9 research outputs found
Coherent Coupled Qubits for Quantum Annealing
Quantum annealing is an optimization technique which potentially leverages quantum tunneling to enhance computational performance. Existing quantum annealers use superconducting flux qubits with short coherence times limited primarily by the use of large persistent currents I[subscript p]. Here, we examine an alternative approach using qubits with smaller I[subscript p] and longer coherence times. We demonstrate tunable coupling, a basic building block for quantum annealing, between two flux qubits with small (approximately 50-nA) persistent currents. Furthermore, we characterize qubit coherence as a function of coupler setting and investigate the effect of flux noise in the coupler loop on qubit coherence. Our results provide insight into the available design space for next-generation quantum annealers with improved coherence.United States. Office of the Director of National IntelligenceUnited States. Intelligence Advanced Research Projects ActivityUnited States. Dept. of Defense. Assistant Secretary of Defense for Research & Engineering (FA8721-05-C-0002
Realization of high-fidelity CZ and ZZ-free iSWAP gates with a tunable coupler
High-fidelity two-qubit gates at scale are a key requirement to realize the
full promise of quantum computation and simulation. The advent and use of
coupler elements to tunably control two-qubit interactions has improved
operational fidelity in many-qubit systems by reducing parasitic coupling and
frequency crowding issues. Nonetheless, two-qubit gate errors still limit the
capability of near-term quantum applications. The reason, in part, is the
existing framework for tunable couplers based on the dispersive approximation
does not fully incorporate three-body multi-level dynamics, which is essential
for addressing coherent leakage to the coupler and parasitic longitudinal
() interactions during two-qubit gates. Here, we present a systematic
approach that goes beyond the dispersive approximation to exploit the
engineered level structure of the coupler and optimize its control. Using this
approach, we experimentally demonstrate CZ and -free iSWAP gates with
two-qubit interaction fidelities of % and %,
respectively, which are close to their limits.Comment: 28 pages, 32 figure
Quantum coherent control of a hybrid superconducting circuit made with graphene-based van der Waals heterostructures
Quantum coherence and control is foundational to the science and engineering
of quantum systems. In van der Waals (vdW) materials, the collective coherent
behavior of carriers has been probed successfully by transport measurements.
However, temporal coherence and control, as exemplified by manipulating a
single quantum degree of freedom, remains to be verified. Here we demonstrate
such coherence and control of a superconducting circuit incorporating
graphene-based Josephson junctions. Furthermore, we show that this device can
be operated as a voltage-tunable transmon qubit, whose spectrum reflects the
electronic properties of massless Dirac fermions traveling ballistically. In
addition to the potential for advancing extensible quantum computing
technology, our results represent a new approach to studying vdW materials
using microwave photons in coherent quantum circuits
Experimental Demonstration of Lindblad Tomography on a Superconducting Quantum Device
Information loss in experimental quantum devices is traditionally characterized using metrics such as T₁ and T₂, which are readily accessible from standard time-domain measurement. While T₁ and T₂ times provide rough heuristics for interaction between single qubits and their lossy environments, these numbers stand in as mere proxies for the full multi-qubit loss channel of interest, which can be described more fully with a Lindbladian operator in the master equation formalism. In this thesis, I outline and present the results of the first experimental demonstration of Lindblad Tomography, a novel technique for tomographically reconstructing the Hamiltonian and Lindbladian operators of an arbitrary quantum channel from an ensemble of time-domain measurements. Starting from a theoretically minimal set of assumptions, I show that this method is resilient to state-preparation and measurement (SPAM) errors and places strong bounds on the degree of non-Markovianity in the channel of interest. Comparing the results for single- and two-qubit tomography of a superconducting quantum processor, I demonstrate how Lindblad Tomography can be used to identify sources of crosstalk on large quantum processors, particularly in the presence of always-on qubit-qubit interactions.S.M
Lindblad Tomography of a Superconducting Quantum Processor
As progress is made towards the first generation of error-corrected quantum computers, robust characterization and validation protocols are required to assess the noise environments of physical quantum processors. While standard coherence metrics and characterization protocols such as T1 and T2, process tomography, and randomized benchmarking are now ubiquitous, these techniques provide only partial information about the dynamic multiqubit loss channels responsible for processor errors, which can be described more fully by a Lindblad operator in the master equation formalism. Here, we introduce and experimentally demonstrate Lindblad tomography, a hardware-agnostic characterization protocol for tomographically reconstructing the Hamiltonian and Lindblad operators of a quantum noise environment from an ensemble of time-domain measurements. Performing Lindblad tomography on a small superconducting quantum processor, we show that this technique naturally builds on standard process tomography and T1/T2 measurement protocols, characterizes and accounts for state-preparation and measurement errors, and allows one to place bounds on the fit to a Markovian model. Comparing the results of single- and two-qubit measurements on a superconducting quantum processor, we demonstrate that Lindblad tomography can also be used to identify and quantify sources of crosstalk on quantum processors, such as the presence of always-on qubit-qubit interactions. </p
Lindblad Tomography of a Superconducting Quantum Processor
As progress is made towards the first generation of error-corrected quantum computers, robust characterization and validation protocols are required to assess the noise environments of physical quantum processors. While standard coherence metrics and characterization protocols such as T1 and T2, process tomography, and randomized benchmarking are now ubiquitous, these techniques provide only partial information about the dynamic multiqubit loss channels responsible for processor errors, which can be described more fully by a Lindblad operator in the master equation formalism. Here, we introduce and experimentally demonstrate Lindblad tomography, a hardware-agnostic characterization protocol for tomographically reconstructing the Hamiltonian and Lindblad operators of a quantum noise environment from an ensemble of time-domain measurements. Performing Lindblad tomography on a small superconducting quantum processor, we show that this technique naturally builds on standard process tomography and T1/T2 measurement protocols, characterizes and accounts for state-preparation and measurement errors, and allows one to place bounds on the fit to a Markovian model. Comparing the results of single- and two-qubit measurements on a superconducting quantum processor, we demonstrate that Lindblad tomography can also be used to identify and quantify sources of crosstalk on quantum processors, such as the presence of always-on qubit-qubit interactions. QN/Borregaard groe