A quantum engineer's guide to superconducting qubits

Krantz, Philip; Kjærgaard, Morten; Yan, Fei; Orlando, Terry P.; Gustavsson, Simon; Oliver, William

doi:10.1063/1.5089550

Cited by 1,261 publications

(892 citation statements)

References 410 publications

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“…π-superconducting RF-SQUIDS with ferromagnetic-insulating barriers were only theoretically suggested as quiet qubits efficiently decoupled from the fluctuations of an external magnetic field [20,21]. A spin-filter JJ with t = 3.0 nm and an A ∼ 50 µm 2 has an estimated charg-ing energy E c ∼ 900 µK, and a Josephson energy at 20 mK E J ∼ 100 K, which means E J /E c ∼ 10 5 , suitable for a fluxqubit [39,40].…”

Section: Discussion and Concluding Remarksmentioning

confidence: 99%

“…The order of magnitude of the ratio E J /E c for the investigated junctions scales with the thickness from 10 6 to 10. Adapting the area of the devices to conventional dimensions in transmon qubits (A ∼ 1 µm 2 ), lower values of E J /E c can be achieved, falling in the typical range of transmon qubit [39,40,61]. As an example, reducing the cross section to A ∼ 1 µm 2 , E J of the spin-filter JJ with t = 3.5 nm becomes ∼ 280 mK, while E c becomes ∼ 180 mK, so that E J /E c ∼ 2.…”

Section: Discussion and Concluding Remarksmentioning

confidence: 99%

“…This allows us to explore substantially quite different transport regimes. If one wants to place the junction in a circuit or to couple it to a cavity [39,40], knowledge of the electrodynamic parameters and how they scale with the barrier thickness is fundamental. Therefore, this study provides a pathway to the engineering of tunnel-ferromagnetic JJs for specific applications.…”

Section: Introductionmentioning

confidence: 99%

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Electrodynamics of Highly Spin-Polarized Tunnel Josephson Junctions

et al. 2020

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The continuous development of superconducting electronics is encouraging several studies on hybrid Josephson junctions (JJs) based on superconductor/ferromagnet/superconductor (SFS) heterostructures, as either spintronic devices or switchable elements in quantum and classical circuits. Recent experimental evidence of macroscopic quantum tunneling and of an incomplete 0-π transition in tunnel-ferromagnetic spin-filter JJs could enhance the capabilities of SFS JJs also as active elements. Here, we provide a self-consistent electrodynamic characterization of NbN/GdN/NbN spin-filter JJs as a function of the barrier thickness, disentangling the highfrequency dissipation effects due to the environment from the intrinsic low-frequency dissipation processes. The fitting of the I −V characteristics at 4.2 K and at 300 mK by using the Tunnel Junction Microscopic model allows us to determine the subgap resistance R sg , the quality factor Q and the junction capacitance C. These results provide the scaling behavior of the electrodynamic parameters as a function of the barrier thickness, which represents a fundamental step for the feasibility of tunnel-ferromagnetic JJs as active elements in classical and quantum circuits, and are of general interest for tunnel junctions other than conventional SIS JJs.

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Section: Discussion and Concluding Remarksmentioning

confidence: 99%

Section: Discussion and Concluding Remarksmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Electrodynamics of Highly Spin-Polarized Tunnel Josephson Junctions

et al. 2020

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“…Efficient tools for engineering control sequences that drive a quantum system to undertake a desired evolution are critical for quantum computing, sensing, and spectroscopy. In the case of quantum computing [1][2][3], it is imperative that the effective evolution corresponds, as closely as possible, to that of the experimenter's best characterization of the system Hamiltonian, as only this can reliably lead to high fidelity unitary operations. In realistic settings this requires successful suppression of numerous unwanted, yet unavoidable, physical effects: couplings to uncharted or unaccountable external degrees of freedom [4][5][6], leakage out of the computational subspace [7][8][9], as well as uncertainties and stochastic variations in the system's internal and control Hamiltonians [10][11][12].…”

Section: Introductionmentioning

confidence: 99%

Engineering effective Hamiltonians

et al. 2019

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In the field of quantum control, effective Hamiltonian engineering is a powerful tool that utilizes perturbation theory to mitigate or enhance the effect that a variation in the Hamiltonian has on the evolution of the system. Here, we provide a general framework for computing arbitrary timedependent perturbation theory terms, as well as their gradients with respect to control variations, enabling the use of gradient methods for optimizing these terms. In particular, we show that effective Hamiltonian engineering is an instance of a bilinear control problem-the same general problem class as that of standard unitary design-and hence the same optimization algorithms apply. We demonstrate this method in various examples, including decoupling, recoupling, and robustness to control errors and stochastic errors. We also present a control engineering example that was used in experiment, demonstrating the practical feasibility of this approach.

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“…The transition rates obey the detailed balance condition, Γ ↑ = e −β ωQ Γ ↓ . To further connect the results with a concrete circuit, we note that the quality factor relates to the standard T 1 relaxation time of the qubit by T 1 = Q/ω Q at low temperature [18]. As a sanity check, we return to the stochastic wavefunction and calculate the quantity J(t) ≡ |ψ(t + dt) ψ(t + dt)|, the average over many trajectories, which is expected to mimic the density matrix.…”

mentioning

confidence: 99%

Quantum Trajectory Analysis of Single Microwave Photon Detection by Nanocalorimetry

Karimi

Pekola

2020

Phys. Rev. Lett.

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We apply quantum trajectory techniques to analyze a realistic set-up of a superconducting qubit coupled to a heat bath formed by a resistor, a system that yields explicit expressions of the relevant transition rates to be used in the analysis. We discuss the main characteristics of the jump trajectories and relate them to the expected outcomes ("clicks") of a fluorescence measurement using the resistor as a nanocalorimeter. As the main practical outcome we present a model that predicts the time-domain response of a realistic calorimeter subject to single microwave photons, incorporating the intrinsic noise due to the fundamental thermal fluctuations of the absorber and finite bandwidth of a thermometer.Quantum trajectories provide a way to predict the stochastic behaviour of an open quantum system experiencing the subtle influence of the environment via a non-Hermitian Hamiltonian, and jumps between eigenstates. Initially developed about 30 years ago as a computational aid [1][2][3][4], the trajectories are nowadays routinely used for interpretation of experiments even in modern macroscopic quantum systems [5][6][7][8][9][10][11]. For instance, in the currently active field of quantum thermodynamics, quantum trajectories provide an invaluable tool to describe the stochastic thermodynamics properties of open quantum systems [12][13][14][15][16][17]. In this paper we present an analysis of an archetypical basic set-up: a two-level system (qubit) coupled to a heat bath. In particular, we take a concrete system of a solid-state superconducting qubit [18] and resistive environment forming an equilibrium heat bath, which is readily realizable experimentally [19,20]. We focus here on the expected outcomes of a fluorescence measurement based on observing emitted and absorbed microwave photons by a nanocalorimeter that presents a circuit realization of a photoreceiver discussed in general terms, e.g. in [21]. We demonstrate that the common interpretation of the outcome of a projective measurement ("collapse") is consistent with the quantum jump trajectories. We present a stochastic simulation of the output of this detector in the presence of qubit-calorimeter interaction and coupling of the calorimeter to the phonon heat bath including thermal noise on the detector. This analysis illustrates the feasibility of such an experiment under realistic conditions, and its potential to detect not only the arrival times but also the energies of the quanta in a continuous measurement in the challenging regime of microwave photons.We consider a qubit coupled to a heat bath as schematically shown in the inset of Fig. 1a. The stochastic wave function of this system,is written in the basis of the ground |g and excited |e states. The non-Hermitian Hamiltonian of the system is 0.0 0.2 0.4 0.6 0.8 1.0 || 2 ,J ee , r ee , P no-jump | ۧ | ۧ Bath 0 2 4 6 8 0.0 0.5 1.0 G¯ t (a) (b) 0 20 40 60 80 clicks 0 20 40 60 80 100 repetition # (c) FIG. 1. Two level system (qubit) coupled to a heat bath, shown in the inset. (a) Time evolution of the ...

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A quantum engineer's guide to superconducting qubits

Cited by 1,261 publications

References 410 publications

Electrodynamics of Highly Spin-Polarized Tunnel Josephson Junctions

Electrodynamics of Highly Spin-Polarized Tunnel Josephson Junctions

Engineering effective Hamiltonians

Quantum Trajectory Analysis of Single Microwave Photon Detection by Nanocalorimetry

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