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
Apr
2022
Two-qubit gate using conditional driving for highly detuned Kerr-nonlinear parametric oscillators
A Kerr-nonlinear parametric oscillator (KPO) is one of the promising devices to realize qubits for universal quantum computing. The KPO can stabilize two coherent states with opposite
phases, yielding a quantum superposition called a Schrödinger cat state. Universal quantum computing with KPOs requires three kinds of quantum gates: Rz,Rx, and Rzz gates. We theoretically propose a two-qubit gate Rzz for highly detuned KPOs. In the proposed scheme, we add another two-photon drive for the first KPO. This leads to the Rzz gate based on the driving of the second KPO depending on the first-KPO state, which we call „conditional driving.“ First, we perform simulations using a conventional KPO Hamiltonian derived from a superconducting-circuit model under some approximations and evaluate the gate fidelity. Next, we also perform numerical simulations of the two-qubit gate using the superconducting-circuit model without the approximations. The simulation results indicate that two-qubit gates can be implemented with high fidelity (>99.9%) for rotation angles required for universality.
06
Apr
2022
Cancelling microwave crosstalk with fixed-frequency qubits
Scalable quantum information processing requires that modular gate operations can be executed in parallel. The presence of crosstalk decreases the individual addressability, causing
erroneous results during simultaneous operations. For superconducting qubits which operate in the microwave regime, electromagnetic isolation is often limited due to design constraints, leading to signal crosstalk that can deteriorate the quality of simultaneous gate operations. Here, we propose and demonstrate a method based on AC Stark effect for calibrating the microwave signal crosstalk. The method is suitable for processors based on fixed-frequency qubits which are known for high coherence and simple control. The optimal compensation parameters can be reliably identified from a well-defined interference pattern. We implement the method on an array of 7 superconducting qubits, and show its effectiveness in removing the majority of crosstalk errors.
Accelerated quantum adiabatic transfer in superconducting qubits
Quantum adiabatic transfer is widely used in quantum computation and quantum simulation. However, the transfer speed is limited by the quantum adiabatic approximation condition, which
hinders its application in quantum systems with a short decoherence time. Here we demonstrate quantum adiabatic state transfers that jump along geodesics in one-qubit and two-qubit superconducting transmons. This approach possesses the advantages of speed, robustness, and high fidelity compared with the usual adiabatic process. Our protocol provides feasible strategies for improving state manipulation and gate operation in superconducting quantum circuits.
Towards a controllable SQUID
Josephson junctions and superconducting quantum interference devices (SQUID) are important electronic elements, which are based on normal conductor sandwiched between two superconductors.
These junctions are produced by evaporation techniques, and once they are embedded in an electronic circuit, their properties are fixed. Using SQUIDs as a tunable component requires the ability to generate Josephson junctions in situ in a reversible controllable manner. In this work we demonstrated how a normal (metallic) region along a line traversing a superconductor can be turned on and off externally thus potentially generating a controllable Josephson junction or a SQUID. The concept is based on a long, current-carrying excitation coil, piercing a ring shaped superconductor with nucleation points. The vector potential produced by this coil generates a circular current that destroys superconductivity along a radial line starting at the nucleation point. Unlike the destruction of superconductivity with magnetic field, the vector potential method is reversible and reproducible; full superconductivity is recovered upon removing the current from the coil and different cool-downs yield the same normal lines.
05
Apr
2022
A gate-tunable graphene Josephson parametric amplifier
With a large portfolio of elemental quantum components, superconducting quantum circuits have contributed to dramatic advances in microwave quantum optics. Of these elements, quantum-limited
parametric amplifiers have proven to be essential for low noise readout of quantum systems whose energy range is intrinsically low (tens of μeV ). They are also used to generate non classical states of light that can be a resource for quantum enhanced detection. Superconducting parametric amplifiers, like quantum bits, typically utilize a Josephson junction as a source of magnetically tunable and dissipation-free nonlinearity. In recent years, efforts have been made to introduce semiconductor weak links as electrically tunable nonlinear elements, with demonstrations of microwave resonators and quantum bits using semiconductor nanowires, a two dimensional electron gas, carbon nanotubes and graphene. However, given the challenge of balancing nonlinearity, dissipation, participation, and energy scale, parametric amplifiers have not yet been implemented with a semiconductor weak link. Here we demonstrate a parametric amplifier leveraging a graphene Josephson junction and show that its working frequency is widely tunable with a gate voltage. We report gain exceeding 20 dB and noise performance close to the standard quantum limit. Our results complete the toolset for electrically tunable superconducting quantum circuits and offer new opportunities for the development of quantum technologies such as quantum computing, quantum sensing and fundamental science.
01
Apr
2022
A quantum Szilard engine for two-level systems coupled to a qubit
The innate complexity of solid state physics exposes superconducting quantum circuits to interactions with uncontrolled degrees of freedom degrading their coherence. By using a simple
stabilization sequence we show that a superconducting fluxonium qubit is coupled to a two-level system (TLS) environment of unknown origin, with a relatively long energy relaxation time exceeding 50ms. Implementing a quantum Szilard engine with an active feedback control loop allows us to decide whether the qubit heats or cools its TLS environment. The TLSs can be cooled down resulting in a four times lower qubit population, or they can be heated to manifest themselves as a negative temperature environment corresponding to a qubit population of ∼80%. We show that the TLSs and the qubit are each other’s dominant loss mechanism and that the qubit relaxation is independent of the TLS populations. Understanding and mitigating TLS environments is therefore not only crucial to improve qubit lifetimes but also to avoid non-Markovian qubit dynamics.
A fast tunable 3D-transmon architecture for superconducting qubit-based hybrid devices
Superconducting qubits utilize the strong non-linearity of the Josephson junctions. Control over the Josephson nonlinearity, either by a current bias or by the magnetic flux, can be
a valuable resource that brings tunability in the hybrid system consisting of superconducting qubits. To enable such a control, here we incorporate a fast-flux line for a frequency tunable transmon qubit in 3D cavity architecture. We investigate the flux-dependent dynamic range, relaxation from unconfined states, and the bandwidth of the flux-line. Using time-domain measurements, we probe transmon’s relaxation from higher energy levels after populating the cavity with ≈2.1×104 photons. For the device used in the experiment, we find a resurgence time corresponding to the recovery of coherence to be 4.8~μs. We use a fast-flux line to tune the qubit frequency and demonstrate the swap of a single excitation between cavity and qubit mode. By measuring the deviation in the transferred population from the theoretical prediction, we estimate the bandwidth of the flux line to be ≈~100~MHz, limited by the parasitic effect in the design. These results suggest that the approach taken here to implement a fast-flux line in a 3D cavity could be helpful for the hybrid devices based on the superconducting qubit.
22
Mä
2022
Double-transmon coupler: Fast two-qubit gate with no residual coupling for highly detuned superconducting qubits
Although two-qubit entangling gates are necessary for universal quantum computing, they are notoriously difficult to implement with high fidelity. Recently, tunable couplers have become
a key component for realizing high-fidelity two-qubit gates in superconducting quantum computers. However, it is still difficult to achieve tunable coupling free of unwanted residual coupling for highly detuned qubits, which are desirable for mitigating qubit-frequency crowding or errors due to crosstalk between qubits. We thus propose a design for this kind of tunable coupler, which we call a double-transmon coupler, because this is composed of two transmon qubits coupled through a common loop with an additional Josephson junction. Controlling the magnetic flux in the loop, we can achieve not only fast high-fidelity two-qubit gates, but also no residual coupling during idle time, where computational qubits are highly detuned fixed-frequency transmons. The proposed coupler is expected to offer an alternative approach to higher-performance superconducting quantum computers.
Experimental demonstration of robustness under scaling errors for superadiabatic population transfer in a superconducting circuit
We study experimentally and theoretically the transfer of population between the ground state and the second excited state in a transmon circuit by the use of superadiabatic stimulated
Raman adiabatic passage (saSTIRAP). We show that the transfer is remarkably resilient against variations in the amplitudes of the pulses (scaling errors), thus demostrating that the superadiabatic process inherits certain robustness features from the adiabatic one. In particular, we put in evidence a new plateau that appears at high values of the counterdiabatic pulse strength, which goes beyond the usual framework of saSTIRAP.
Analog quantum control of magnonic cat states on-a-chip by a superconducting qubit
We propose to directly and quantum-coherently couple a superconducting transmon qubit to magnons – the quanta of the collective spin excitations, in a nearby magnetic particle.
The magnet’s stray field couples to the qubit via a superconducting interference device (SQUID). We predict a resonant qubit-magnon exchange and a nonlinear radiation-pressure interaction that are both stronger than dissipation rates and tunable by an external flux bias. We additionally demonstrate a quantum control scheme that generates qubit-magnon entanglement and magnonic Schrödinger cat states with high fidelity.