I am going to post here all newly submitted articles on the arXiv related to superconducting circuits. If your article has been accidentally forgotten, feel free to contact me
07
Dez
2021
Pokemon: Protected Logic Qubit Derived from the 0-π Qubit
We propose a new protected logic qubit called pokemon, which is derived from the 0-π qubit by harnessing one capacitively shunted inductor and two capacitively shunted Josephson junctions
embedded in a superconducting loop. Similar to the 0-π qubit, the two basis states of the proposed qubit are separated by a high barrier, but their wave functions are highly localized along both axis directions of the two-dimensional parameter space, instead of the highly localized wave functions along only one axis direction in the 0-π qubit. This makes the pokemon qubit more protected. For instance, the relaxation of the pokemon qubit is exponentially reduced by two equally important factors, while the relaxation of the 0-π qubit is exponentially reduced by only one factor. Moreover, we show that the inductor in the pokemon can be replaced by a nonlinear inductor using, e.g., a pair or two pairs of Josephson junctions. This offers an experimentally promising way to implement next-generation superconducting qubits with even higher quantum coherence.
06
Dez
2021
ICARUS-Q: A scalable RFSoC-based control system for superconducting quantum computers
We present a control and measurement setup for superconducting qubits based on Xilinx 16-channel radio frequency system on chip (RFSoC) device. The proposed setup consists of four parts:
multiple RFSoC FPGA boards, a setup to synchronise every DAC and ADC channel across multiple boards, a low-noise DC current supply for qubit biasing and cloud access for remotely performing experiments. We also design the setup to be free of physical mixers. The FPGA boards directly generate microwave pulses using sixteen DAC channels up to the third Nyquist zone which are directly sampled by its eight ADC channels between the fifth and the ninth zones.
Building Blocks of a Flip-Chip Integrated Superconducting Quantum Processor
We have integrated single and coupled superconducting transmon qubits into flip-chip modules. Each module consists of two chips – one quantum chip and one control chip –
that are bump-bonded together. We demonstrate time-averaged coherence times exceeding 90μs, single-qubit gate fidelities exceeding 99.9%, and two-qubit gate fidelities above 98.6%. We also present device design methods and discuss the sensitivity of device parameters to variation in interchip spacing. Notably, the additional flip-chip fabrication steps do not degrade the qubit performance compared to our baseline state-of-the-art in single-chip, planar circuits. This integration technique can be extended to the realisation of quantum processors accommodating hundreds of qubits in one module as it offers adequate input/output wiring access to all qubits and couplers.
Homointerface planar Josephson junction based on inverse proximity effect
The quality of a superconductor-normal metal-superconductor (SNS) Josephson junction (JJ) depends crucially on the transparency of the superconductor-normal metal (S/N) interface. We
demonstrate a technique for fabricating planar JJs with perfect interfaces. The technique utilizes a strong inverse proximity effect (IPE) discovered in Al/V5S8 bilayers, by which Al is driven into the normal state. The highly transparent homointerface enables the flow of Josephson supercurrent across a 2.9 μm long weak link. Moreover, our JJ exhibits a giant critical current and a large product of the critical current and the normal state resistance. The latter exceeds the theoretical bound, which is probably related to the unusual normal metal weak link.
05
Dez
2021
Swap-test interferometry with biased ancilla noise
The Mach–Zehnder interferometer is a powerful device for detecting small phase shifts between two light beams. Simple input states — such as coherent states or single photons
— can reach the standard quantum limit of phase estimation while more complicated states can be used to reach Heisenberg scaling; the latter, however, require complex states at the input of the interferometer which are difficult to prepare. The quest for highly sensitive phase estimation therefore calls for interferometers with nonlinear devices which would make the preparation of these complex states more efficient. Here, we show that the Heisenberg scaling can be recovered with simple input states (including Fock and coherent states) when the linear mirrors in the interferometer are replaced with controlled-swap gates and measurements on ancilla qubits. These swap tests project the input Fock and coherent states onto NOON and entangled coherent states, respectively, leading to improved sensitivity to small phase shifts in one of the interferometer arms. We perform detailed analysis of ancilla errors, showing that biasing the ancilla towards phase flips offers a great advantage, and perform thorough numerical simulations of a possible implementation in circuit quantum electrodynamics. Our results thus present a viable approach to phase estimation approaching Heisenberg-limited sensitivity.
03
Dez
2021
Qubit–Photon Bound States in Superconducting Metamaterials
We study quantum features of electromagnetic radiation propagating in a one–dimensional superconducting quantum metamaterial comprised of an infinite chain of charge qubits placed
within two–stripe massive superconductive resonators. The Quantum–mechanical model is derived assuming weak fields and that, at low temperatures, each qubit is either unoccupied (N=0) or occupied by a single Cooper pair (N=1). Based on this assumption we demonstrate the emergence of two bands of single photon–qubit bound states with the energy lying within (lower branch) or outside (higher) the photon continuum. The emergence of bound states may cause radiation trapping which could be of interest for the control of photon transport in these systems.
Microwave Amplification in a PT -symmetric-like Cavity Magnomechanical System
We propose a scheme that can generate tunable magnomechanically induced amplification in a double-cavity parity-time-(PT -) symmetric-like magnomechanical system under a strong control
and weak probe field. The system consists of a ferromagnetic-material yttrium iron garnet (YIG) sphere placed in a passive microwave cavity which is connected with another active cavity. We reveal that ideally induced amplification of the microwave probe signal may reach the maximum value 1000000 when cavity-cavity, cavity-magnon and magnomechanical coupling strengths are nonzero simultaneously. The phenomenon might have potential applications in the field of quantum information processing and quantum optical devices. Besides, we also find the phenomena of slow-light propagation. In this case, group speed delay of the light can achieve 0.000035s, which can enhance some nonlinear effect. Moreover, due to the relatively flat dispersion curve, the proposal may be applied to sensitive optical switches, which plays an important role in storing photons and quantum optical chips.
02
Dez
2021
A flying Schrödinger cat in multipartite entangled states
Schrödinger’s cat originates from the famous thought experiment querying the counterintuitive quantum superposition of macroscopic objects. As a natural extension, several „cats“
(quasi-classical objects) can be prepared into coherent quantum superposition states, which is known as multipartite cat states demonstrating quantum entanglement among macroscopically distinct objects. Here we present a highly scalable approach to deterministically create flying multipartite Schrödinger cat states, by reflecting coherent state photons from a microwave cavity containing a superconducting qubit. We perform full quantum state tomography on the cat states with up to four photonic modes and confirm the existence of quantum entanglement among them. We also witness the hybrid entanglement between discrete-variable states (the qubit) and continuous-variable states (the flying multipartite cat) through a joint quantum state tomography. Our work demonstrates an important experimental control method in the microwave region and provides an enabling step for implementing a series of quantum metrology and quantum information processing protocols based on cat states.
Optimizing frequency allocation for fixed-frequency superconducting quantum processors
Fixed-frequency superconducting quantum processors are one of the most mature quantum computing architectures with high-coherence qubits and low-complexity controls. However, high-fidelity
multi-qubit gates pose tight requirements on individual qubit frequencies in these processors and their fabrication suffers from the large dispersion in the fabrication of Josephson junctions. It is inefficient to make a large number of processors because degeneracy in frequencies can degrade the processors‘ quality. In this article, we propose an optimization scheme based on mixed-integer programming to maximize the fabrication yield of quantum processors. We study traditional qubit and qutrit (three-level) architectures with cross-resonance interaction processors. We compare these architectures to a differential AC-Stark shift based on entanglement gates and show that our approach greatly improves the fabrication yield and also increases the scalability of these devices. Our approach is general and can be adapted to problems where one must avoid specific frequency collisions.
01
Dez
2021
Entanglement between charge qubit states and coherent states of nanomechanical resonator generated by AC Josephson effect
We considered a nanoelectromechanical system consisting of a movable Cooper-pair box qubit which is subject to an electrostatic field, and coupled to the two bulk superconductors via
tunneling processes. We suggest that qubit dynamics is related to the one of a quantum oscillator and demonstrate that a bias voltage applied between superconductors generates states represented by the entanglement of qubit states and coherent states of the oscillator if certain resonant conditions are fulfilled. It is shown that a structure of this entanglement may be controlled by the bias voltage in a way that gives rise to the entanglement incorporating so-called cat-states – the superposition of coherent states. We characterize the formation and development of such states analyzing the entropy of entanglement and corresponding Wigner function. The experimentally feasible detection of the effect by measuring the average current is also considered.