122 research outputs found
Simple preparation of Bell and GHZ states using ultrastrong-coupling circuit QED
The ability to entangle quantum systems is crucial for many applications in
quantum technology, including quantum communication and quantum computing.
Here, we propose a new, simple, and versatile setup for deterministically
creating Bell and Greenberger-Horne-Zeilinger (GHZ) states between photons of
different frequencies in a two-step protocol. The setup consists of a quantum
bit (qubit) coupled ultrastrongly to three photonic resonator modes. The only
operations needed in our protocol are to put the qubit in a superposition
state, and then tune its frequency in and out of resonance with sums of the
resonator-mode frequencies. By choosing which frequency we tune the qubit to,
we select which entangled state we create. We show that our protocol can be
implemented with high fidelity using feasible experimental parameters in
state-of-the-art circuit quantum electrodynamics. One possible application of
our setup is as a node distributing entanglement in a quantum network.Comment: 15 pages, 7 figure
Efficient creation of multipartite entanglement in flux qubits
We investigate three superconducting flux qubits coupled in a loop. In this
setup, tripartite entanglement can be created in a natural, controllable, and
stable way. Both generic kinds of tripartite entanglement -the W type as well
as the GHZ type entanglement- can be identified among the eigenstates. We also
discuss the violation of Bell inequalities in this system and show the impact
of a limited measurement fidelity on the detection of entanglement and quantum
nonlocality.Comment: 15 pages, 7 figures; extended sections on coupling strength, system
preparation, and entanglement detectio
Generation of Three-Qubit Entangled States using Superconducting Phase Qubits
Entanglement is one of the key resources required for quantum computation, so
experimentally creating and measuring entangled states is of crucial importance
in the various physical implementations of a quantum computer. In
superconducting qubits, two-qubit entangled states have been demonstrated and
used to show violations of Bell's Inequality and to implement simple quantum
algorithms. Unlike the two-qubit case, however, where all maximally-entangled
two-qubit states are equivalent up to local changes of basis, three qubits can
be entangled in two fundamentally different ways, typified by the states
and . Here we demonstrate the operation of three coupled
superconducting phase qubits and use them to create and measure
and states. The states are fully characterized
using quantum state tomography and are shown to satisfy entanglement witnesses,
confirming that they are indeed examples of three-qubit entanglement and are
not separable into mixtures of two-qubit entanglement.Comment: 9 pages, 5 figures. Version 2: added supplementary information and
fixed image distortion in Figure 2
Generating Bell states and -partite states of long-distance qubits in superconducting waveguide QED
We show how to generate Bell states and -partite states of
long-distance superconducting (SC) qubits in a SC waveguide quantum
electrodynamical (QED) system, where SC qubits are coupled to an open microwave
transmission line. In the two-qubit case, the Bell state of two long-distance
qubits can be a dark state of the system by choosing appropriate system
parameters. If one proper microwave pulse drives one of two qubits, the two
qubits will evolve from their ground states to a Bell state. Further, we extend
this scheme to the multi-qubit case. We show that states of
long-distance qubits can also be generated. Because both the Bell and
states are decoupled from the waveguide (i.e., dark states of the system), they
are steady and have very long lifetimes in the ideal case without decoherence
of qubits. In contrast to the ideal case, the presence of decoherence of qubits
limits the lifetimes of the Bell and states. Our study provides a novel
scheme for generating Bell states and -partite states in SC waveguide
QED, which can be used to entangle long-distance nodes in waveguide quantum
networks.Comment: 12 pages, 9 figure
Bare-excitation ground state of a spinless-fermion -- boson model and W-state engineering in an array of superconducting qubits and resonators
This work unravels an interesting property of a one-dimensional lattice model
that describes a single itinerant spinless fermion (excitation) coupled to
zero-dimensional (dispersionless) bosons through two different
nonlocal-coupling mechanisms. Namely, below a critical value of the effective
excitation-boson coupling strength the exact ground state of this model is the
zero-quasimomentum Bloch state of a bare (i.e., completely undressed)
excitation. It is demonstrated here how this last property of the lattice model
under consideration can be exploited for a fast, deterministic preparation of
multipartite states in a readily realizable system of inductively-coupled
superconducting qubits and microwave resonators.Comment: final, published versio
Generating entanglement between microwave photons and qubits in multiple cavities coupled by a superconducting qutrit
We discuss how to generate entangled coherent states of four
\textrm{microwave} resonators \textrm{(a.k.a. cavities)} coupled by a
superconducting qubit. We also show \textrm{that} a GHZ state of four
superconducting qubits embedded in four different resonators \textrm{can be
created with this scheme}. In principle, \textrm{the proposed method} can be
extended to create an entangled coherent state of resonators and to prepare
a Greenberger-Horne-Zeilinger (GHZ) state of qubits distributed over
cavities in a quantum network. In addition, it is noted that four resonators
coupled by a coupler qubit may be used as a basic circuit block to build a
two-dimensional quantum network, which is useful for scalable quantum
information processing.Comment: 13 pages, 7 figure
Generation of entanglement in systems of intercoupled qubits
We consider systems of two and three qubits, mutually coupled by
Heisenberg-type exchange interaction and interacting with external laser
fields. We show that these systems allow one to create maximally entangled Bell
states, as well as three qubit Greenberger-Horne-Zeilinger and W states. In
particular, we point out that some of the target states are the eigenstates of
the initial bare system. Due to this, one can create entangled states by means
of pulse area and adiabatic techniques, when starting from a separable
(non-entangled) ground state. On the other hand, for target states, not present
initially in the eigensystem of the model, we apply the robust stimulated Raman
adiabatic passage and pulse techniques, that create desired coherent
superpositions of non-entangled eigenstates.Comment: 9 pages, 7 figures. Updated version for publicatio
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