Superconducting Cavity Research Articles

RF design and frequency sensitivity analysis of a 1500 MHz passive third harmonic superconducting cavity for the SILF

To reduce Touschek scattering in the beam and extend the lifetime of the beam in the Shenzhen Innovation Light-source Facility (a fourth-generation light source), a 1500 MHz passive third harmonic system will be used. The present work is devoted to the RF design and frequency sensitivity analysis of the 1500 MHz passive third harmonic superconducting cavity, optimizing the RF parameters of the cavity, and giving a frequency scheme for the cavity in fabrication. The reliability of the cavity during operation is taken as the main goal for the optimization of the geometrical parameters, and its structural and RF parameters are obtained with E peak/E acc of 2.17, B peak/E acc of 5.12 mT/(MV/m) and G · R/Q of 25052 at 2 K. A pair of fluted beam pipes are used to propagate the higher order modes (HOMs) of the harmonic cavity, allowing a smooth transition of the first dipole mode to the absorber located at room temperature area. For the frequency sensitivity analysis, the frequency variations of the cavity in fabrication are obtained by multi-physics coupling simulation in the paper, which gives the target frequency of the cavity between welding and preloading. After electron beam welding (EBW), the resonant frequency of the cavity should be maintained at 1500.096 MHz, and the pre-tuning target frequency is 1497.456 MHz. The slow tuning range is ±400 kHz, the fast-tuning range is 600 Hz, and the maximum allowable tuning force is 15 kN.

Tuning of superconducting cavities using the FFT of transmitted power

We implemented a method to tune the ESS superconducting cavities based on the spectral analysis of the high resolution data available from the Low Level RF system (LLRF) for the transmitted power, without the need of connecting a network analyzer or any other dedicated instrumentation along the RF chain. A frequency peak up to 4 MHz off from the resonating frequency can be detected and used to control the stepper motor of the tuner until the cavity is stretched to the proper length to reach the correct operation frequency. Experience of its use at the ESS Test Stand 2 (TS2) facility at Lund during cryomodule acceptance testing is presented.

Multipacting study for fundamental power coupler of a superconducting cavity

The research work of Hefei Advanced Light Facility (HALF) is in progress. A 499.8 MHz superconducting cavity will be used in the HALF RF system. The preliminary RF design and thermal analysis for the fundamental power coupler (FPC) of HALF 499.8 MHz superconducting cavity have been completed. Multipacting (MP) is one of the major causes of the breakdown in many high power input couplers and RF cavities, thus, MP simulation for our FPC is necessary. In this paper, the MP simulation for the FPC was performed by using the CST. The MP performance of the FPC was studied. The formation process of the MP in the FPC was investigated and the physical mechanism of the delay of the MP establishment was analyzed. The simulation results show that MP will occur when the power level is higher than 175 kW, and the MP location is near the ceramic. Therefore, MP suppression is required in the FPC. In order to suppress the MP, the effect of the biasing DC electric voltage to MP in the FPC was analyzed in this paper. The simulation results indicates that the biasing DC electric voltage not only can suppress the MP, but also can excites new MP in the coupler. Thus, it is crucial to select the appropriate biasing DC electric voltage for the MP suppression in the coupler. The results show that the MP in the FPC can be suppressed very well by using 2 kV or 3 kV biasing DC electric voltage.

Optimal Control of a Cavity-Mediated iSWAP Gate between Silicon Spin Qubits

Semiconductor spin qubits may be coupled through a superconducting cavity to generate an entangling two-qubit gate. However, the fidelity of such an operation will be reduced by a variety of error mechanisms such as charge and magnetic noise, phonons, cavity loss, transitions to nonqubit states, and, for electrons in silicon, excitation into other valley eigenstates. Here, we model the effects of these error sources and the valley degree of freedom on the performance of a cavity-mediated two-qubit $\mathrm{i}$swap gate. For valley splittings inadequately large relative to the interdot tunnel coupling within each qubit, we find that valley excitation may be a limiter to the fidelity of this two-qubit gate. In addition, we show trade-offs between gating times and exposure to various error sources, identifying optimal operating regimes and device improvements that would have the greatest impact on the fidelity of the cavity-mediated spin $\mathrm{i}$swap gate. Importantly, we find that, while the impact of charge noise and phonon relaxation favor operation in the regime where the qubits are most spinlike to reduce sensitivity to these sources of noise, the combination of hyperfine noise and valley physics shifts the optimal regime to chargelike qubits with stronger effective spin-photon coupling so that gate times can be made as short as possible. In this regime, the primary limitation is the need to avoid Landau-Zener transitions as the gate is implemented.

Transfer of arbitrary quantum states between separated superconducting cavities via an ensemble of nitrogen-vacancy centers

Transfer of quantum states between distant nodes is one of the most important preconditions for realizing large-scale quantum information processing. We here propose two ways to transfer an arbitrary quantum state between two separated superconducting cavities coupled to an ensemble of nitrogen-vacancy centers (NV ensemble). By using the double-swap scheme and theory of the dispersive regime, quantum states can be deterministically transferred between two superconducting cavities. With current hybrid circuit QED technology, the numerical simulations are performed to demonstrate that the single-photon state and cat state can be high-fidelity transferred between two superconducting cavities.

State initialization of a hot spin qubit in a double quantum dot by measurement-based quantum feedback control

A measurement-based quantum feedback protocol is developed for spin-state initialization in a gate-defined double quantum dot spin qubit coupled to a superconducting cavity. The protocol improves qubit state initialization as it is able to robustly prepare the spin in shorter time and reach a higher fidelity, which can be preset. Being able to preset the fidelity aimed at is a highly desired feature enabling qubit initialization to be more deterministic. The protocol developed herein is also effective at high temperatures, which is critical for the current efforts toward scaling up the number of qubits in quantum computers.

Commissioning and First Results of the SSRF Phase-II Beamline Project

The SSRF Phase-II Beamline Project started November 20, 2016 with a 6-year construction period. This project is mainly to construct 18 new beamlines and more than 30 end-stations, as well as the corresponding storage ring upgrades, the user auxiliary laboratories, the beamline technology support laboratories, and the infrastructures. Till now, 11 beamlines have passed the technical acceptance test and been in running. The remaining seven beamlines will be finishing their technical acceptance test soon. The commissioning of the upgraded accelerator is successful including the lattice modification, the new 3rd harmonic superconducting cavity installation and the 650 W helium cryogenic system construction.

Hybrid states of a cavity-photon–vortex coupled system in a superconductive cavity

As the Abrikosov vortex lattice has recently been found in van der Waals heterostructures constructed by a two-dimensional (2D) ferromagnet and a superconductor, we propose the realization of cavity-photon–vortex coupling in a superconductive cavity to construct a new hybrid quantum system in this paper. We study the corresponding hybrid states therein, including the exceptional lines (ELs) in the parameter space. Considering that the parameters of our system are adjustable by external magnetic field and temperature, our system and the ELs are much easier to be realized in experiments. Furthermore, the numerical results show that the corresponding hybrid states can be switched by tuning the source of AC, which makes this hybrid system more advantageous to realize hybrid quantum computing in the future. Moreover, for practical use in detecting hybrid states and the vortex dynamics, the transmission amplitude of an external transverse electric wave through the cavity is also studied.

Numerical gate synthesis for quantum heuristics on bosonic quantum processors

There is a recent surge of interest and insights regarding the interplay of quantum optimal control and variational quantum algorithms. We study the framework in the context of qudits which are, for instance, definable as controllable electromagnetic modes of a superconducting cavity system coupled to a transmon. By employing recent quantum optimal control approaches described in (Petersson and Garcia, 2021), we showcase control of single-qudit operations up to eight states, and two-qutrit operations, mapped respectively onto a single mode and two modes of the resonator. We discuss the results of numerical pulse engineering on the closed system for parametrized gates useful to implement Quantum Approximate Optimization Algorithm (QAOA) for qudits. The results show that high fidelity (>0.99) is achievable with sufficient computational effort for most cases under study, and extensions to multiple modes and open, noisy systems are possible. The tailored pulses can be stored and used as calibrated primitives for a future compiler in circuit quantum electrodynamics (cQED) systems.

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.

Characterizing Multipartite non-Gaussian Entanglement for a Three-Mode Spontaneous Parametric Down-Conversion Process

Very recently, strongly non-Gaussian states have been observed via a direct three-mode spontaneous parametric down-conversion in a superconducting cavity [Phys. Rev. X 10, 011011 (2020)]. The created multi-photon non-Gaussian correlations are attractive and useful for various quantum information tasks. However, how to detect and classify multipartite non-Gaussian entanglement has not yet been completely understood. Here, we present an experimentally practical method to characterize continuous-variable multipartite non-Gaussian entanglement, by introducing a class of nonlinear squeezing parameters involving accessible higher-order moments of phase-space quadratures. As these parameters can depend on arbitrary operators, we consider their analytical optimization over a set of practical measurements, in order to detect different classes of multipartite non-Gaussian entanglement ranging from fully separable to fully inseparable. We demonstrate that the nonlinear squeezing parameters act as an excellent approximation to the quantum Fisher information within accessible third-order moments. The level of the nonlinear squeezing quantifies the metrological advantage provided by those entangled states. Moreover, by analyzing the above mentioned experiment, we show that our method can be readily used to confirm fully inseparable tripartite non-Gaussian entangled states by performing a limited number of measurements without requiring full knowledge of the quantum state.

Design of injectors and stay-alone cryostats with superconducting cavities for high RF powers applications

Injectors and stay-alone cryostats with superconducting (sc) cavities are sometimes applied at modern particle accelerators. Typically, very stringent requirements on beam quality are foreseen at injectors, and application of sc cavities is advantageous, particularly if operation at high current densities and high accelerating gradients is foreseen. Stay-alone cryostats with one or few sc cavities also find applications, e.g. at ELBE, DIAMOND, S-DALINAC accelerators, and accelerate particle beams in High Frequency fields. In the future, very high RF fields will be needed, and improvement of the cooling of cavity surfaces will improve cavity performance. Application of a sub-cooled superfluid helium gives several advantages, e.g. higher heat flux densities, longer time for onset of a film boiling regime and shorter recovery time, reduced Kapitza resistances, etc [2]. In the present paper, an application of sub-cooled superfluid helium for injector and stay alone cryostats is considered. Design of the cryogenic system is presented in some details and possibility of adapting of such injector for future applications is also discussed.

Improvement of Magnet and Cavities cooling at Heavy Ion or Rare Isotopes Accelerators due to Application of sub-cooled Superfluid Helium

Several modern Heavy Ion or Rare Iosotopes Accelerator uses superconducting (sc) cavities for an acceleration of the beam particles. Operation of sc magnets with higher magnetic fields and improved field gradients also leads to a better beam focusing (also called “beam optics”) and improves the overall accelerator compactness. In many accelerators, like XFEL, SNS, LCLS-II, HIE-ISOLDE, ATLAS, ISAC-II/ARIEL, SPIRAL-2, FLASH, Jlab-FEL/ERL, the operation is limited either by cooling capability of sc cavities or refrigeration capacities. In most of the cases, increasing of refrigerator capacities is a challenging task due to available free space or investments, and, therefore, improvement of cavity cooling due to increasing of a quality factor (Q 0) or heat transfer at cavity surfaces is considered. Application of a sub-cooled superfluid (sf) helium gives several advantages, like higher heat flux densities, longer time for onset of a film boiling regime and shorter recovery time, reduced Kapitza resistances, etc. In the present paper, application of sub-cooled superfluid helium for cooling of cavities with half and quarter-wave resonators, which are typically applied at Heavy Ion and Rare Isotopes Accelerators or sometimes at electron and proton ones, is considered. In order to limit the present discussion, its application to FRIB-style cryomodules is discussed in details.

Quantum Otto cycle in a superconducting cavity in the nonadiabatic regime

We analyze the efficiency of the quantum Otto cycle applied to a superconducting cavity. We consider its description in terms of a full quantum scalar field in a one-dimensional cavity with a time dependent boundary condition that can be externally controlled to perform and extract work unitarily from the system. We study the performance of this machine when acting as a heat engine as well as a refrigerator. It is shown that, in a non-adiabatic regime, the efficiency of the quantum cycle is affected by the dynamical Casimir effect, that induces a sort of quantum friction that diminishes the efficiency. We also find regions of parameters where the effect is so strong that the machine can no longer function as an engine since the work that would be produced is completely consumed by the quantum friction. However, this effect can be avoided for some particular temporal evolutions of the boundary conditions that do not change the occupation number of the modes in the cavity, leading to a highly improved efficiency.

Strong coupling and active cooling in a finite-temperature hybrid atom-cavity system

For a standard two-level atom coupled to the quantized field of a resonant cavity, finite temperature effects lead to thermal occupation of the cavity modes that obfuscates measurement of the quantum nature of the atom-light interaction. In this paper we demonstrate that using a hybrid system of a superconducting cavity coupled to a multi-level Rydberg atom it is possible to observe the quantum nature of strong coupling even at finite temperatures, and to exploit this coupling to permit cooling of the thermal microwave mode towards the ground-state, enabling observation of coherent vacuum Rabi oscillations even at 4~K for realistic experimental parameters. Cooling for multiple atoms is also explored, showing maximal cooling for small samples, making this a viable approach to cavity cooling with potential applications in long-range coupling of superconducting qubits via thermal waveguides.

Superconducting niobium coating deposited using cold spray

New fabrication techniques associated with cost reduction is gaining attention in the field of superconducting (SC) accelerators. Owing to the low process temperature, cold spraying is a suitable technique to obtain SC cavities. In this work, superconducting properties of cold sprayed Niobium (Nb) is discussed and compared with other techniques. Critical temperature (Tc) for superconductivity of cold sprayed coatings were estimated to be ∼ 8.6 K as against 9.2 K for the same of bulk Nb. Residual Resistivity Ratio (RRR), critical fields (Hc1 and Hc2) were also compared. Cold spraying can be an alternate route to obtain Nb superconducting cavities.

Heterodyne broadband detection of axion dark matter

We propose a new broadband search strategy for ultralight axion dark matter that interacts with electromagnetism. An oscillating axion field induces transitions between two quasi-degenerate resonant modes of a superconducting cavity. In two broadband runs optimized for high and low masses, this setup can probe unexplored parameter space for axion-like particles covering fifteen orders of magnitude in mass, including astrophysically long-ranged fuzzy dark matter.

A dynamic collimation and alignment system for the Helmholtz linear accelerator.

The upcoming commissioning of the superconducting (SC) continuous wave Helmholtz linear accelerators first of series cryomodule is going to demand precise alignment of the four internal SC cavities and two SC solenoids. For optimal results, a beam-based alignment method is used to reduce the misalignment of the whole cryomodule, as well as its individual components. A symmetric beam of low transverse emittance is required for this method, which is to be formed by a collimation system. It consists of two separate plates with milled slits, aligned in the horizontal and vertical direction. The collimation system and alignment measurements are proposed, investigated, and realized. The complete setup of this system and its integration into the existing environment at the GSI High Charge State Injector are presented, as well as the results of the recent reference measurements.

Probing CP violation in photon self-interactions with cavities

In this paper we study CP violation in photon self-interactions at low energy. These interactions, mediated by the effective operator FFFtilde{F} , where ( tilde{F} ) F is the (dual) electromagnetic field strength, have yet to be directly probed experimentally. Possible sources for such interactions are weakly coupled light scalars with both scalar and pseudoscalar couplings to photons (for instance, complex Higgs-portal scalars or the relaxion), or new light fermions coupled to photons via dipole operators. We propose a method to isolate the CP-violating contribution to the photon self-interactions using Superconducting Radio-Frequency cavities and vacuum birefringence experiments. In addition, we consider several theoretical and experimental indirect bounds on the scale of new physics associated with the above effective operator, and present projections for the sensitivity of the proposed experiments to this scale. We also discuss the implications of these bounds on the CP-violating couplings of new light particles coupled to photons.

Simulating Quantum Particles on a Lattice

A new quantum simulator uses microwave photons in a superconducting cavity to simulate particles on a lattice similar to those found in superconductors or atomic nuclei.