19 research outputs found
Time-dependent Hamiltonian estimation for Doppler velocimetry of trapped ions
The time evolution of a closed quantum system is connected to its Hamiltonian
through Schroedinger's equation. The ability to estimate the Hamiltonian is
critical to our understanding of quantum systems, and allows optimization of
control. Though spectroscopic methods allow time-independent Hamiltonians to be
recovered, for time-dependent Hamiltonians this task is more challenging. Here,
using a single trapped ion, we experimentally demonstrate a method for
estimating a time-dependent Hamiltonian of a single qubit. The method involves
measuring the time evolution of the qubit in a fixed basis as a function of a
time-independent offset term added to the Hamiltonian. In our system the
initially unknown Hamiltonian arises from transporting an ion through a static,
near-resonant laser beam. Hamiltonian estimation allows us to estimate the
spatial dependence of the laser beam intensity and the ion's velocity as a
function of time. This work is of direct value in optimizing transport
operations and transport-based gates in scalable trapped ion quantum
information processing, while the estimation technique is general enough that
it can be applied to other quantum systems, aiding the pursuit of high
operational fidelities in quantum control.Comment: 10 pages, 8 figure
High-rate, high-fidelity entanglement of qubits across an elementary quantum network
We demonstrate remote entanglement of trapped-ion qubits via a
quantum-optical fiber link with fidelity and rate approaching those of local
operations. Two Sr qubits are entangled via the polarization
degree of freedom of two photons which are coupled by high-numerical-aperture
lenses into single-mode optical fibers and interfere on a beamsplitter. A novel
geometry allows high-efficiency photon collection while maintaining unit
fidelity for ion-photon entanglement. We generate remote Bell pairs with
fidelity at an average rate (success
probability ).Comment: v2 updated to include responses to reviewers, as published in PR
Probing Qubit Memory Errors at the Part-per-Million Level
Robust qubit memory is essential for quantum computing, both for near-term
devices operating without error correction, and for the long-term goal of a
fault-tolerant processor. We directly measure the memory error for
a Ca trapped-ion qubit in the small-error regime and find
for storage times t\lesssim50\,\mbox{ms}. This exceeds
gate or measurement times by three orders of magnitude. Using randomized
benchmarking, at t=1\,\mbox{ms} we measure ,
around ten times smaller than that extrapolated from the time,
and limited by instability of the atomic clock reference used to benchmark the
qubit.Comment: 8 pages, 5 figure
Device-Independent Quantum Key Distribution
Cryptographic key exchange protocols traditionally rely on computational
conjectures such as the hardness of prime factorisation to provide security
against eavesdropping attacks. Remarkably, quantum key distribution protocols
like the one proposed by Bennett and Brassard provide information-theoretic
security against such attacks, a much stronger form of security unreachable by
classical means. However, quantum protocols realised so far are subject to a
new class of attacks exploiting implementation defects in the physical devices
involved, as demonstrated in numerous ingenious experiments. Following the
pioneering work of Ekert proposing the use of entanglement to bound an
adversary's information from Bell's theorem, we present here the experimental
realisation of a complete quantum key distribution protocol immune to these
vulnerabilities. We achieve this by combining theoretical developments on
finite-statistics analysis, error correction, and privacy amplification, with
an event-ready scheme enabling the rapid generation of high-fidelity
entanglement between two trapped-ion qubits connected by an optical fibre link.
The secrecy of our key is guaranteed device-independently: it is based on the
validity of quantum theory, and certified by measurement statistics observed
during the experiment. Our result shows that provably secure cryptography with
real-world devices is possible, and paves the way for further quantum
information applications based on the device-independence principle.Comment: 5+1 pages in main text and methods with 4 figures and 1 table; 37
pages of supplementary materia
High-rate high-fidelity entanglement of qubits across an elementary quantum network
We demonstrate remote entanglement of trapped-ion qubits via a quantum-optical fiber link with fidelity and rate approaching those of local operations. Two 88Sr+ qubits are entangled via the polarization degree of freedom of two spontaneously emitted 422 nm photons which are coupled by high-numerical-aperture lenses into single-mode optical fibers and interfere on a beam splitter. A novel geometry allows high-efficiency photon collection while maintaining unit fidelity for ion-photon entanglement. We generate heralded Bell pairs with fidelity 94% at an average rate 182 s−1 (success probability 2.18×10−4).</p
An elementary quantum network of entangled optical atomic clocks
Optical atomic clocks are our most precise tools to measure time and frequency1,2,3. Precision frequency comparisons between clocks in separate locations enable one to probe the space–time variation of fundamental constants4,5 and the properties of dark matter6,7, to perform geodesy8,9,10 and to evaluate systematic clock shifts. Measurements on independent systems are limited by the standard quantum limit; measurements on entangled systems can surpass the standard quantum limit to reach the ultimate precision allowed by quantum theory—the Heisenberg limit. Although local entangling operations have demonstrated this enhancement at microscopic distances11,12,13,14,15,16, comparisons between remote atomic clocks require the rapid generation of high-fidelity entanglement between systems that have no intrinsic interactions. Here we report the use of a photonic link17,18 to entangle two 88Sr+ ions separated by a macroscopic distance19 (approximately 2 m) to demonstrate an elementary quantum network of entangled optical clocks. For frequency comparisons between the ions, we find that entanglement reduces the measurement uncertainty by nearly 2‾√, the value predicted for the Heisenberg limit. Today’s optical clocks are typically limited by dephasing of the probe laser20; in this regime, we find that entanglement yields a factor of 2 reduction in the measurement uncertainty compared with conventional correlation spectroscopy techniques20,21,22. We demonstrate this enhancement for the measurement of a frequency shift applied to one of the clocks. This two-node network could be extended to additional nodes23, to other species of trapped particles or—through local operations—to larger entangled systems
Fast, high-fidelity addressed single-qubit gates using efficient composite pulse sequences
We use electronic microwave control methods to implement addressed
single-qubit gates with high speed and fidelity, for
hyperfine ''atomic clock'' qubits in a cryogenic (100K) surface trap. For a
single qubit, we benchmark an error of per Clifford
gate (implemented using -pulses). For two qubits in the
same trap zone (ion separation ), we use a spatial microwave
field gradient, combined with an efficient 4-pulse scheme, to implement
independent addressed gates. Parallel randomized benchmarking on both qubits
yields an average error per logical gate
An elementary quantum network of entangled optical atomic clocks
Optical atomic clocks are our most precise tools to measure time and frequency1,2,3. Precision frequency comparisons between clocks in separate locations enable one to probe the space–time variation of fundamental constants4,5 and the properties of dark matter6,7, to perform geodesy8,9,10 and to evaluate systematic clock shifts. Measurements on independent systems are limited by the standard quantum limit; measurements on entangled systems can surpass the standard quantum limit to reach the ultimate precision allowed by quantum theory—the Heisenberg limit. Although local entangling operations have demonstrated this enhancement at microscopic distances11,12,13,14,15,16, comparisons between remote atomic clocks require the rapid generation of high-fidelity entanglement between systems that have no intrinsic interactions. Here we report the use of a photonic link17,18 to entangle two 88Sr+ ions separated by a macroscopic distance19 (approximately 2 m) to demonstrate an elementary quantum network of entangled optical clocks. For frequency comparisons between the ions, we find that entanglement reduces the measurement uncertainty by nearly 2‾√, the value predicted for the Heisenberg limit. Today’s optical clocks are typically limited by dephasing of the probe laser20; in this regime, we find that entanglement yields a factor of 2 reduction in the measurement uncertainty compared with conventional correlation spectroscopy techniques20,21,22. We demonstrate this enhancement for the measurement of a frequency shift applied to one of the clocks. This two-node network could be extended to additional nodes23, to other species of trapped particles or—through local operations—to larger entangled systems