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
09
Dez
2021
Microwave calibration of qubit drive line components at millikelvin temperatures
Systematic errors in qubit state preparation arise due to non-idealities in the qubit control lines such as impedance mismatch. Using a data-based methodology of short-open-load calibration
at a temperature of 30 mK, we report calibrated 1-port scattering parameter data of individual qubit drive line components. At 5~GHz, cryogenic return losses of a 20-dB-attenuator, 10-dB-attenuator, a 230-mm-long 0.86-mm silver-plated cupronickel coaxial cable, and a 230-mm-long 0.86-mm NbTi coaxial cable were found to be 35+3−2 dB, 33+3−2 dB, 34+3−2 dB, and 29+2−1 dB respectively. For the same frequency, we also extract cryogenic insertion losses of 0.99+0.04−0.04 dB and 0.02+0.04−0.04 dB for the coaxial cables. We interpret the results using a master equation simulation of all XY gates performed on a single qubit. For example, we simulate a sequence of two 5 ns gate pulses (X & Y) through a 2-element Fabry-Pérot cavity with 400-mm path length directly preceding the qubit, and establish that the return loss of its reflective elements must be >9.42 dB (> 14.3 dB) to obtain 99.9 % (99.99 %) gate fidelity.
Low-loss high-impedance circuit for quantum transduction between optical and microwave photons
Quantum transducers between microwave and optical photons are essential for long-distance quantum networks based on superconducting qubits. An optically active self-assembled quantum
dot molecule (QDM) is an attractive platform for the implementation of a quantum transducer because an exciton in a QDM can be efficiently coupled to both optical and microwave fields at the single-photon level. Recently, the transduction between microwave and optical photons has been demonstrated with a QDM integrated with a superconducting resonator. In this paper, we present a design of a QD-high impedance resonator device with a low microwave loss and an expected large single-microwave photon coupling strength of 100s of MHz. We integrate self-assembled QDs onto a high-impedance superconducting resonator using a transfer printing technique and demonstrate a low-microwave loss rate of 1.8 MHz and gate tunability of the QDs. The microwave loss rate is much lower than the expected QDM-resonator coupling strength as well as the typical transmon-resonator coupling strength. This feature will facilitate efficient quantum transduction between an optical and microwave qubit.
08
Dez
2021
Ancilla-Error-Transparent Controlled Beam Splitter Gate
In hybrid circuit QED architectures containing both ancilla qubits and bosonic modes, a controlled beam splitter gate is a powerful resource. It can be used to create (up to a controlled-parity
operation) an ancilla-controlled SWAP gate acting on two bosonic modes. This is the essential element required to execute the `swap test‘ for purity, prepare quantum non-Gaussian entanglement and directly measure nonlinear functionals of quantum states. It also constitutes an important gate for hybrid discrete/continuous-variable quantum computation. We propose a new realization of a hybrid cSWAP utilizing `Kerr-cat‘ qubits — anharmonic oscillators subject to strong two-photon driving. The Kerr-cat is used to generate a controlled-phase beam splitter (cPBS) operation. When combined with an ordinary beam splitter one obtains a controlled beam-splitter (cBS) and from this a cSWAP. The strongly biased error channel for the Kerr-cat has phase flips which dominate over bit flips. This yields important benefits for the cSWAP gate which becomes non-destructive and transparent to the dominate error. Our proposal is straightforward to implement and, based on currently existing experimental parameters, should achieve controlled beam-splitter gates with high fidelities comparable to current ordinary beam-splitter operations available in circuit QED.
07
Dez
2021
Realizing Repeated Quantum Error Correction in a Distance-Three Surface Code
Quantum computers hold the promise of solving computational problems which are intractable using conventional methods. For fault-tolerant operation quantum computers must correct errors
occurring due to unavoidable decoherence and limited control accuracy. Here, we demonstrate quantum error correction using the surface code, which is known for its exceptionally high tolerance to errors. Using 17 physical qubits in a superconducting circuit we encode quantum information in a distance-three logical qubit building up on recent distance-two error detection experiments. In an error correction cycle taking only 1.1μs, we demonstrate the preservation of four cardinal states of the logical qubit. Repeatedly executing the cycle, we measure and decode both bit- and phase-flip error syndromes using a minimum-weight perfect-matching algorithm in an error-model-free approach and apply corrections in postprocessing. We find a low error probability of 3% per cycle when rejecting experimental runs in which leakage is detected. The measured characteristics of our device agree well with a numerical model. Our demonstration of repeated, fast and high-performance quantum error correction cycles, together with recent advances in ion traps, support our understanding that fault-tolerant quantum computation will be practically realizable.
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.