21 research outputs found
Nanoscale electrical conductivity imaging using a nitrogen-vacancy center in diamond
The electrical conductivity of a material can feature subtle, nontrivial, and
spatially-varying signatures with critical insight into the material's
underlying physics. Here we demonstrate a conductivity imaging technique based
on the atom-sized nitrogen-vacancy (NV) defect in diamond that offers local,
quantitative, and noninvasive conductivity imaging with nanoscale spatial
resolution. We monitor the spin relaxation rate of a single NV center in a
scanning probe geometry to quantitatively image the magnetic fluctuations
produced by thermal electron motion in nanopatterned metallic conductors. We
achieve 40-nm scale spatial resolution of the conductivity and realize a
25-fold increase in imaging speed by implementing spin-to-charge conversion
readout of a shallow NV center. NV-based conductivity imaging can probe
condensed-matter systems in a new regime, and as a model example, we project
readily achievable imaging of nanoscale phase separation in complex oxides.Comment: Supplementary information at en
Compiling Quantum Circuits for Dynamically Field-Programmable Neutral Atoms Array Processors
Dynamically field-programmable qubit arrays (DPQA) have recently emerged as a
promising platform for quantum information processing. In DPQA, atomic qubits
are selectively loaded into arrays of optical traps that can be reconfigured
during the computation itself. Leveraging qubit transport and parallel,
entangling quantum operations, different pairs of qubits, even those initially
far away, can be entangled at different stages of the quantum program
execution. Such reconfigurability and non-local connectivity present new
challenges for compilation, especially in the layout synthesis step which
places and routes the qubits and schedules the gates. In this paper, we
consider a DPQA architecture that contains multiple arrays and supports 2D
array movements, representing cutting-edge experimental platforms. Within this
architecture, we discretize the state space and formulate layout synthesis as a
satisfactory modulo theories problem, which can be solved by existing solvers
optimally in terms of circuit depth. For a set of benchmark circuits generated
by random graphs with complex connectivities, our compiler OLSQ-DPQA reduces
the number of two-qubit entangling gates on small problem instances by 1.7x
compared to optimal compilation results on a fixed planar architecture. To
further improve scalability and practicality of the method, we introduce a
greedy heuristic inspired by the iterative peeling approach in classical
integrated circuit routing. Using a hybrid approach that combined the greedy
and optimal methods, we demonstrate that our DPQA-based compiled circuits
feature reduced scaling overhead compared to a grid fixed architecture,
resulting in 5.1X less two-qubit gates for 90 qubit quantum circuits. These
methods enable programmable, complex quantum circuits with neutral atom quantum
computers, as well as informing both future compilers and future hardware
choices.Comment: An extended abstract of this work was presented at the 41st
International Conference on Computer-Aided Design (ICCAD '22
Constant-Overhead Fault-Tolerant Quantum Computation with Reconfigurable Atom Arrays
Quantum low-density parity-check (qLDPC) codes can achieve high encoding
rates and good code distance scaling, providing a promising route to
low-overhead fault-tolerant quantum computing. However, the long-range
connectivity required to implement such codes makes their physical realization
challenging. Here, we propose a hardware-efficient scheme to perform
fault-tolerant quantum computation with high-rate qLDPC codes on reconfigurable
atom arrays, directly compatible with recently demonstrated experimental
capabilities. Our approach utilizes the product structure inherent in many
qLDPC codes to implement the non-local syndrome extraction circuit via atom
rearrangement, resulting in effectively constant overhead in practically
relevant regimes. We prove the fault tolerance of these protocols, perform
circuit-level simulations of memory and logical operations with these codes,
and find that our qLDPC-based architecture starts to outperform the surface
code with as few as several hundred physical qubits at a realistic physical
error rate of . We further find that less than 3000 physical qubits
are sufficient to obtain over an order of magnitude qubit savings compared to
the surface code, and quantum algorithms involving thousands of logical qubits
can be performed using less than physical qubits. Our work paves the way
for explorations of low-overhead quantum computing with qLDPC codes at a
practical scale, based on current experimental technologies
A quantum processor based on coherent transport of entangled atom arrays
The ability to engineer parallel, programmable operations between desired
qubits within a quantum processor is central for building scalable quantum
information systems. In most state-of-the-art approaches, qubits interact
locally, constrained by the connectivity associated with their fixed spatial
layout. Here, we demonstrate a quantum processor with dynamic, nonlocal
connectivity, in which entangled qubits are coherently transported in a highly
parallel manner across two spatial dimensions, in between layers of single- and
two-qubit operations. Our approach makes use of neutral atom arrays trapped and
transported by optical tweezers; hyperfine states are used for robust quantum
information storage, and excitation into Rydberg states is used for
entanglement generation. We use this architecture to realize programmable
generation of entangled graph states such as cluster states and a 7-qubit
Steane code state. Furthermore, we shuttle entangled ancilla arrays to realize
a surface code with 19 qubits and a toric code state on a torus with 24 qubits.
Finally, we use this architecture to realize a hybrid analog-digital evolution
and employ it for measuring entanglement entropy in quantum simulations,
experimentally observing non-monotonic entanglement dynamics associated with
quantum many-body scars. Realizing a long-standing goal, these results pave the
way toward scalable quantum processing and enable new applications ranging from
simulation to metrology.Comment: 23 pages, 14 figures; movie attached as ancillary fil
Probing many-body noise in a strongly interacting two-dimensional dipolar spin system
The most direct approach for characterizing the quantum dynamics of a
strongly-interacting system is to measure the time-evolution of its full
many-body state. Despite the conceptual simplicity of this approach, it quickly
becomes intractable as the system size grows. An alternate framework is to
think of the many-body dynamics as generating noise, which can be measured by
the decoherence of a probe qubit. Our work centers on the following question:
What can the decoherence dynamics of such a probe tell us about the many-body
system? In particular, we utilize optically addressable probe spins to
experimentally characterize both static and dynamical properties of
strongly-interacting magnetic dipoles. Our experimental platform consists of
two types of spin defects in diamond: nitrogen-vacancy (NV) color centers
(probe spins) and substitutional nitrogen impurities (many-body system). We
demonstrate that signatures of the many-body system's dimensionality, dynamics,
and disorder are naturally encoded in the functional form of the NV's
decoherence profile. Leveraging these insights, we directly characterize the
two-dimensional nature of a nitrogen delta-doped diamond sample. In addition,
we explore two distinct facets of the many-body dynamics: First, we address a
persistent debate about the microscopic nature of spin dynamics in
strongly-interacting dipolar systems. Second, we demonstrate direct control
over the spectral properties of the many-body system, including its correlation
time. Our work opens the door to new directions in both quantum sensing and
simulation.Comment: 10 + 8 + 5 pages; 3 + 5 figure
High-fidelity parallel entangling gates on a neutral atom quantum computer
The ability to perform entangling quantum operations with low error rates in
a scalable fashion is a central element of useful quantum information
processing. Neutral atom arrays have recently emerged as a promising quantum
computing platform, featuring coherent control over hundreds of qubits and
any-to-any gate connectivity in a flexible, dynamically reconfigurable
architecture. The major outstanding challenge has been to reduce errors in
entangling operations mediated through Rydberg interactions. Here we report the
realization of two-qubit entangling gates with 99.5% fidelity on up to 60 atoms
in parallel, surpassing the surface code threshold for error correction. Our
method employs fast single-pulse gates based on optimal control, atomic dark
states to reduce scattering, and improvements to Rydberg excitation and atom
cooling. We benchmark fidelity using several methods based on repeated gate
applications, characterize the physical error sources, and outline future
improvements. Finally, we generalize our method to design entangling gates
involving a higher number of qubits, which we demonstrate by realizing
low-error three-qubit gates. By enabling high-fidelity operation in a scalable,
highly connected system, these advances lay the groundwork for large-scale
implementation of quantum algorithms, error-corrected circuits, and digital
simulations.Comment: 5 pages, 4 figures. Methods: 13 pages, 10 figure