755 research outputs found
Deterministic quantum teleportation of photonic quantum bits by a hybrid technique
Quantum teleportation allows for the transfer of arbitrary, in principle,
unknown quantum states from a sender to a spatially distant receiver, who share
an entangled state and can communicate classically. It is the essence of many
sophisticated protocols for quantum communication and computation. In order to
realize flying qubits in these schemes, photons are an optimal choice, however,
teleporting a photonic qubit has been limited due to experimental
inefficiencies and restrictions. Major disadvantages have been the
fundamentally probabilistic nature of linear-optics Bell measurements as well
as the need for either destroying the teleported qubit or attenuating the input
qubit when the detectors do not resolve photon numbers. Here we experimentally
realize fully deterministic, unconditional quantum teleportation of photonic
qubits. The key element is to make use of a "hybrid" technique:
continuous-variable (CV) teleportation of a discrete-variable, photonic qubit.
By optimally tuning the receiver's feedforward gain, the CV teleporter acts as
a pure loss channel, while the input dual-rail encoded qubit, based on a single
photon, represents a quantum error detection code against amplitude damping and
hence remains completely intact for most teleportation events. This allows for
a faithful qubit transfer even with imperfect CV entangled states: the overall
transfer fidelities range from 0.79 to 0.82 for four distinct qubits, all of
them exceeding the classical limit of teleportation. Furthermore, even for a
relatively low level of the entanglement, qubits are teleported much more
efficiently than in previous experiments, albeit post-selectively (taking into
account only the qubit subspaces), with a fidelity comparable to the previously
reported values
Complete temporal mode characterization of non-Gaussian states by dual homodyne measurement
Optical quantum states defined in temporal modes, especially non-Gaussian
states like photon-number states, play an important role in quantum computing
schemes. In general, the temporal-mode structures of these states are
characterized by one or more complex functions called temporal-mode functions
(TMFs). Although we can calculate TMF theoretically in some cases, experimental
estimation of TMF is more advantageous to utilize the states with high purity.
In this paper, we propose a method to estimate complex TMFs. This method can be
applied not only to arbitrary single-temporal-mode non-Gaussian states but also
to two-temporal-mode states containing two photons. This method is implemented
by continuous-wave (CW) dual homodyne measurement and doesn't need prior
information of the target states nor state reconstruction procedure. We
demonstrate this method by analyzing several experimentally created
non-Gaussian states
Scalar polarization window in gravitational-wave signals
Scalar polarization modes of gravitational waves, which are often introduced
in the context of the viable extension of gravity, have been actively searched.
However, couplings of the scalar modes to the matter are strongly constrained
by the fifth-force experiments. Thus, the amplitude of scalar polarization in
the observed gravitational-wave signal must be significantly suppressed
compared to that of the tensor modes. Here, we discuss the implications of the
experiments in the solar system on the detectability of scalar modes in
gravitational waves from compact binary coalescences, taking into account the
whole processes from the generation to the observation of gravitational waves.
We first claim that the energy carried by the scalar modes at the generation
is, at most, that of the tensor modes from the observed phase evolution of the
inspiral gravitational waves. Next, we formulate general gravitational-wave
propagation and point out that the energy flux hardly changes through
propagation as long as the background changes slowly compared to the wavelength
of the propagating waves. Finally, we show that the possible magnitude of
scalar polarization modes detected by the ground-based gravitational-wave
telescopes is already severely constrained by the existing gravity tests in the
solar system.Comment: 18 page
Schr\"odinger's cat in an optical sideband
We propose a method to subtract a photon from a double sideband mode of
continuous-wave light. The central idea is to use phase modulation as a
frequency sideband beamsplitter in the heralding photon subtraction scheme,
where a small portion of the sideband mode is downconverted to the carrier
frequency to provide a trigger photon. An optical Schr\"odinger's cat state is
created by applying the propesed method to a squeezed state at 500MHz sideband,
which is generated by an optical parametric oscillator. The Wigner function of
the cat state reconstructed from a direct homodyne measurement of the 500MHz
sideband modes shows the negativity of without any
loss corrections.Comment: 11 pages, 9 figure
Quantum mode filtering of non-Gaussian states for teleportation-based quantum information processing
We propose and demonstrate an effective mode-filtering technique of
non-Gaussian states generated by photon-subtraction. More robust non-Gaussian
states have been obtained by removing noisy low frequencies from the original
mode spectrum. We show that non-Gaussian states preserve their non-classicality
after quantum teleportation to a higher degree, when they have been
mode-filtered. This is indicated by a stronger negativity of
the Wigner function at the origin, compared to for states
that have not been mode-filtered. This technique can be straightforwardly
applied to various kinds of photon-subtraction protocols, and can be a key
ingredient in a variety of applications of non-Gaussian states, especially
teleportation-based protocols towards universal quantum information processing
Superlattice formation lifting degeneracy protected by non-symmorphic symmetry through a metal-insulator transition in RuAs
The single crystal of RuAs obtained by Bi-flux method shows obvious
successive metal-insulator transitions at T_MI1~255 K and T_MI2~195$ K. The
X-ray diffraction measurement reveals a formation of superlattice of 3x3x3 of
the original unit cell below T_MI2, accompanied by a change of the crystal
system from the orthorhombic structure to the monoclinic one. Simple
dimerization of the Ru ions is nor seen in the ground state. The multiple As
sites observed in nuclear quadrupole resonance (NQR) spectrum also demonstrate
the formation of the superlattice in the ground state, which is clarified to be
nonmagnetic. The divergence in 1/T_1 at T_MI1 shows that a symmetry lowering by
the metal-insulator transition is accompanied by strong critical fluctuations
of some degrees of freedom. Using the structural parameters in the insulating
state, the first principle calculation reproduces successfully the reasonable
size of nuclear quadrupole frequencies for the multiple As sites, ensuring the
high validity of the structural parameters. The calculation also gives a
remarkable suppression in the density of states (DOS) near the Fermi level,
although the gap opening is insufficient. A coupled modulation of the
calculated Ru d electron numbers and the crystal structure proposes a formation
of charge density wave (CDW) in RuAs. Some lacking factors remain, but it shows
that a lifting of degeneracy protected by the non-symmorphic symmetry through
the superlattice formation is a key ingredient for the metal-insulator
transition in RuAs.Comment: 10 pages, 10 figure
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