69 research outputs found
Synthesis of linear quantum stochastic systems via quantum feedback networks
Recent theoretical and experimental investigations of coherent feedback
control, the feedback control of a quantum system with another quantum system,
has raised the important problem of how to synthesize a class of quantum
systems, called the class of linear quantum stochastic systems, from basic
quantum optical components and devices in a systematic way. The synthesis
theory sought in this case can be naturally viewed as a quantum analogue of
linear electrical network synthesis theory and as such has potential for
applications beyond the realization of coherent feedback controllers. In
earlier work, Nurdin, James and Doherty have established that an arbitrary
linear quantum stochastic system can be realized as a cascade connection of
simpler one degree of freedom quantum harmonic oscillators, together with a
direct interaction Hamiltonian which is bilinear in the canonical operators of
the oscillators. However, from an experimental perspective and based on current
methods and technologies, direct interaction Hamiltonians are challenging to
implement for systems with more than just a few degrees of freedom. In order to
facilitate more tractable physical realizations of these systems, this paper
develops a new synthesis algorithm for linear quantum stochastic systems that
relies solely on field-mediated interactions, including in implementation of
the direct interaction Hamiltonian. Explicit synthesis examples are provided to
illustrate the realization of two degrees of freedom linear quantum stochastic
systems using the new algorithm.Comment: 21 pages, 6 figure
On the infeasibility of entanglement generation in Gaussian quantum systems via classical control
This paper uses a system theoretic approach to show that classical linear
time invariant controllers cannot generate steady state entanglement in a
bipartite Gaussian quantum system which is initialized in a Gaussian state. The
paper also shows that the use of classical linear controllers cannot generate
entanglement in a finite time from a bipartite system initialized in a
separable Gaussian state. The approach reveals connections between system
theoretic concepts and the well known physical principle that local operations
and classical communications cannot generate entangled states starting from
separable states.Comment: 6 pages, 3 figures. To appear in IEEE Transactions on Automatic
Control, 201
Network Synthesis of Linear Dynamical Quantum Stochastic Systems
The purpose of this paper is to develop a synthesis theory for linear
dynamical quantum stochastic systems that are encountered in linear quantum
optics and in phenomenological models of linear quantum circuits. In
particular, such a theory will enable the systematic realization of
coherent/fully quantum linear stochastic controllers for quantum control,
amongst other potential applications. We show how general linear dynamical
quantum stochastic systems can be constructed by assembling an appropriate
interconnection of one degree of freedom open quantum harmonic oscillators and,
in the quantum optics setting, discuss how such a network of oscillators can be
approximately synthesized or implemented in a systematic way from some linear
and non-linear quantum optical elements. An example is also provided to
illustrate the theory.Comment: Revised and corrected version, published in SIAM Journal on Control
and Optimization, 200
Modular Quantum Memories Using Passive Linear Optics and Coherent Feedback
In this paper, we show that quantum memory for qudit states encoded in a single photon pulsed optical field has a conceptually simple modular realization using only passive linear optics and coherent feedback. We exploit the idea that two decaying optical cavities can be coupled in a coherent feedback configuration to create an internal mode of the coupled system which is isolated and decoherence-free for the purpose of qubit storage. The qubit memory can then be switched between writing/read-out mode and storage mode simply by varying the routing of certain freely propagating optical fields in the network. It is then shown that the qubit memories can be interconnected with one another to form a qudit quantum memory. We explain each of the phase of writing, storage, and read-out for this modular quantum memory scheme. The results point a way towards modular architectures for complex compound quantum memories
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