arXiv subjects

U. Vool

Publications and source records attributed to U. Vool.

14 recordsLinked to original sources

arXiv2023

Circuit QED detection of induced two-fold anisotropic pairing in a hybrid superconductor-ferromagnet bilayer

Hybrid systems represent one of the frontiers in the study of unconventional superconductivity and are a promising platform to realize topological superconducting states. Owing to their mesoscopic dimensions, these materials are challenging to probe using many conventional measurement techniques, and require new experimental probes to successfully characterize. In this work, we develop a probe that enables us to measure the superfluid density of micron-size superconductors using microwave techniques drawn from circuit quantum electrodynamics (cQED). We apply this technique to a paradigmatic hybrid system, the superconductor/ferromagnet bilayer, and find that the proximity-induced superfluid density is two-fold anisotropic within the plane of the sample and exhibits power law temperature-scaling which is indicative of a nodal superconducting state. These experimental results are consistent with the theoretically predicted signatures of induced triplet pairing with a nodal $p$-wave order parameter. Moreover, we unexpectedly observe drastic modifications to the microwave response at frequencies near the ferromagnetic resonance, suggesting a coupling between the spin dynamics and induced superconducting order in the ferromagnetic layer. Our results offer new insights into the unconventional superconducting states induced in superconductor/ferromagnet heterostructures and simultaneously establish a new avenue for the study of fragile unconventional superconductivity in low-dimensional materials such as van der Waals heterostructures.

cond-mat.supr-con↗

arXiv2021

Parametric longitudinal coupling between a high-impedance superconducting resonator and a semiconductor quantum dot singlet-triplet spin qubit

Long-distance two-qubit coupling, mediated by a superconducting resonator, is a leading paradigm for performing entangling operations in a quantum computer based on spins in semiconducting materials. Here, we demonstrate a novel, controllable spin-photon coupling based on a longitudinal interaction between a spin qubit and a resonator. We show that coupling a singlet-triplet qubit to a high-impedance superconducting resonator can produce the desired longitudinal coupling when the qubit is driven near the resonator's frequency. We measure the energy splitting of the qubit as a function of the drive amplitude and frequency of a microwave signal applied near the resonator antinode, revealing pronounced effects close to the resonator frequency due to longitudinal coupling. By tuning the amplitude of the drive, we reach a regime with longitudinal coupling exceeding $1$ MHz. This demonstrates a new mechanism for qubit-resonator coupling, and represents a stepping stone towards producing high-fidelity two-qubit gates mediated by a superconducting resonator.

cond-mat.mes-hall↗

arXiv2019

Superconducting circuit protected by two-Cooper-pair tunneling

We present a protected superconducting qubit based on an effective circuit element that only allows pairs of Cooper pairs to tunnel. These dynamics give rise to a nearly degenerate ground state manifold indexed by the parity of tunneled Cooper pairs. We show that, when the circuit element is shunted by a large capacitance, this manifold can be used as a logical qubit that we expect to be insensitive to multiple relaxation and dephasing mechanisms.

quant-ph↗

arXiv2017

Driving forbidden transitions in the fluxonium artificial atom

Atomic systems display a rich variety of quantum dynamics due to the different possible symmetries obeyed by the atoms. These symmetries result in selection rules that have been essential for the quantum control of atomic systems. Superconducting artificial atoms are mainly governed by parity symmetry. Its corresponding selection rule limits the types of quantum systems that can be built using electromagnetic circuits at their optimal coherence operation points ("sweet spots"). Here, we use third-order nonlinear coupling between the artificial atom and its readout resonator to drive transitions forbidden by the parity selection rule for linear coupling to microwave radiation. A Lambda-type system emerges from these newly accessible transitions, implemented here in the fluxonium artificial atom coupled to its "antenna" resonator. We demonstrate coherent manipulation of the fluxonium artificial atom at its sweet spot by stimulated Raman transitions. This type of transition enables the creation of new quantum operations, such as the control and readout of physically protected artificial atoms.

quant-ph↗

arXiv2017

Simultaneous monitoring of fluxonium qubits in a waveguide

Most quantum-error correcting codes assume that the decoherence of each physical qubit is independent of the decoherence of any other physical qubit. We can test the validity of this assumption in an experimental setup where a microwave feedline couples to multiple qubits by examining correlations between the qubits. Here, we investigate the correlations between fluxonium qubits located in a single waveguide. Despite being in a wide-bandwidth electromagnetic environment, the qubits have measured relaxation times in excess of 100 us. We use cascaded Josephson parametric amplifiers to measure the quantum jumps of two fluxonium qubits simultaneously. No correlations are observed between the relaxation times of the two fluxonium qubits, which indicates that the sources of relaxation are local to each qubit. Our architecture can easily be scaled to monitor larger numbers of qubits.

quant-ph↗

arXiv2017

3-Wave Mixing Josephson Dipole Element

Parametric conversion and amplification based on three-wave mixing are powerful primitives for efficient quantum operations. For superconducting qubits, such operations can be realized with a quadrupole Josephson junction element, the Josephson Ring Modulator (JRM), which behaves as a loss-less three-wave mixer. However, combining multiple quadrupole elements is a difficult task so it would be advantageous to have a pure three-wave dipole element that could be tessellated for increased power handling and/or information throughput. Here, we present a dipole circuit element with third-order nonlinearity, which implements three-wave mixing. Experimental results for a non-degenerate amplifier based on the proposed pure third-order nonlinearity are reported.

cond-mat.supr-con↗

arXiv2016

Generating higher order quantum dissipation from lower order parametric processes

Stabilization of quantum manifolds is at the heart of error-protected quantum information storage and manipulation. Nonlinear driven-dissipative processes achieve such stabilization in a hardware efficient manner. Josephson circuits with parametric pump drives implement these nonlinear interactions. In this article, we propose a scheme to engineer a four-photon drive and dissipation on a harmonic oscillator by cascading experimentally demonstrated two-photon processes. This would stabilize a four-dimensional degenerate manifold in a superconducting resonator. We analyze the performance of the scheme using numerical simulations of a realizable system with experimentally achievable parameters.

quant-ph↗

arXiv2016

A fluxonium-based artificial molecule with a tunable magnetic moment

Engineered quantum systems allow us to observe phenomena that are not easily accessible naturally. The LEGO-like nature of superconducting circuits makes them particularly suited for building and coupling artificial atoms. Here, we introduce an artificial molecule, composed of two strongly coupled fluxonium atoms, which possesses a tunable magnetic moment. Using an applied external flux, one can tune the molecule between two regimes: one in which the ground-excited state manifold has a magnetic dipole moment and one in which the ground-excited state manifold has only a magnetic quadrupole moment. By varying the applied external flux, we find the coherence of the molecule to be limited by local flux noise. The ability to engineer and control artificial molecules paves the way for building more complex circuits for protected qubits and quantum simulation.

quant-ph↗

arXiv2016

Continuous quantum nondemolition measurement of the transverse component of a qubit

Quantum jumps of a qubit are usually observed between its energy eigenstates, also known as its longitudinal pseudo-spin component. Is it possible, instead, to observe quantum jumps between the transverse superpositions of these eigenstates? We answer positively by presenting the first continuous quantum nondemolition measurement of the transverse component of an individual qubit. In a circuit QED system irradiated by two pump tones, we engineer an effective Hamiltonian whose eigenstates are the transverse qubit states, and a dispersive measurement of the corresponding operator. Such transverse component measurements are a useful tool in the driven-dissipative operation engineering toolbox, which is central to quantum simulation and quantum error correction.

quant-ph↗

arXiv2016

Quantization of inductively-shunted superconducting circuits

We present a method for calculating the energy levels of superconducting circuits that contain highly anharmonic, inductively-shunted modes with arbitrarily strong coupling. Our method starts by calculating the normal modes of the linearized circuit and proceeds with numerical diagonalization in this basis. As an example, we analyze the Hamiltonian of a fluxonium qubit inductively coupled to a readout resonator. While elementary, this simple example is nontrivial because it cannot be efficiently treated by the method known as "black-box quantization," numerical diagonalization in the bare harmonic oscillator basis, or perturbation theory. Calculated spectra are compared to measured spectroscopy data, demonstrating excellent quantitative agreement between theory and experiment.

quant-ph↗

arXiv2015

Single-photon Resolved Cross-Kerr Interaction for Autonomous Stabilization of Photon-number States

Quantum states can be stabilized in the presence of intrinsic and environmental losses by either applying active feedback conditioned on an ancillary system or through reservoir engineering. Reservoir engineering maintains a desired quantum state through a combination of drives and designed entropy evacuation. We propose and implement a quantum reservoir engineering protocol that stabilizes Fock states in a microwave cavity. This protocol is realized with a circuit quantum electrodynamics platform where a Josephson junction provides direct, nonlinear coupling between two superconducting waveguide cavities. The nonlinear coupling results in a single photon resolved cross-Kerr effect between the two cavities enabling a photon number dependent coupling to a lossy environment. The quantum state of the microwave cavity is discussed in terms of a net polarization and is analyzed by a measurement of its steady state Wigner function.

quant-ph↗

arXiv2013

Stabilizing entanglement autonomously between two superconducting qubits

Quantum error-correction codes would protect an arbitrary state of a multi-qubit register against decoherence-induced errors, but their implementation is an outstanding challenge for the development of large-scale quantum computers. A first step is to stabilize a non-equilibrium state of a simple quantum system such as a qubit or a cavity mode in the presence of decoherence. Several groups have recently accomplished this goal using measurement-based feedback schemes. A next step is to prepare and stabilize a state of a composite system. Here we demonstrate the stabilization of an entangled Bell state of a quantum register of two superconducting qubits for an arbitrary time. Our result is achieved by an autonomous feedback scheme which combines continuous drives along with a specifically engineered coupling between the two-qubit register and a dissipative reservoir. Similar autonomous feedback techniques have recently been used for qubit reset and the stabilization of a single qubit state, as well as for creating and stabilizing states of multipartite quantum systems. Unlike conventional, measurement-based schemes, an autonomous approach counter-intuitively uses engineered dissipation to fight decoherence, obviating the need for a complicated external feedback loop to correct errors, simplifying implementation. Instead the feedback loop is built into the Hamiltonian such that the steady state of the system in the presence of drives and dissipation is a Bell state, an essential building-block state for quantum information processing. Such autonomous schemes, broadly applicable to a variety of physical systems as demonstrated by a concurrent publication with trapped ion qubits, will be an essential tool for the implementation of quantum-error correction.

quant-ph↗

arXiv2013

Stabilizing a Bell state of two superconducting qubits by dissipation engineering

We propose a dissipation engineering scheme that prepares and protects a maximally entangled state of a pair of superconducting qubits. This is done by off-resonantly coupling the two qubits to a low-Q cavity mode playing the role of a dissipative reservoir. We engineer this coupling by applying six continuous-wave microwave drives with appropriate frequencies. The two qubits need not be identical. We show that our approach does not require any fine-tuning of the parameters and requires only that certain ratios between them be large. With currently achievable coherence times, simulations indicate that a Bell state can be maintained over arbitrary long times with fidelities above 94%. Such performance leads to a significant violation of Bell's inequality (CHSH correlation larger than 2.6) for arbitrary long times.

quant-ph↗

arXiv2012

Cavity-assisted quantum bath engineering

We demonstrate quantum bath engineering for a superconducting artificial atom coupled to a microwave cavity. By tailoring the spectrum of microwave photon shot noise in the cavity, we create a dissipative environment that autonomously relaxes the atom to an arbitrarily specified coherent superposition of the ground and excited states. In the presence of background thermal excitations, this mechanism increases the state purity and effectively cools the dressed atom state to a low temperature.

quant-ph↗