Circuit Quantum Electrodynamics Architecture Research Articles

Exceptional Entanglement Phenomena: Non-Hermiticity Meeting Nonclassicality.

Non-Hermitian (NH) extension of quantum-mechanical Hamiltonians represents one of the most significant advancements in physics. During the past two decades, numerous captivating NH phenomena have been revealed and demonstrated, but all of which can appear in both quantum and classical systems. This leads to the fundamental question: what NH signature presents a radical departure from classical physics? The solution of this problem is indispensable for exploring genuine NH quantum mechanics, but remains experimentally untouched so far. Here, we resolve this basic issue by unveiling distinct exceptional entanglement phenomena, exemplified by an entanglement transition, occurring at the exceptional point of NH interacting quantum systems. We illustrate and demonstrate such purely quantum-mechanical NH effects with a naturally dissipative light-matter system, engineered in a circuit quantum electrodynamics architecture. Our results lay the foundation for studies of genuinely quantum-mechanical NH physics, signified by exceptional-point-enabled entanglement behaviors.

Quantum-noise-limited microwave amplification using a graphene Josephson junction.

Josephson junctions (JJs) and their tunable properties, including their nonlinearities, play an important role in superconducting qubits and amplifiers. JJs together with the circuit quantum electrodynamics architecture form many key components of quantum information processing1. In quantum circuits, low-noise amplification of feeble microwave signals is essential, and Josephson parametric amplifiers (JPAs)2 are the widely used devices. The existing JPAs are based on Al-AlOx-Al tunnel junctions realized in a superconducting quantum interference device geometry, where magnetic flux is the knob for tuning the frequency. Recent experimental realizations of two-dimensional (2D) van der Waals JJs3-5 provide an opportunity to implement various circuit quantum electrodynamics devices6-8 with the added advantage of tuning the junction properties and the operating point using a gate potential. While other components of a possible 2D van der Waals circuit quantum electrodynamics architecture have been demonstrated, a quantum-noise-limited amplifier, an essential component, has not been realized, to the best of our knowledge. Here we implement a quantum-noise-limited JPA using a graphene JJ, that has a linear resonance gate tunability of 3.5 GHz. We report 24 dB amplification with 10 MHz bandwidth and -130 dBm saturation power, a performance on par with the best single-junction JPAs2,9. Importantly, our gate-tunable JPA works in the quantum-limited noise regime, which makes it an attractive option for highly sensitive signal processing. Our work has implications for novel bolometers; the low heat capacity of graphene together with JJ nonlinearity can result in an extremely sensitive microwave bolometer embedded inside a quantum-noise-limited amplifier. In general, this work will open up the exploration of scalable device architectures of 2D van der Waals materials by integrating a sensor with the quantum amplifier.

High cooperativity coupling to nuclear spins on a circuit quantum electrodynamics architecture

Nuclear spins are candidates to encode qubits or qudits due to their isolation from magnetic noise and potentially long coherence times. However, their weak coupling to external stimuli makes them hard to integrate into circuit quantum electrodynamics architectures, the leading technology for solid-state quantum processors. Here, we study the coupling of 173Yb(III) nuclear spin states in an [Yb(trensal)] molecule to superconducting cavities. Experiments have been performed on magnetically dilute single crystals placed on the inductors of lumped-element LC superconducting resonators with characteristic frequencies spanning the range of nuclear and electronic spin transitions. We achieve a high cooperative coupling to all electronic and most nuclear [173Yb(trensal)] spin transitions, a necessary ingredient for the implementation of qudit protocols with molecular spins using a hybrid architecture.

Photon generation and entanglement in a double superconducting cavity

We study the dynamical Casimir effect in a double superconducting cavity in a circuit quantum electrodynamics architecture. Parameters in the quantum circuit are chosen in such a way the superconducting cavity can mimic a double cavity, formed by two perfectly conducting outer walls and a dielectric one, with arbitrary permittivity separating both halves. We undertake a spectral analysis of the cavity, showing that the spectrum varies significantly depending on the values of the susceptibility of the dielectric mirror and the relative lengths of both cavities. We study the creation of photons when the walls oscillate harmonically with a small amplitude. Furthermore, we explore the possibility of entangling two uncoupled cavities, starting from a symmetric double cavity and having both of its halves become uncoupled at a later given instant. We consider both cases: (i) when the field is initially in a vacuum state and (ii) the situation in which photon creation via the dynamical Casimir effect has already taken place. We show that the cavities become entangled in both cases but, in the latter, the quantum correlation between individual modes can be greatly increased at the cost of diminishing the entanglement between most pairs of modes.

Shortcuts to Adiabaticity for Fast Qubit Readout in Circuit Quantum Electrodynamics

We propose how to engineer the longitudinal coupling to accelerate the measurement of a qubit longitudinally coupled to a cavity, motivated by the concept of shortcuts to adiabaticity. Different modulations are inversely designed from two methods of inverse engineering and counter-diabatic driving, for achieving larger values of the signal-to-noise ratio (SNR) at nanosecond scale. By comparison, we demonstrate that our protocols outperform the usual periodic modulations on the pointer state separation and SNR. Finally, we show a possible implementation considering state-of-the-art circuit quantum electrodynamics architecture, estimating the minimal time allowed for the measurement process.

Single electrons on solid neon as a solid-state qubit platform.

Progress towards the realization of quantum computers requires persistent advances in their constituent building blocks-qubits. Novel qubit platforms that simultaneously embody long coherence, fast operation and large scalability offer compelling advantages in the construction of quantum computers and many other quantum information systems1-3. Electrons, ubiquitous elementary particles of non-zero charge, spin and mass, have commonly been perceived as paradigmatic local quantum information carriers. Despite superior controllability and configurability, their practical performance as qubits through either motional or spin states depends critically on their material environment3-5. Here we report our experimental realization of a qubit platform based on isolated single electrons trapped on an ultraclean solid neon surface in vacuum6-13. By integrating an electron trap in a circuit quantum electrodynamics architecture14-20, we achieve strong coupling between the motional states of a single electron and a single microwave photon in an on-chip superconducting resonator. Qubit gate operations and dispersive readout are implemented to measure the energy relaxation time T1 of 15 μs and phase coherence time T2 over 200 ns. These results indicate that the electron-on-solid-neon qubit already performs near the state of the art for a charge qubit21.

Front Cover: Characterization of a Triple Quantum Dot via an On‐chip Microwave Resonator (Adv. Quantum Technol. 2/2022)

Circuit quantum electrodynamics architecture provides a controllable artificial system and enriches the physics with the participation of electron-photon interaction. The cover shows this hybrid architecture comprised of a superconducting microwave resonator and a triple quantum dot. In article number 2100104, Si-Si Gu and colleagues investigate quantum cellular automata and “photon-assisted” quadruple point phenomena in this system, which involve the Coulomb interaction among multiple dots and the simultaneously-tunneling processes assisted with cavity photons. These results suggest that this hybrid system has the potential for studying exotic many-body effects and electron-photon interaction.

Characterization of a Triple Quantum Dot via an On‐chip Microwave Resonator

AbstractThe hybrid circuit quantum electrodynamics architecture integrated quantum dots with a microwave resonator results in the creation of a controllable artificial system and enriches its physics through electron–photon interaction. In this study, a hybrid device is investigated, wherein a triple quantum dot (TQD) is dipole coupled with electric field of a superconducting quantum interference device array resonator. The quantum cellular automata (QCA) process of TQD, related to charge reconfiguration as a result of Coulomb interaction between each dot, is observed by probing reflectance of the resonator. The suppressed cavity reflectance signal of the QCA process may experimentally provide the evidence for the different measurement mechanism of the cavity from external electrometers like quantum point contact. Furthermore, on careful tuning of multiple energy level differences of the TQD to match the cavity photon frequency, four tunneling processes assisted by the absorption of the cavity photons occur simultaneously, contributing to “photon‐assisted” quadruple point. Consequently, these two specific phenomena are found to be closely dependent on the relationship between the various Coulomb energy in TQD and the cavity photon energy. The results are consistent with the theoretical model presented and imply that this hybrid system has the potential for investigating exotic many‐body effects and matter–light interaction.

Kondo induced π -phase shift of microwave photons in a circuit quantum electrodynamics architecture

Mesoscopic systems constitute appealing platforms to study many-body physics with light and matter degrees of freedom. The Kondo effect refers to the screening of a spin-1/2 impurity by a cloud of conduction electrons, then forming a many-body Fermi liquid ground state. The Kondo resonance produces a phase shift in the transmitted electronic wave packet which depends on the symmetry and nature of the many-body ground state. Theoretical calculations suggest that the Kondo resonance can interact with the irradiation photon field and should give rise to a {\pi}-phase shift of the photon signal in the case where the ground state is a Fermi liquid. This {\pi}-phase shift of microwave photon is driven from the Korringa-Shiba relation of quantum impurity Fermi-liquid ground states. We report the first observation of such a {\pi}-phase shift in a graphene double quantum dot within a circuit quantum electrodynamics architecture where the microwave photons couple to the pseudo-spin or charge degrees of freedom. The observed Kondo temperature TK ~ 550 mK is in agreement with DC conductance measurements. All our results support the formation of a Kondo resonance located above the Fermi level of the electronic reservoirs and the occurrence of an SU(4) Fermi-liquid ground state. We finally study how the Kondo-photon interactions can be tuned by inter-dot electron tunnel coupling strengths. Our experimental achievements may contribute to a better understanding of many-body physics in hybrid circuit systems, and open up new applications in atomic thin materials from the light-matter interaction.

Phase-sensitive Landau-Zener-Stückelberg interference in superconducting quantum circuit* *Project supported by the Key-Area Research and Development Program of Guangdong Province, China (Grant No. 2018B030326001), the National Natural Science Foundation of China (Grant Nos. U1801661, 11874065, and Youth Project No. 11904158), the Guangdong Provincial Key Laboratory (Grant No. 2019B121203002), the Natural Science Foundation of Hunan Province, China (Grant No. 2018JJ1031), and the

Superconducting circuit quantum electrodynamics (QED) architecture composed of superconducting qubit and resonator is a powerful platform for exploring quantum physics and quantum information processing. By employing techniques developed for superconducting quantum computing, we experimentally investigate phase-sensitive Landau–Zener–Stückelberg (LZS) interference phenomena in a circuit QED. Our experiments cover an extensive range of LZS transition parameters and demonstrate the LZS induced Rabi-like oscillation as well as phase-dependent steady-state population.

Probing XY phase transitions in a Josephson junction array with tunable frustration

The seminal theoretical works of Berezinskii, Kosterlitz, and Thouless presented a new paradigm for phase transitions in condensed matter that are driven by topological excitations. These transitions have been extensively studied in the context of two-dimensional XY models -- coupled compasses -- and have generated interest in the context of quantum simulation. Here, we use a circuit quantum-electrodynamics architecture to study the critical behavior of engineered XY models through their dynamical response. In particular, we examine not only the unfrustrated case but also the fully-frustrated case which leads to enhanced degeneracy associated with the spin rotational [U$(1)$] and discrete chiral ($Z_2$) symmetries. The nature of the transition in the frustrated case has posed a challenge for theoretical studies while direct experimental probes remain elusive. Here we identify the transition temperatures for both the unfrustrated and fully-frustrated XY models by probing a Josephson junction array close to equilibrium using weak microwave excitations and measuring the temperature dependence of the effective damping obtained from the complex reflection coefficient. We argue that our probing technique is primarily sensitive to the dynamics of the U$(1)$ part.

Observation of a Strongly Enhanced Relaxation Time of an In-situ Tunable Transmon on a Silicon Substrate up to the Purcell Limit Approaching 100 μs

The remarkable advances of quantum computation technology with superconducting qubits based on circuit quantum electrodynamics (QED) architecture have been achieved by improving control, protection and measurement of the quantum states at the same time. At the heart of all these quantum operations, the significant enhancement of the qubit coherence time during the last decades was the key. Even after all these advances, the coherence and relaxation time of superconducting qubits still requires further improvements toward fault-tolerant quantum computation. Here, we report our observation of a strongly enhanced lifetime of an in-situ tunable superconducting transmon qubit on a silicon substrate that is embedded in a three-dimensional copper cavity. We measured a lifetime of the qubit of up to 84 µs, which is the best reported value of an in-situ tunable transmon on a silicon substrate. In our experiment, the in-situ frequency tunability over a broad range enabled the Purcell factor to be controlled continuously by detuning the qubit frequency against the resonator frequency in the strong dispersive regime. The silicon substrate has its own importance because the substrate should be fully compatible with the conventional semiconductor processes so that scalability and multi-chip module capability are guaranteed. In order to control the Purcell factor with another parameter, we displaced the qubit position in the cavity and observed a longer relaxation time with a smaller coupling coefficient. We believe that this systematic study and the control technique of the Purcell effect in the circuit QED design, together with minimizing the microwave photon loss through an improvement of the fabrication, will contribute to the realization of a practical large-scale quantum computer based on superconducting qubit technology.

On-demand generation and characterization of a microwave time-bin qubit

Superconducting circuits offer a scalable platform for the construction of large-scale quantum networks, where information can be encoded in multiple temporal modes of propagating microwaves. Characterization of such microwave signals with a method extendable to an arbitrary number of temporal modes with a single detector and demonstration of their phase-robust nature are of great interest. Here, we show the on-demand generation and Wigner tomography of a microwave time-bin qubit with superconducting circuit quantum electrodynamics architecture. We perform the tomography with a single heterodyne detector by dynamically switching the measurement quadrature independently for two temporal modes through the pump phase of a phase-sensitive amplifier. We demonstrate that the time-bin encoding scheme relies on the relative phase between the two modes and does not need a shared phase reference between sender and receiver.

Automated analysis of single-tone spectroscopic data for cQED systems

Physical systems for quantum computation require calibration of the control parameters based on their physical characteristics by performing a chain of experiments that gather most precise information about the given device. It follows that there is a need for automated data acquisition and interpretation. In this work, we have developed a tool that allows for automatic analysis of single-tone spectroscopy (STS) results for a single cell consisting of qubit and resonator in the circuit quantum electrodynamics architecture. Using analytic approaches and maximum likelihood estimation, our algorithm is capable of finding all relevant physical characteristics of the cell by using only the measured STS data. The described approach is fast and robust to noise, and its open-source Python implementation can readily be used to calibrate transmon qubits coupled to notch-port resonators.

Multiplexed readout of four qubits in 3D circuit QED architecture using a broadband Josephson parametric amplifier

We propose and demonstrate a frequency-multiplexed readout scheme in 3D circuit-quantum electrodynamics (cQED) architecture. We use four transmon qubits coupled to individual rectangular cavities which are aperture-coupled to a common rectangular waveguide feedline. A coaxial to waveguide transformer at the other end of the feedline allows one to launch and collect the multiplexed signal. The reflected readout signal is amplified by an impedance-engineered broadband parametric amplifier with 380 MHz bandwidth. This provides us high fidelity single-shot readout of multiple qubits using compact microwave circuitry, an efficient way for scaling up to more qubits in 3D cQED.

Charge noise acting on graphene double quantum dots in circuit quantum electrodynamics architecture**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFA0301700), the National Natural Science Foundation of China (Grant Nos. 61674132, 11674300, 11575172, and 11625419), and the Anhui Initiative in Quantum information Technologies, China (Grant No. AHY080000). This work was partially carried out at

We investigate the dephasing mechanisms induced by the charge noise and microwave heating effect acting on a graphene double quantum dot (DQD) capacitively coupled to a microwave resonator. The charge noise is obtained from DC transport current, and its contribution to dephasing is simultaneously determined by the amplitude response of the microwave resonator. A lowfrequency 1/f-type noise is demonstrated to be the dominant factor of the dephasing of graphene DQD. Furthermore, when the applied microwave power is larger than −90 dBm, the dephasing rate of graphene DQD increases rapidly with the increase of microwave power, and fluctuates slightly with the applied microwave power smaller than −90 dBm. Our results can be applied to suppress the impeditive influence on the dephasing of graphene-based devices associated with microwave input in the perspective investigations.

Characterization of hidden modes in networks of superconducting qubits

We present a method for detecting electromagnetic (EM) modes that couple to a superconducting qubit in a circuit quantum electrodynamics architecture. Based on measurement-induced dephasing, this technique allows the measurement of modes that have a high quality factor (Q) and may be difficult to detect through standard transmission and reflection measurements at the device ports. In this scheme, the qubit itself acts as a sensitive phase meter, revealing modes that couple to it through measurements of its coherence time. Such modes are indistinguishable from EM modes that do not couple to the qubit using a vector network analyzer. Moreover, this technique provides useful characterization parameters including the quality factor and the coupling strength of the unwanted resonances. We demonstrate the method for detecting both high-Q coupling resonators in planar devices as well as spurious modes produced by a 3D cavity.

Strong Coupling of Microwave Photons to Antiferromagnetic Fluctuations in an Organic Magnet

Coupling between a crystal of di(phenyl)-(2,4,6-trinitrophenyl)iminoazanium radicals and a superconducting microwave resonator is investigated in a circuit quantum electrodynamics (circuit QED) architecture. The crystal exhibits paramagnetic behavior above 4K, with antiferromagnetic correlations appearing below this temperature, and we demonstrate strong coupling at base temperature. The magnetic resonance acquires a field angle dependence as the crystal is cooled down, indicating anisotropy of the exchange interactions. These results show that multispin modes in organic crystals are suitable for circuit QED, offering a platform for their coherent manipulation. They also utilize the circuit QED architecture as a way to probe spin correlations at low temperature.

Cutoff-Free Circuit Quantum Electrodynamics.

Any quantum-confined electronic system coupled to the electromagnetic continuum is subject to radiative decay and renormalization of its energy levels. When coupled to a cavity, these quantities can be strongly modified with respect to their values in vacuum. Generally, this modification can be accurately captured by including only the closest resonant mode of the cavity. In the circuit quantum electrodynamics architecture, it is, however, found that the radiative decay rates are strongly influenced by far off-resonant modes. A multimode calculation accounting for the infinite set of cavity modes leads to divergences unless a cutoff is imposed. It has so far not been identified what the source of divergence is. We show here that unless gauge invariance is respected, any attempt at the calculation of circuit QED quantities is bound to diverge. We then present a theoretical approach to the calculation of a finite spontaneous emission rate and the Lamb shift that is free of cutoff.

Scalable quantum computing model in the circuit-QED lattice with circulator function

We propose a model for a scalable quantum computing in the circuit-quantum electrodynamics(QED) architecture. In the Kagome lattice of qubits three qubits are connected to each other through a superconducting three-junction flux qubit at the vertices of the lattice. By controlling one of the three Josephson junction energies of the intervening flux qubit we can achieve the circulator function that couples arbitrary pair of two qubits among three. This selective coupling enables the interaction between two nearest neighbor qubits in the Kagome lattice, and further the two-qubit gate operation between any pair of qubits in the whole lattice by performing consecutive nearest neighbor two-qubit gates.