18,034 research outputs found
Finite quantum tomography via semidefinite programming
Using the the convex semidefinite programming method and superoperator
formalism we obtain the finite quantum tomography of some mixed quantum states
such as: qudit tomography, N-qubit tomography, phase tomography and coherent
spin state tomography, where that obtained results are in agreement with those
of References \cite{schack,Pegg,Barnett,Buzek,Weigert}.Comment: 25 page
Quantum Tomography twenty years later
A sample of some relevant developments that have taken place during the last
twenty years in classical and quantum tomography are displayed. We will present
a general conceptual framework that provides a simple unifying mathematical
picture for all of them and, as an effective use of it, three subjects have
been chosen that offer a wide panorama of the scope of classical and quantum
tomography: tomography along lines and submanifolds, coherent state tomography
and tomography in the abstract algebraic setting of quantum systems
Quantum process tomography with coherent states
We develop an enhanced technique for characterizing quantum optical processes
based on probing unknown quantum processes only with coherent states. Our
method substantially improves the original proposal [M. Lobino et al., Science
322, 563 (2008)], which uses a filtered Glauber-Sudarshan decomposition to
determine the effect of the process on an arbitrary state. We introduce a new
relation between the action of a general quantum process on coherent state
inputs and its action on an arbitrary quantum state. This relation eliminates
the need to invoke the Glauber-Sudarshan representation for states; hence it
dramatically simplifies the task of process identification and removes a
potential source of error. The new relation also enables straightforward
extensions of the method to multi-mode and non-trace-preserving processes. We
illustrate our formalism with several examples, in which we derive analytic
representations of several fundamental quantum optical processes in the Fock
basis. In particular, we introduce photon-number cutoff as a reasonable
physical resource limitation and address resource vs accuracy trade-off in
practical applications. We show that the accuracy of process estimation scales
inversely with the square root of photon-number cutoff.Comment: 18 pages, 2 figure
Quantum optomechanics beyond the quantum coherent oscillation regime
Interaction with a thermal environment decoheres the quantum state of a
mechanical oscillator. When the interaction is sufficiently strong, such that
more than one thermal phonon is introduced within a period of oscillation,
quantum coherent oscillations are prevented. This is generally thought to
preclude a wide range of quantum protocols. Here, we introduce a pulsed
optomechanical protocol that allows ground state cooling, general linear
quantum non-demolition measurements, optomechanical state swaps, and quantum
state preparation and tomography without requiring quantum coherent
oscillations. Finally we show how the protocol can break the usual thermal
limit for sensing of impulse forces.Comment: 6 pages, 3 figure
Two-photon quantum walks in an elliptical direct-write waveguide array
Integrated optics provides an ideal test bed for the emulation of quantum
systems via continuous-time quantum walks. Here we study the evolution of
two-photon states in an elliptic array of waveguides. We characterise the
photonic chip via coherent-light tomography and use the results to predict
distinct differences between temporally indistinguishable and distinguishable
two-photon inputs which we then compare with experimental observations. Our
work highlights the feasibility for emulation of coherent quantum phenomena in
three-dimensional waveguide structures.Comment: 8 pages, 7 figure
Maximum-likelihood coherent-state quantum process tomography
Coherent-state quantum process tomography (csQPT) is a method of completely
characterizing a quantum-optical "black box" by probing it with coherent states
and performing homodyne measurements on the output [M. Lobino et al, Science
322, 563 (2008)]. We present a technique for csQPT that is fully based on
statistical inference, specifically, quantum expectation-maximization. The
method relies on the Jamiolkowski isomorphism and iteratively reconstructs the
process tensor in the Fock basis directly from the experimental data. This
approach permits incorporation of a priori constraints into the reconstruction
procedure, thereby guaranteeing that the resulting process tensor is physically
consistent. Furthermore, our method is easier to implement and requires a
narrower range of coherent states than its predecessors. We test its
feasibility using simulations on several experimentally relevant processes.Comment: 17 pages, 4 figure
On-chip quantum tomography of mechanical nano-scale oscillators with guided Rydberg atoms
Nano-mechanical oscillators as well as Rydberg-atomic waveguides hosted on
micro-fabricated chip surfaces hold promise to become pillars of future quantum
technologies. In a hybrid platform with both, we show that beams of Rydberg
atoms in waveguides can quantum-coherently interrogate and manipulate
nanomechanical elements, allowing full quantum state tomography. Central to the
tomography are quantum non-demolition measurements using the Rydberg atoms as
probes. Quantum coherent displacement of the oscillator is also made possible,
by driving the atoms with external fields while they interact with the
oscillator. We numerically demonstrate the feasibility of this fully integrated
on-chip control and read-out suite for quantum nano-mechanics, taking into
account noise and error sources.Comment: 11 pages, 5 figures, 1 tabl
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