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Physical review. B./Physical review. B

Photon number splitting in a multilevel transmon with different microwave photon sources

Zishuo Li, Tingting Guo, Kaixuan Zhang, Wenqu Xu, Quan Zuo, Jinpeng Li, Jiazheng Pan, Hancong Sun, Chunhai Cao, Guozhu Sun, Peiheng Wu

Published Feb 28, 2025• 0 citations
Publisher

Abstract

We present the observation of photon number splitting (PNS) in a superconducting transmon qubit with different microwave photon sources. Such PNS is not limited to the lower energy levels and can also be observed in the higher energy levels of a superconducting transmon qubit. The sideband transition, Autler-Townes splitting, and the absence of PNS are demonstrated under certain conditions. Furthermore, we investigate the PNS induced by the nonclassical microwave photons emitted from a dual-mode Josephson parametric amplifier, which can operate in either the single mode or dual mode. The evolution from the dual-mode squeezing to the self-oscillation light emission is demonstrated. Our results provide a useful reference for future investigations, such as the exploration of more nonlinear effects in strongly driven circuit QED systems, the manipulation of quantum states, and the characterization of the quantum microwave source by utilizing PNS.

DOI
10.1103/physrevb.111.054525

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A superconducting qubit in the strong dispersive regime of circuit quantum electrodynamics is a powerful probe for microwave photons in a cavity mode. In this regime, a qubit excitation spectrum is split into multiple peaks, with each peak corresponding to an individual photon number in the cavity (discrete ac Stark shift). Here, we measure the qubit spectrum in a cavity that is driven continuously with a squeezed vacuum generated by a Josephson parametric amplifier. By fitting the obtained spectrum with a model which takes into account the finite qubit excitation power, we determine the photon number distribution, which reveals an even-odd photon number oscillation and quantitatively fulfills Klyshko's criterion for nonclassicality.
We address recent advances in quantum optics with propagating microwaves\nin superconducting circuits. This research field exploits on the fact that the coupling between a superconducting artificial atom and propagating microwave photons in a one-dimensional (1D) open transmission line can be made strong enough to observe quantum effects, without using any cavity to confine the microwave photons.\nWe embed an artificial atom, a superconducting transmon qubit, in a 1D open\ntransmission line and investigate the scattering properties of coherent microwaves. When an input coherent state, with an average photon number much less than 1, is on resonance with the artificial atom, we observe extinction of up to 99% in the forward propagating field. We observe the strong nonlinearity of the artificial atom and under strong driving we observe the Mollow triplet. We also study the statistics of the reflected and transmitted beams, which are predicted to be nonclassical states. In particular, we demonstrate photon antibunching in the reflected beam by measuring the second-order correlation function. By applying a second control tone, we observe the Autler-Townes splitting and a giant cross-Kerr effect.\nFurthermore, we demonstrate fast operation of a single-photon router using\nthe Autler-Townes splitting. This device provides important steps towards the realization of a quantum network. This thesis describes the motivation, theoretical background, design, implementation and measurement results.
Abstract We present a detailed study of a microwave single-photon source utilizing a transmon qubit asymmetrically coupled to two transmission lines: a weakly coupled drive line and a strongly coupled emission line. Our design allows for the generation of single photons in the microwave regime, which are essential for quantum communication, computing, and sensing applications. In addition to single-photon generation, the same design functions as a power sensor and enables precise measurement of the system’s attenuation. This dual functionality allows us to calibrate the noise of our amplification chain. This enables a self-calibrated architecture in which the same device acts simultaneously as the photon source and as a reference standard, eliminating the need for external tones or calibrated inputs. With this calibration we reconstruct the density matrix of the emitted photon state, for which we achieve a fidelity of 61.4% and measure a second-order correlation function of g ( 2 ) ( 0 ) = 0.06 − 0.06 + 0.45 , indicating quantum nature and single-photon characteristics. These results confirm generation of single photons and demonstrate the potential of transmon qubits in advancing quantum technologies in the microwave domain.
Quantum computing requires a novel approach to store data as quantum states, opposite to classical bits. One of the most promising candidates is entangled photons. In this manuscript, we show the photon emission in the range of microwave frequencies of three different types of superconducting circuits, a SQUID, a JPA, and a JTWPA, often used as low-noise parametric amplifiers. These devices can be operated as sources of entangled photons. We report the experimental protocol used to produce and measure microwave radiation from these circuits, as well as data simulations. The collected spectra are obtained by performing single-tone measurements with a direct rf pump on the devices; the output spectra at low powers (below −100 dBm) are well interpreted by the dynamical Casimir model, while at high powers (above −100 dBm) the system is well described by the Autler–Townes fluorescence of a three-level atom.
We have observed the Autler-Townes doublet in a superconducting Al/AlOX{}_{X}X​/Al transmon qubit that acts as an artificial atom embedded in a three-dimensional Cu microwave cavity at a temperature of 22 mK. Using pulsed microwave spectroscopy, the three lowest transmon levels are isolated, eliminating unwanted effects of higher qubit modes and cavity modes. The long coherence time ($\ensuremath{\sim}$$40\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\text{s}$) of the transmon enables us to observe a clear Autler-Townes splitting at drive amplitudes much smaller than the transmon level anharmonicity (177 MHz). Three-level density matrix simulations with no free parameters provide excellent fits to the data. At maximum separation, the fidelity of a dark state achieved in this experiment is estimated to be 99.699.6%--99.9%99.6.
We measure photon occupancy in a thin-film superconducting lumped element resonator coupled to a transmon qubit at 20mK20\phantom{\rule{0.16em}{0ex}}\mathrm{mK}20mK and find a nonlinear dependence on the applied microwave power. The transmon-resonator system was operated in the strong dispersive regime, where the ac Stark shift $(2\ensuremath{\chi})$ due to a single microwave photon present in the resonator was larger than the linewidth $(\mathrm{\ensuremath{\Gamma}})$ of the qubit transition. When the resonator was coherently driven at 5.474 325 GHz, the transition spectrum of the transmon at 4.982 GHz revealed well-resolved peaks, each corresponding to an individual photon number-state of the resonator. From the relative peak heights we obtain the occupancy of the photon states and the average photon occupancy n‾\overline{n}n of the resonator. We observe a nonlinear variation of n‾\overline{n}n with the applied drive power Prf{P}_{\mathrm{rf}}Prf​ for n‾<5\overline{n}<5n<5 and compare our results to numerical simulations of the system-bath master equation in the steady state, as well as to a semiclassical model for the resonator that includes the Jaynes-Cummings interaction between the transmon and the resonator. We find good quantitative agreement using both models and analysis reveals that the nonlinear behavior is principally due to shifts in the resonant frequency caused by a qubit-induced Jaynes-Cummings nonlinearity.