Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session S75: Advances in Superconducting Qubit Readout and ResetFocus
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Sponsoring Units: DQI Chair: Christian Andersen, Delft University of Technology Room: Room 401/402 |
Thursday, March 9, 2023 8:00AM - 8:36AM |
S75.00001: Parametric cQED: a new approach to realizing strong light-matter interactions Invited Speaker: Zhihao Xiao Strong light-matter interactions form the standard paradigm for quantum information processing. Most QIP platforms, however, employ static interactions where both the strength and the form of interactions are `hard-wired’ in the design. In this talk, I will describe a new framework based on time-dependent strong parametric interactions, a.k.a. parametric cQED, which realizes new and powerful functionalities for in-situ tunable qubit readout and control. To analyze such strong parametric couplings, we developed a perturbative Hamiltonian diagonalization technique based on sequential time-dependent Schrieffer-Wolff transformations [1], which allows accounting for inertial terms at successive orders independently. When applying our technique to canonical cQED setups, we predict astonishingly large dispersive shifts compared to those attainable with static interactions. Crucially, these dressed shifts are tunable in magnitude and sign with the frequency and the amplitude of the pump mediating the interactions; this tunability supports exact cancellation of static and qubit-induced dispersive shifts on the cavity for specific pump frequencies (“blind spots”) even for strong interaction strengths. We find the presence of blind spots to be a robust feature in parametric cQED, under both two-level and multi-level approximations, that can be exploited to realize a readout switch with high on/off ratio. Furthermore, our results indicate that Bloch-Siegert shifts can be rendered large even in the dispersive regime, and hence the validity of the rotating-wave needs to be revisited in the presence of time-dependent interactions. Finally, we identify a new “parametric straddling regime” in multi-level atoms, such as transmons, realizable even when the detuning between the transmon and cavity frequencies is large. In support of our theory, I will present experimental results [2] showing high-contrast single- and joint-qubit readout in a system comprising two transmons parametrically coupled to a lumped-element cavity. |
Thursday, March 9, 2023 8:36AM - 8:48AM |
S75.00002: Observation of Extremely Large Lamb Shift in a Multi-mode Circuit QED System in Deep-strong Coupling Regime Ziqiao Ao, Sahel Ashhab, Fumiki Yoshihara, Tomoko Fuse, Kosuke Kakuyanagi, Shiro Saito, Takao Aoki, Kouichi Semba We report experimental results on an extremely large Lamb shift observed in a multi-mode circuit QED system in the deep-strong coupling (DSC) regime, where the qubit-resonator coupling strength exceeds the qubit energy and the resonator frequency. The system comprises a superconducting flux qubit (FQ) and a quarter-wavelength coplanar waveguide resonator (CPWR) which are coupled inductively through a shared edge that contains a Josephson junction to achieve the DSC regime [1,2]. The FQ is placed at the open end of the CPWR to couple with every mode in the CPWR. Spectroscopy results around the frequency of the fundamental mode of the CPWR is measured and fitted by the single-mode quantum Rabi model Hamiltonian as an approximation to obtain the parameters of the system. The renormalized qubit energy is calculated through the difference between the frequencies of the 03, from the ground to the third excited state, and 13, from the first excited to the third excited state, transitions. The fundamental mode contributes an 85.4% Lamb-shift suppression of the qubit energy. Since the qubit is also coupled to an infinite number of higher modes in the resonator, the single-mode fitting does not provide the bare qubit energy. If we use the designed qubit energy as reference, we find that the Lamb shift is 99.7% of the bare qubit energy. This result shows that the coupling to an infinite number of modes induces a huge Lamb shift to the qubit energy. We also provide theoretical formulas for the Lamb shift in our circuit, which is consistent with previous results in the literature [3,4] despite differences in the circuit design and qubit-resonator coupling mechanism. |
Thursday, March 9, 2023 8:48AM - 9:00AM |
S75.00003: Measuring a superconducting qubit linearly decoupled from the readout resonator (1/2) Wei Dai, Sumeru Hazra, Zhixin Wang, Rodrigo G Cortinas, Jayameenakshi Venkatraman, Xu Xiao, Alec W Eickbusch, Luigi Frunzio, Michel H Devoret Dispersive readout of superconducting qubit is now a standard measurement scheme in circuit quantum electrodynamics (cQED) systems. It is realized by linearly coupling a readout resonator to a qubit in the dispersive regime. However, undesired qubit state transition induced by readout photons (the so-called “T1 versus nbar” effect) tends to limit further improvement of readout performance. To overcome this obstacle, we introduce a superconducting artificial molecule that integrates a “dark” transmon qubit and a “bright” meter mode with quartic coupling between each other. A two-stage readout of the qubit state is mediated by the intra-cavity meter mode. The absence of linear coupling between the qubit and the readout cavity improves readout fidelity and speed. |
Thursday, March 9, 2023 9:00AM - 9:12AM |
S75.00004: Measuring a superconducting qubit linearly decoupled from the readout resonator (2/2) Sumeru Hazra, Wei Dai, Zhixin Wang, Rodrigo G Cortinas, Jayameenakshi Venkatraman, Xu Xiao, Alec W Eickbusch, Luigi Frunzio, Michel H Devoret Dispersive readout of superconducting qubit is now a standard measurement scheme in circuit quantum electrodynamics (cQED) systems. It is realized by linearly coupling a readout resonator to a qubit in the dispersive regime. However, undesired qubit state transition induced by readout photons (the so-called “T1 versus nbar” effect) tends to limit further improvement of readout performance. To overcome this obstacle, we introduce a superconducting artificial molecule that integrates a “dark” transmon qubit and a “bright” meter mode with quartic coupling between each other. A two-stage readout of the qubit state is mediated by the intra-cavity meter mode. The absence of linear coupling between the qubit and the readout cavity improves readout fidelity and speed. |
Thursday, March 9, 2023 9:12AM - 9:24AM |
S75.00005: Measurement-Induced State Transitions in a Superconducting Qubit: Part I, Experiment Alexander M Opremcak, Mostafa Khezri, Zijun Chen, Andreas Bengtsson, Ted White, Ofer Naaman, Daniel T Sank, Alexander Korotkov Dispersive readout of superconducting qubits is performed by driving a resonator that is coupled to the qubit in order to measure the state-dependent frequency shift of the resonator and infer the qubit state. Experiments have shown that when the resonator photon number exceeds a certain threshold, the qubit is excited out of its computational subspace, which we refer to as a measurement-induced state transition. These transitions degrade readout fidelity, and constitute leakage which precludes further operation of the qubit in e.g. error correction. Here we study these transitions using a transmon qubit by experimentally measuring their dependence on qubit frequency and average photon number, in the regime where the resonator frequency is lower than the qubit frequency. We observe signatures of resonant transitions between levels in the coupled qubit-resonator system that exhibit noisy behavior when measured repeatedly in time. We provide a semi-classical model of these transitions based on the rotating wave approximation and use it to predict the onset of transitions versus frequency and photon number. Our results suggest the transmon is excited to levels near the top of its cosine potential, and that offset charge dispersion can explain the observed noisy dynamics of these transitions. |
Thursday, March 9, 2023 9:24AM - 9:36AM |
S75.00006: Measurement-Induced State Transitions in a Superconducting Qubit: Part II, Theory Mostafa Khezri, Alex Opremcak, Zijun Chen, Andreas Bengtsson, Ted White, Ofer Naaman, Daniel T Sank, Alexander Korotkov Dispersive readout of superconducting qubits is performed by driving a resonator that is coupled to the qubit in order to measure the state-dependent frequency shift of the resonator and infer the qubit state. Experiments have shown that when the resonator photon number exceeds a certain threshold, the qubit is excited out of its computational subspace, which we refer to as a measurement-induced state transition. These transitions degrade readout fidelity, and constitute leakage which precludes further operation of the qubit in e.g. error correction. Here we study these transitions using a transmon qubit by experimentally measuring their dependence on qubit frequency and average photon number, in the regime where the resonator frequency is lower than the qubit frequency. We observe signatures of resonant transitions between levels in the coupled qubit-resonator system that exhibit noisy behavior when measured repeatedly in time. We provide a semi-classical model of these transitions based on the rotating wave approximation and use it to predict the onset of transitions versus frequency and photon number. Our results suggest the transmon is excited to levels near the top of its cosine potential, and that offset charge dispersion can explain the observed noisy dynamics of these transitions. |
Thursday, March 9, 2023 9:36AM - 9:48AM |
S75.00007: Bolometric readout of superconducting qubits, Part 1/2 András Gunyhó, Priyank Singh, Suman Kundu, Jian Ma, Wei Liu, Giacomo Catto, Sakari Niemelä, Qiming Chen, Mikko Möttönen Measuring the state of qubits is a fundamental operation in quantum computing, quantum error correction, and quantum communication. Recently, nanobolometers have been developed that allow the thermal detection of a low number of microwave photons [1,2]. We use such a bolometer in place of a parametric amplifier (paramp) in a dispersive-readout setup to measure the state of a superconducting qubit. |
Thursday, March 9, 2023 9:48AM - 10:00AM |
S75.00008: Bolometric readout of superconducting qubits, Part 2/2 Priyank Singh Singh, András Gunyhó, Suman Kundu, Jian Ma, Wei Liu, Giacomo Catto, Sakari Niemelä, Qiming Chen, Mikko Möttönen Measuring the state of qubits is a fundamental operation in quantum computing, quantum error correction, and quantum communication. Recently, nanobolometers have been developed that allow the thermal detection of a low number of microwave photons [1,2]. We use such a bolometer in place of a parametric amplifier (paramp) in a dispersive-readout setup to measure the state of a superconducting qubit. |
Thursday, March 9, 2023 10:00AM - 10:12AM |
S75.00009: Measurement of critical photon number of the 2nd excited state of a Transmon qubit JeaKyung Choi, Hyeok Hwang, Eunseong Kim Two level quantum states in a transmon qubit can be distinguished by their interaction with a dispersive coupled cavity. When the interaction between the cavity and the qubit is relatively smaller than exchange energy of cavity photon and qubit excitation, the combined cavity-qubit system can be assumed as a linear system and perturbation. Non-linear contribution at higher Fock numbers and higher order qubit state becomes more relevant in the interaction Hamiltonian, H=g(a†b+ab†), as the photon number in the cavity increases. The resonance frequency of cavity shifts gradually with increasing power and splits finally when exceeding the critical point. These shift and splitting, non-linear behavior, can be understood by the non-linearity of interaction Hamiltonian. In addition to previous research[1], we study the power-dependent dynamics of ground, primary, and secondary excited states of a transmon qubit in the dispersive regimes up to punch-out. We try to obtain the critical photon number of nonlinear behavior and explain how it is determined. |
Thursday, March 9, 2023 10:12AM - 10:24AM |
S75.00010: Exploring dispersive qubit readout in the strong driving limit. Luciano I Pereira Dispersive readout in superconducting circuits is a limiting factor in the performance of current quantum processors. Experimentally, it has been observed that increasing the intensity of the readout pulses improves the signal-to-noise ratio of the measurement up to some threshold [1,2], where non-dispersive effects and leakage to higher levels enter into play. In this work, we perform a numerical study of the dispersive measurement of superconducting qubits to find the optimal calibration point in the strong driving limit. Moreover, using QND measurement tomography [3,4], we identify the physical processes and error sources that affect the QND nature of the measurement. |
Thursday, March 9, 2023 10:24AM - 10:36AM |
S75.00011: Novel wideband Purcell filters for circuit QED Basil M Smitham, Christie S Chiu, Jeronimo G Martinez, Andrew A Houck On-chip Purcell filters — which suppress the spontaneous emission of superconducting qubits into their control and readout lines — are a key component of many quantum processors. Here we present a new design methodology for Purcell filters, which can have compact size, while also maintaining large frequency bandwidths for both qubits and readout resonators — with extensive control over the frequency scaling of readout resonator linewidths. We experimentally demonstrate the functionality of these filters in devices containing transmon qubits. |
Thursday, March 9, 2023 10:36AM - 10:48AM |
S75.00012: Rapid high-fidelity readout of a strongly Purcell-filtered transmon qubit Yoshiki Sunada, Shuhei Tamate, Peter Spring, Arjan F Van Loo, Jesper Ilves, Shingo Kono, Yasunobu Nakamura High-fidelity quantum-nondemolition (QND) readout of qubit states is crucial for fault-tolerant quantum computation using superconducting qubits. Such a readout is commonly realized using the dispersive interaction between a qubit and a resonator. To achieve a high readout fidelity, the resonator is often accompanied by a Purcell filter, which suppresses the Purcell effect—the energy decay of the qubit mediated by the resonator. However, the rate of this decay has been found to significantly increase while a readout pulse is being applied [1]. |
Thursday, March 9, 2023 10:48AM - 11:00AM |
S75.00013: Enhancing the Dispersive Interaction between a Qubit and Squeezed Photons Marius Villiers, Clarke Smith, Alexandru Petrescu, Alvise Borgognoni, Matthieu Delbecq, Alain Sarlette, Mazyar Mirrahimi, Philippe Campagne-Ibarcq, Takis Kontos, Zaki Leghtas The interaction strength of an oscillator to a qubit is proportional to its vacuum field fluctuations. The well known degenerate parametric oscillator has revived interest in the regime of strongly detuned squeezing, where its eigenstates are squeezed Fock states. Owing to these amplified fluctuations, it was recently proposed that such an oscillator, coined the Bogoliubov oscillator, would couple more strongly to a qubit. In a superconducting circuit experiment, we have observed that squeezing a Bogoliubov oscillator amplifies its dispersive coupling to a qubit. We measure a two-fold increase in the dispersive interaction strength at 5.5 dB of squeezing. This work initiates the experimental coupling of Bogoliubov oscillators to qubits, and cautiously motivates their dissemination in experimental platforms seeking enhanced interactions. |
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