47 research outputs found
Circuit QED with Hole-Spin Qubits in Ge/Si Nanowire Quantum Dots
We propose a setup for universal and electrically controlled quantum
information processing with hole spins in Ge/Si core/shell nanowire quantum
dots (NW QDs). Single-qubit gates can be driven through electric-dipole-induced
spin resonance, with spin-flip times shorter than 100 ps. Long-distance
qubit-qubit coupling can be mediated by the cavity electric field of a
superconducting transmission line resonator, where we show that operation times
below 20 ns seem feasible for the entangling square-root-of-iSWAP gate. The
absence of Dresselhaus spin-orbit interaction (SOI) and the presence of an
unusually strong Rashba-type SOI enable precise control over the transverse
qubit coupling via an externally applied, perpendicular electric field. The
latter serves as an on-off switch for quantum gates and also provides control
over the g factor, so single- and two-qubit gates can be operated
independently. Remarkably, we find that idle qubits are insensitive to charge
noise and phonons, and we discuss strategies for enhancing noise-limited gate
fidelities.Comment: 6 pages main article + 12 pages supplement, 4 figure
Quantum state engineering with flux-biased Josephson phase qubits by Stark-chirped rapid adiabatic passages
In this paper, the scheme of quantum computing based on Stark chirped rapid
adiabatic passage (SCRAP) technique [L. F. Wei et al., Phys. Rev. Lett. 100,
113601 (2008)] is extensively applied to implement the quantum-state
manipulations in the flux-biased Josephson phase qubits. The broken-parity
symmetries of bound states in flux-biased Josephson junctions are utilized to
conveniently generate the desirable Stark-shifts. Then, assisted by various
transition pulses universal quantum logic gates as well as arbitrary
quantum-state preparations could be implemented. Compared with the usual
PI-pulses operations widely used in the experiments, the adiabatic population
passage proposed here is insensitive the details of the applied pulses and thus
the desirable population transfers could be satisfyingly implemented. The
experimental feasibility of the proposal is also discussed.Comment: 9 pages, 4 figure
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 Electrodynamics in a Topological Waveguide
While designing the energy-momentum relation of photons is key to many linear, nonlinear, and quantum optical phenomena, a new set of light-matter properties may be realized by employing the topology of the photonic bath itself. In this work we experimentally investigate the properties of superconducting qubits coupled to a metamaterial waveguide based on a photonic analog of the Su-Schrieffer-Heeger model. We explore topologically induced properties of qubits coupled to such a waveguide, ranging from the formation of directional qubit-photon bound states to topology-dependent cooperative radiation effects. Addition of qubits to this waveguide system also enables direct quantum control over topological edge states that form in finite waveguide systems, useful for instance in constructing a topologically protected quantum communication channel. More broadly, our work demonstrates the opportunity that topological waveguide-QED systems offer in the synthesis and study of many-body states with exotic long-range quantum correlations
Phase transition in Random Circuit Sampling
Quantum computers hold the promise of executing tasks beyond the capability
of classical computers. Noise competes with coherent evolution and destroys
long-range correlations, making it an outstanding challenge to fully leverage
the computation power of near-term quantum processors. We report Random Circuit
Sampling (RCS) experiments where we identify distinct phases driven by the
interplay between quantum dynamics and noise. Using cross-entropy benchmarking,
we observe phase boundaries which can define the computational complexity of
noisy quantum evolution. We conclude by presenting an RCS experiment with 70
qubits at 24 cycles. We estimate the computational cost against improved
classical methods and demonstrate that our experiment is beyond the
capabilities of existing classical supercomputers
Quantum Electrodynamics in a Topological Waveguide
While designing the energy-momentum relation of photons is key to many linear, nonlinear, and quantum optical phenomena, a new set of light-matter properties may be realized by employing the topology of the photonic bath itself. In this work we experimentally investigate the properties of superconducting qubits coupled to a metamaterial waveguide based on a photonic analog of the Su-Schrieffer-Heeger model. We explore topologically induced properties of qubits coupled to such a waveguide, ranging from the formation of directional qubit-photon bound states to topology-dependent cooperative radiation effects. Addition of qubits to this waveguide system also enables direct quantum control over topological edge states that form in finite waveguide systems, useful for instance in constructing a topologically protected quantum communication channel. More broadly, our work demonstrates the opportunity that topological waveguide-QED systems offer in the synthesis and study of many-body states with exotic long-range quantum correlations
Quantum Memory: A Missing Piece in Quantum Computing Units
Memory is an indispensable component in classical computing systems. While
the development of quantum computing is still in its early stages, current
quantum processing units mainly function as quantum registers. Consequently,
the actual role of quantum memory in future advanced quantum computing
architectures remains unclear. With the rapid scaling of qubits, it is
opportune to explore the potential and feasibility of quantum memory across
different substrate device technologies and application scenarios. In this
paper, we provide a full design stack view of quantum memory. We start from the
elementary component of a quantum memory device, quantum memory cells. We
provide an abstraction to a quantum memory cell and define metrics to measure
the performance of physical platforms. Combined with addressing functionality,
we then review two types of quantum memory devices: random access quantum
memory (RAQM) and quantum random access memory (QRAM). Building on top of these
devices, quantum memory units in the computing architecture, including building
a quantum memory unit, quantum cache, quantum buffer, and using QRAM for the
quantum input-output module, are discussed. We further propose the programming
model for the quantum memory units and discuss their possible applications. By
presenting this work, we aim to attract more researchers from both the Quantum
Information Science (QIS) and classical memory communities to enter this
emerging and exciting area.Comment: 41 pages, 11 figures, 7 table
Quantum information processing with tunable and low-loss superconducting circuits
The perhaps most promising platform for quantum information processing is the circuit-QED architecture based on superconducting circuits representing quantum bits. These circuits must be made with low losses so that the quantum information is retained for as long as possible. We developed fabrication processes achieving state-of-the-art coherence times of over 100 \ub5s. We identified the primary source of loss to be parasitic two-level systems by studying fluctuations of qubit relaxation times.Using our high-coherence circuits, we implemented a quantum processor built on fixed-frequency qubits and frequency-tunable couplers. The tunable couplers were lumped-element LC resonators, where the inductance came from a superconducting quantum interference device (SQUID). We achieved a controlled-phase gate with a fidelity of 99% by parametric modulation of the coupler frequency. Using this device, and another similar to it, we demonstrated two different quantum algorithms, the quantum approximate optimization algorithm, and density matrix exponentiation. We achieved high algorithmic fidelities, aided by our carefully calibrated gates.Additionally, we researched parametric oscillations using frequency-tunable resonators. Previously, degenerate parametric oscillations have been demonstrated by modulation of the resonant frequency at twice that frequency. We use this phenomenon to implement a readout method for a superconducting qubit with a fidelity of 98.7%. We demonstrated correlated radiation in nondegenerate parametric oscillations by modulating at the sum of two resonant frequencies of a multimode resonator. We showed an excellent quantitative agreement between the classical properties of the oscillations with a theoretical model. Moreover, we studied higher-order modulation at up to five times their resonant frequencies. These types of parametric oscillation states might be used as a quantum resource for continuous-variable quantum computing