We present a detailed study of the coherence of a tunable capacitively-shunted flux qubit, designed for coherent quantum annealing applications. The measured relaxation at the qubitsymmetry point is mainly due to intrinsic flux noise in the main qubit loop for qubit frequencies below ∼3 GHz. At higher frequencies, thermal noise in the bias line makes a significant contribution to the relaxation, arising from the design choice to experimentally explore both fast annealing and high-frequency control. The measured dephasing rate is primarily due to intrinsic low-frequency flux noise in the two qubit loops, with additional contribution from the low-frequency noise of control electronics used for fast annealing. The flux-bias dependence of the dephasing time also reveals apparent noise correlation between the two qubit loops, possibly due to non-local sources of flux noise or junction critical-current noise. Our results are relevant for ongoing efforts toward building superconducting quantum annealers with increased coherence.
Landau-Zener (LZ) tunneling, describing transitions in a two-level system during a sweep through an anti-crossing, is a model applicable to a wide range of physical phenomena, suchas atomic collisions, chemical reactions, and molecular magnets, and has been extensively studied theoretically and experimentally. Dissipation due to coupling between the system and environment is an important factor in determining the transition rates. Here we report experimental results on the dissipative LZ transition. Using a tunable superconducting flux qubit, we observe for the first time the crossover from weak to strong coupling to the environment. The weak coupling limit corresponds to small system-environment coupling and leads to environment-induced thermalization. In the strong coupling limit, environmental excitations dress the system and transitions occur between the dressed states. Our results confirm previous theoretical studies of dissipative LZ tunneling in the weak and strong coupling limits. Our results for the intermediate regime are novel and could stimulate further theoretical development of open system dynamics. This work provides insight into the role of open system effects on quantum annealing, which employs quantum tunneling to search for low-energy solutions to hard computational problems.
Dynamical decoupling (DD) is a widely-used quantum control technique that takes advantage of temporal symmetries in order to partially suppress quantum errors without the need resource-intensiveerror detection and correction protocols. This and other open-loop error mitigation techniques are critical for quantum information processing in the era of Noisy Intermediate-Scale Quantum technology. However, despite its utility, dynamical decoupling does not address errors which occur at unstructured times during a circuit, including certain commonly-encountered noise mechanisms such as cross-talk and imperfectly calibrated control pulses. Here, we introduce and demonstrate an alternative technique – `quantum measurement emulation‘ (QME) – that effectively emulates the measurement of stabilizer operators via stochastic gate application, leading to a first-order insensitivity to coherent errors. The QME protocol enables error suppression based on the stabilizer code formalism without the need for costly measurements and feedback, and it is particularly well-suited to discrete coherent errors that are challenging for DD to address.
A quantum algorithm consists of a sequence of operations and measurements applied to a quantum processor. To date, the instruction set which defines this sequence has been providedby a classical computer and passed via control hardware to the quantum processor. Here, we demonstrate the first experimental realization of a quantum instruction set, in which a fixed sequence of classically-defined gates perform an operation that is fully determined only by a quantum input to the fixed sequence. Specifically, we implement the density matrix exponentiation algorithm, which consumes N copies of the instruction state ρ to approximate the operation e−iρθ (θ an arbitrary angle). Our implementation relies on a 99.7\% fidelity controlled-phase gate between two superconducting transmon qubits. We achieve an average algorithmic fidelity ≈0.9, independent of the setting of ρ, to circuit depth nearly 90. This new paradigm for quantum instructions has applications to resource-efficient protocols for validating entanglement spectra, principal component analysis of large quantum states, and universal quantum emulation.
Developing a packaging scheme that meets all of the requirements for operation of solid-state qubits in a cryogenic environment can be a formidable challenge. In this article, we discusswork being done in our group as well as in the broader community, focusing on the role of 3D integration and packaging in quantum processing with solid-state qubits.
As the field of superconducting quantum computing advances from the few-qubit stage to larger-scale processors, qubit addressability and extensibility will necessitate the use of 3Dintegration and packaging. While 3D integration is well-developed for commercial electronics, relatively little work has been performed to determine its compatibility with high-coherence solid-state qubits. Of particular concern, qubit coherence times can be suppressed by the requisite processing steps and close proximity of another chip. In this work, we use a flip-chip process to bond a chip with superconducting flux qubits to another chip containing structures for qubit readout and control. We demonstrate that high qubit coherence (T1, T2,echo>20μs) is maintained in a flip-chip geometry in the presence of galvanic, capacitive, and inductive coupling between the chips.
We present an experimental realization of resonance fluorescence in squeezed vacuum. We strongly couple microwave-frequency squeezed light to a superconducting artificial atom and detectthe resulting fluorescence with high resolution enabled by a broadband traveling-wave parametric amplifier. We investigate the fluorescence spectra in the weak and strong driving regimes, observing up to 3.1 dB of reduction of the fluorescence linewidth below the ordinary vacuum level and a dramatic dependence of the Mollow triplet spectrum on the relative phase of the driving and squeezed vacuum fields. Our results are in excellent agreement with predictions for spectra produced by a two-level atom in squeezed vacuum [Phys. Rev. Lett. \textbf{58}, 2539-2542 (1987)], demonstrating that resonance fluorescence offers a resource-efficient means to characterize squeezing in cryogenic environments.
Gate operations in a quantum information processor are generally realized by tailoring specific periods of free and driven evolution of a quantum system. Unwanted environmental noise,which may in principle be distinct during these two periods, acts to decohere the system and increase the gate error rate. While there has been significant progress characterizing noise processes during free evolution, the corresponding driven-evolution case is more challenging as the noise being probed is also extant during the characterization protocol. Here we demonstrate the noise spectroscopy (0.1 – 200 MHz) of a superconducting flux qubit during driven evolution by using a robust spin-locking pulse sequence to measure relaxation (T1rho) in the rotating frame. In the case of flux noise, we resolve spectral features due to coherent fluctuators, and further identify a signature of the 1MHz defect in a time-domain spin-echo experiment. The driven-evolution noise spectroscopy complements free-evolution methods, enabling the means to characterize and distinguish various noise processes relevant for universal quantum control.
The scalable application of quantum information science will stand on reproducible and controllable high-coherence quantum bits (qubits). In this work, we revisit the design and fabricationof the superconducting flux qubit, achieving a planar device with broad frequency tunability, strong anharmonicity, high reproducibility, and coherence times in excess of 40 us at its flux-insensitive point. Qubit relaxation times across 21 qubits of widely varying designs are consistently matched with a single model involving ohmic charge noise, quasiparticle fluctuations, resonator loss, and 1/f flux noise, a noise source previously considered primarily in the context of dephasing. We furthermore demonstrate that qubit dephasing at the flux-insensitive point is dominated by residual thermal photons in the readout resonator. The resulting photon shot noise is mitigated using a dynamical decoupling protocol, reaching T2 ~ 80 us , approximately the 2T1 limit. In addition to realizing a dramatically improved flux qubit, our results uniquely identify photon shot noise as limiting T2 in contemporary state-of-art qubits based on transverse qubit-resonator interaction.
We present a new read-out technique for a superconducting qubit dispersively coupled to a Josephson parametric oscillator. We perform degenerate parametric flux pumping of the Josephsoninductance with a pump amplitude surpassing the threshold for parametric instability. We map the qubit states onto two distinct states of classical parametric oscillations: one oscillating state, with on average 180 photons in the resonator, and one with zero oscillation amplitude. We demonstrate single-shot readout performance, with a total state discrimination of 81.5%. When accounting for qubit errors, this gives a corrected fidelity of 98.7%, obviating the need for a following quantum-limited amplifier. An error budget indicates that the readout fidelity is currently limited by spurious switching events between two bistable states of the resonator.