17 research outputs found
Optomechanical state reconstruction and nonclassicality verification beyond the resolved-sideband regime
Quantum optomechanics uses optical means to generate and manipulate quantum
states of motion of mechanical resonators. This provides an intriguing platform
for the study of fundamental physics and the development of novel quantum
devices. Yet, the challenge of reconstructing and verifying the quantum state
of mechanical systems has remained a major roadblock in the field. Here, we
present a novel approach that allows for tomographic reconstruction of the
quantum state of a mechanical system without the need for extremely high
quality optical cavities. We show that, without relying on the usual state
transfer presumption between light an mechanics, the full optomechanical
Hamiltonian can be exploited to imprint mechanical tomograms on a strong
optical coherent pulse, which can then be read out using well-established
techniques. Furthermore, with only a small number of measurements, our method
can be used to witness nonclassical features of mechanical systems without
requiring full tomography. By relaxing the experimental requirements, our
technique thus opens a feasible route towards verifying the quantum state of
mechanical resonators and their nonclassical behaviour in a wide range of
optomechanical systems.Comment: 12 pages + 9 pages of appendices, 4 figure
Operational Gaussian Schmidt-Number Witnesses
The general class of Gaussian Schmidt-number witness operators for bipartite
systems is studied. It is shown that any member of this class is reducible to a
convex combination of two types of Gaussian operators using local operations
and classical communications. This gives rise to a simple operational method,
which is solely based on measurable covariance matrices of quantum states. Our
method bridges the gap between theory and experiment of entanglement
quantification. In particular, we certify lower bounds of the Schmidt number of
squeezed thermal and phase-randomized squeezed vacuum states, as examples of
Gaussian and non-Gaussian quantum states, respectively.Comment: 9 pages, 4 figure
Quantum Correlations in Nonlocal BosonSampling
Determination of the quantum nature of correlations between two spatially
separated systems plays a crucial role in quantum information science. Of
particular interest is the questions of if and how these correlations enable
quantum information protocols to be more powerful. Here, we report on a
distributed quantum computation protocol in which the input and output quantum
states are considered to be classically correlated in quantum informatics.
Nevertheless, we show that the correlations between the outcomes of the
measurements on the output state cannot be efficiently simulated using
classical algorithms. Crucially, at the same time, local measurement outcomes
can be efficiently simulated on classical computers. We show that the only
known classicality criterion violated by the input and output states in our
protocol is the one used in quantum optics, namely, phase-space
nonclassicality. As a result, we argue that the global phase-space
nonclassicality inherent within the output state of our protocol represents
true quantum correlations.Comment: 5 pages, 1 figure, comments are very welcome
Quantum Correlations and Global Coherence in Distributed Quantum Computing
Deviations from classical physics when distant quantum systems become
correlated are interesting both fundamentally and operationally. There exist
situations where the correlations enable collaborative tasks that are
impossible within the classical formalism. Here, we consider the efficiency of
quantum computation protocols compared to classical ones as a benchmark for
separating quantum and classical resources and argue that the computational
advantage of collaborative quantum protocols in the discrete variable domain
implies the nonclassicality of correlations. By analysing a toy model, it turns
out that this argument implies the existence of quantum correlations distinct
from entanglement and discord. We characterize such quantum correlations in
terms of the net global coherence resources inherent within quantum states and
show that entanglement and discord can be understood as special cases of our
general framework. Finally, we provide an operational interpretation of such
correlations as those allowing two distant parties to increase their respective
local quantum computational resources only using locally incoherent operations
and classical communication.Comment: Minor modifications and correction
Measurement-Device-Independent Approach to Entanglement Measures
Within the context of semiquantum nonlocal games, the trust can be removed
from the measurement devices in an entanglement-detection procedure. Here we
show that a similar approach can be taken to quantify the amount of
entanglement. To be specific, first, we show that in this context a small
subset of semiquantum nonlocal games is necessary and sufficient for
entanglement detection in the LOCC paradigm. Second, we prove that the maximum
pay-off for these games is a universal measure of entanglement which is convex
and continuous. Third, we show that for the quantification of
negative-partial-transpose entanglement, this subset can be further reduced
down to a single arbitrary element. Importantly, our measure is operationally
accessible in a measurement-device-independent way by construction. Finally,
our approach is simply extended to quantify the entanglement within any
partitioning of multipartite quantum states.Comment: 8 pages, 2 figures, the main result is split into two theorems with
slight modifications, extended proof
Witnessing entanglement in trapped-ion quantum error correction under realistic noise
Quantum Error Correction (QEC) exploits redundancy by encoding logical
information into multiple physical qubits. In current implementations of QEC,
sequences of non-perfect two-qubit entangling gates are used to codify the
information redundantly into multipartite entangled states. Also, to extract
the error syndrome, a series of two-qubit gates are used to build parity-check
readout circuits. In the case of noisy gates, both steps cannot be performed
perfectly, and an error model needs to be provided to assess the performance of
QEC. We present a detailed microscopic error model to estimate the average gate
infidelity of two-qubit light-shift gates used in trapped-ion platforms. We
analytically derive leading-error contributions in terms of microscopic
parameters and present effective error models that connect the error rates
typically used in phenomenological accounts to the microscopic gate
infidelities hereby derived. We then apply this realistic error model to
quantify the multipartite entanglement generated by circuits that act as QEC
building blocks. We do so by using entanglement witnesses, complementing in
this way the recent studies by exploring the effects of a more realistic
microscopic noise.Comment: 24 pages, 10 figures, 7 table
Efficient and robust certification of genuine multipartite entanglement in noisy quantum error correction circuits
Ensuring the correct functioning of quantum error correction (QEC) circuits
is crucial to achieve fault tolerance in realistic quantum processors subjected
to noise. The first checkpoint for a fully operational QEC circuit is to create
genuine multipartite entanglement across all subsystems of physical qubits. We
introduce a conditional witnessing technique to certify genuine multipartite
entanglement (GME) that is efficient in the number of subsystems and,
importantly, robust against experimental noise and imperfections. Specifically,
we prove that the detection of entanglement in a linear number of bipartitions
by a number of measurements that also scales linearly, suffices to certify GME.
Moreover, our method goes beyond the standard procedure of separating the state
from the convex hull of biseparable states, yielding an improved finesse and
robustness compared to previous techniques. We apply our method to the noisy
readout of stabilizer operators of the distance-three topological color code
and its flag-based fault-tolerant version. In particular, we subject the
circuits to combinations of three types of noise, namely, uniform depolarizing
noise, two-qubit gate depolarizing noise, and bit-flip measurement noise. We
numerically compare our method with the standard, yet generally inefficient,
fidelity test and to a pair of efficient witnesses, verifying the increased
robustness of our method. Last but not least, we provide the full translation
of our analysis to a trapped-ion native gate set that makes it suitable for
experimental applications.Comment: 24 pages including appendices, 19 Figures and 1 Tabl