227 research outputs found
Environment-assisted quantum transport in a 10-qubit network
The way in which energy is transported through an interacting system governs
fundamental properties in many areas of physics, chemistry, and biology.
Remarkably, environmental noise can enhance the transport, an effect known as
environment-assisted quantum transport (ENAQT). In this paper, we study ENAQT
in a network of coupled spins subject to engineered static disorder and
temporally varying dephasing noise. The interacting spin network is realized in
a chain of trapped atomic ions and energy transport is represented by the
transfer of electronic excitation between ions. With increasing noise strength,
we observe a crossover from coherent dynamics and Anderson localization to
ENAQT and finally a suppression of transport due to the quantum Zeno effect. We
found that in the regime where ENAQT is most effective the transport is mainly
diffusive, displaying coherences only at very short times. Further, we show
that dephasing characterized by non-Markovian noise can maintain coherences
longer than white noise dephasing, with a strong influence of the spectral
structure on the transport effciency. Our approach represents a controlled and
scalable way to investigate quantum transport in many-body networks under
static disorder and dynamic noise.Comment: Mai
Compatibility and noncontextuality for sequential measurements
A basic assumption behind the inequalities used for testing noncontextual
hidden variable models is that the observables measured on the same individual
system are perfectly compatible. However, compatibility is not perfect in
actual experiments using sequential measurements. We discuss the resulting
"compatibility loophole" and present several methods to rule out certain hidden
variable models which obey a kind of extended noncontextuality. Finally, we
present a detailed analysis of experimental imperfections in a recent trapped
ion experiment and apply our analysis to that case.Comment: 15 pages, 2 figures, v2: problem with latex solve
Observation of magnon bound states in the long-range, anisotropic Heisenberg model
Over the recent years coherent, time-periodic modulation has been established
as a versatile tool for realizing novel Hamiltonians. Using this approach,
known as Floquet engineering, we experimentally realize a long-ranged,
anisotropic Heisenberg model with tunable interactions in a trapped ion quantum
simulator. We demonstrate that the spectrum of the model contains not only
single magnon excitations but also composite magnon bound states. For the
long-range interactions with the experimentally realized power-law exponent,
the group velocity of magnons is unbounded. Nonetheless, for sufficiently
strong interactions we observe bound states of these unconventional magnons
which possess a non-diverging group velocity. By measuring the configurational
mutual information between two disjoint intervals, we demonstrate the
implications of the bound state formation on the entanglement dynamics of the
system. Our observations provide key insights into the peculiar role of
composite excitations in the non-equilibrium dynamics of quantum many-body
systems
Ultraviolet laser pulses with multigigahertz repetition rate and multiwatt average power for fast trapped-ion entanglement operations
The conventional approach to perform two-qubit gate operations in trapped ions relies on exciting the ions on motional sidebands with laser light, which is an inherently slow process. One way to implement a fast entangling gate protocol requires a suitable pulsed laser to increase the gate speed by orders of magnitude. However, the realization of such a fast entangling gate operation presents a big technical challenge, as such the required laser source is not available off-the-shelf. For this, we have engineered an ultrafast entangling gate source based on a frequency comb. The source generates bursts of several hundred mode-locked pulses with pulse energy ∼800 pJ at 5 GHz repetition rate at 393.3 nm and complies with all requirements for implementing a fast two-qubit gate operation. Using a single, chirped ultraviolet pulse, we demonstrate a rapid adiabatic passage in a Ca+ ion. To verify the applicability and projected performance of the laser system for inducing entangling gates we run simulations based on our source parameters. The gate time can be faster than a trap period with an error approaching 10−4
Sideband thermometry of ion crystals
Coulomb crystals of cold trapped ions are a leading platform for the
realisation of quantum processors and quantum simulations and, in quantum
metrology, for the construction of optical atomic clocks and for fundamental
tests of the Standard Model. For these applications, it is not only essential
to cool the ion crystal in all its degrees of freedom down to the quantum
ground state, but also to be able to determine its temperature with a high
accuracy. However, when a large ground-state cooled crystal is interrogated for
thermometry, complex many-body interactions take place, making it challenging
to accurately estimate the temperature with established techniques. In this
work we present a new thermometry method tailored for ion crystals. The method
is applicable to all normal modes of motion and does not suffer from a
computational bottleneck when applied to large ion crystals. We test the
temperature estimate with two experiments, namely with a 1D linear chain of 4
ions and a 2D crystal of 19 ions and verify the results, where possible, using
other methods. The results show that the new method is an accurate and
efficient tool for thermometry of ion crystals.Comment: 12+5 pages, 9+2 figures, Fig.3(b) was correcte
An Open-System Quantum Simulator with Trapped Ions
The control of quantum systems is of fundamental scientific interest and
promises powerful applications and technologies. Impressive progress has been
achieved in isolating the systems from the environment and coherently
controlling their dynamics, as demonstrated by the creation and manipulation of
entanglement in various physical systems. However, for open quantum systems,
engineering the dynamics of many particles by a controlled coupling to an
environment remains largely unexplored. Here we report the first realization of
a toolbox for simulating an open quantum system with up to five qubits. Using a
quantum computing architecture with trapped ions, we combine multi-qubit gates
with optical pumping to implement coherent operations and dissipative
processes. We illustrate this engineering by the dissipative preparation of
entangled states, the simulation of coherent many-body spin interactions and
the quantum non-demolition measurement of multi-qubit observables. By adding
controlled dissipation to coherent operations, this work offers novel prospects
for open-system quantum simulation and computation.Comment: Pre-review submission to Nature. For an updated and final version see
publication. Manuscript + Supplementary Informatio
A Factorization Law for Entanglement Decay
We present a simple and general factorization law for quantum systems shared
by two parties, which describes the time evolution of entanglement upon passage
of either component through an arbitrary noisy channel. The robustness of
entanglement-based quantum information processing protocols is thus easily and
fully characterized by a single quantity.Comment: 4 pages, 5 figure
Towards fault-tolerant quantum computing with trapped ions
Today ion traps are among the most promising physical systems for
constructing a quantum device harnessing the computing power inherent in the
laws of quantum physics. The standard circuit model of quantum computing
requires a universal set of quantum logic gates for the implementation of
arbitrary quantum operations. As in classical models of computation, quantum
error correction techniques enable rectification of small imperfections in gate
operations, thus allowing for perfect computation in the presence of noise. For
fault-tolerant computation, it is commonly believed that error thresholds
ranging between 10^-4 and 10^-2 will be required depending on the noise model
and the computational overhead for realizing the quantum gates. Up to now, all
experimental implementations have fallen short of these requirements. Here, we
report on a Molmer-Sorensen type gate operation entangling ions with a fidelity
of 99.3(1)% which together with single-qubit operations forms a universal set
of quantum gates. The gate operation is performed on a pair of qubits encoded
in two trapped calcium ions using a single amplitude-modulated laser beam
interacting with both ions at the same time. A robust gate operation, mapping
separable states onto maximally entangled states is achieved by adiabatically
switching the laser-ion coupling on and off. We analyse the performance of a
single gate and concatenations of up to 21 gate operations. The gate mechanism
holds great promise not only for two-qubit but also for multi-qubit operations.Comment: submitted to Nature Physic
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