the spectrum of squeezing. Nonclassicality is observed even when the intensity fluctuations are classical. PACS numbers: 42.50.Dv, 42.50.Ct The seminal work of Hanbury-Brown and Twiss [1] marks the beginning of the systematic study of the quantum fluctuations of light. Two lines of experiments are notable: those measuring correlations between pairs of photodetections (particle aspect of light) [2-6] and squeezing experiments which measure the variance of the electromagnetic field amplitude (wave aspect of light) . No attempt has been made previously to draw the particle and wave aspects together by correlating a photon detection with fluctuations of the electromagnetic field amplitude. We have done this, extending the ideas of Hanbury-Brown and Twiss to record the conditional time evolution of the amplitude fluctuations of an electromagnetic wave. Measurements are made in the strong-coupling regime of cavity quantum electrodynamics (QED) [10] and exhibit the nonclassical fluctuations of light in a dramatic new way.
Relaxation and dephasing in a qubit measurement
arXiv (Cornell University), 2003
We consider a qubit interacting with an environment and continuously monitored by a detector represented by a tunnel junction. Bloch-type equations describing the entire system of qubit and detector are derived. Using these equations we evaluate the detector current and its noise spectrum in terms of decoherence and relaxation rates of the qubit. Simple expressions are obtained which show how these quantities can be measured. We demonstrate that the detector and an environment at zero temperature affect the qubit behavior in a different way. In particular, an environment destroys the Zeno effect, predicted for strong coupling with the detector.