149 research outputs found
Mode-Dependent Loss Model for Multimode Photon-Subtracted States
Multimode photon-subtraction provides an experimentally feasible option to
construct large non-Gaussian quantum states in continuous-variable quantum
optics. The non-Gaussian features of the state can lead towards the more exotic
aspects of quantum theory, such as negativity of the Wigner function. However,
the pay-off for states with such delicate quantum properties is their
sensitivity to decoherence. In this paper, we present a general model that
treats the most important source of decoherence in a purely optical setting:
losses. We use the framework of open quantum systems and master equations to
describe losses in n-photon-subtracted multimode states, where each photon can
be subtracted in an arbitrary mode. As a main result, we find that
mode-dependent losses and photon-subtraction generally do not commute. In
particular, the losses do not only reduce the purity of the state, they also
change the modal structure of its non-Gaussian features. We then conduct a
detailed study of single-photon subtraction from a multimode Gaussian state,
which is a setting that lies within the reach of present-day experiments.Comment: 14 pages, 8 figure
Reversing the Weak Quantum Measurement for a Photonic Qubit
We demonstrate the conditional reversal of a weak (partial-collapse) quantum
measurement on a photonic qubit. The weak quantum measurement causes a
nonunitary transformation of a qubit which is subsequently reversed to the
original state after a successful reversing operation. Both the weak
measurement and the reversal operation are implemented linear optically. The
state recovery fidelity, determined by quantum process tomography, is shown to
be over 94% for partial-collapse strength up to 0.9. We also experimentally
study information gain due to the weak measurement and discuss the role of the
reversing operation as an information erasure
Experimental verification of the commutation relation for Pauli spin operators using single-photon quantum interference
We report experimental verification of the commutation relation for Pauli
spin operators using quantum interference of the single-photon polarization
state. By superposing the quantum operations and on a single-photon polarization state, we have experimentally
implemented the commutator, , and the anticommutator,
, and have demonstrated the relative phase factor
of between and operations. The
experimental quantum operation corresponding to the commutator, , showed process fidelity of 0.94 compared to the ideal
operation and is determined to be .Comment: 4pages, 3 figure
Generation of three-dimensional cluster entangled state
Measurement-based quantum computing is a promising paradigm of quantum
computation, where universal computing is achieved through a sequence of local
measurements. The backbone of this approach is the preparation of multipartite
entanglement, known as cluster states. While a cluster state with
two-dimensional (2D) connectivity is required for universality, a
three-dimensional (3D) cluster state is necessary for additionally achieving
fault tolerance. However, the challenge of making 3D connectivity has limited
cluster state generation up to 2D. Here we experimentally generate a 3D cluster
state in the continuous-variable optical platform. To realize 3D connectivity,
we harness a crucial advantage of time-frequency modes of ultrafast quantum
light: an arbitrary complex mode basis can be accessed directly, enabling
connectivity as desired. We demonstrate the versatility of our method by
generating cluster states with 1D, 2D, and 3D connectivities. For their
complete characterization, we develop a quantum state tomography method for
multimode Gaussian states. Moreover, we verify the cluster state generation by
nullifier measurements, as well as full inseparability and steering tests.
Finally, we highlight the usefulness of 3D cluster state by demonstrating
quantum error detection in topological quantum computation. Our work paves the
way toward fault-tolerant and universal measurement-based quantum computing
Continuous-Variable Nonclassicality Detection under Coarse-Grained Measurement
Coarse graining is a common imperfection of realistic quantum measurement,
obstructing the direct observation of quantum features. Under highly
coarse-grained measurement, we experimentally detect the continuous-variable
nonclassicality of both Gaussian and non-Gaussian states. Remarkably, we find
that this coarse-grained measurement outperforms the conventional fine-grained
measurement for nonclassicality detection: it detects nonclassicality beyond
the reach of the variance criterion, and furthermore, it exhibits stronger
statistical significance than the high-order moments method. Our work shows the
usefulness of coarse-grained measurement by providing a reliable and efficient
way of nonclassicality detection for quantum technologies
Realizing Physical Approximation of the Partial Transpose
The partial transpose by which a subsystem's quantum state is solely
transposed is of unique importance in quantum information processing from both
fundamental and practical point of view. In this work, we present a practical
scheme to realize a physical approximation to the partial transpose using local
measurements on individual quantum systems and classical communication. We then
report its linear optical realization and show that the scheme works with no
dependence on local basis of given quantum states. A proof-of-principle
demonstration of entanglement detection using the physical approximation of the
partial transpose is also reported.Comment: 5 pages with appendix, 3 figure
Experimental Implementation of the Universal Transpose Operation
The universal transpose of quantum states is an anti-unitary transformation
that is not allowed in quantum theory. In this work, we investigate
approximating the universal transpose of quantum states of two-level systems
(qubits) using the method known as the structural physical approximation to
positive maps. We also report its experimental implementation in linear optics.
The scheme is optimal in that the maximal fidelity is attained and also
practical as measurement and preparation of quantum states that are
experimentally feasible within current technologies are solely applied.Comment: 4 pages, 4 figure
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