55 research outputs found
Nearly Deterministic Bell Measurement for Multiphoton Qubits and Its Application to Quantum Information Processing
We propose a Bell measurement scheme by employing a logical qubit in
Greenberger-Horne-Zeilinger (GHZ) entanglement with an arbitrary number of
photons. Remarkably, the success probability of the Bell measurement as well as
teleportation of the GHZ entanglement can be made arbitrarily high using only
linear optics elements and photon on-off measurements as the number of photons
increases. Our scheme outperforms previous proposals using single photon qubits
when comparing the success probabilities in terms of the average photon usages.
It has another important advantage for experimental feasibility that it does
not require photon number resolving measurements. Our proposal provides an
alternative candidate for all-optical quantum information processing.Comment: 7 pages (including supplementary material), 2 figures, to be
published in Phys. Rev. Let
Entangled coherent states versus entangled photon pairs for practical quantum information processing
We compare effects of decoherence and detection inefficiency on entangled
coherent states (ECSs) and entangled photon pairs (EPPs), both of which are
known to be particularly useful for quantum information processing (QIP). When
decoherence effects caused by photon losses are heavy, the ECSs outperform the
EPPs as quantum channels for teleportation both in fidelities and in success
probabilities. On the other hand, when inefficient detectors are used, the
teleportation scheme using the ECSs suffers undetected errors that result in
the degradation of fidelity, while this is not the case for the teleportation
scheme using the EPPs. Our study reveals the merits and demerits of the two
types of entangled states in realizing practical QIP under realistic
conditions.Comment: 9 pages, 6 figures, substantially revised version, to be published in
Phys. Rev.
Controllability-Aware Unsupervised Skill Discovery
One of the key capabilities of intelligent agents is the ability to discover
useful skills without external supervision. However, the current unsupervised
skill discovery methods are often limited to acquiring simple, easy-to-learn
skills due to the lack of incentives to discover more complex, challenging
behaviors. We introduce a novel unsupervised skill discovery method,
Controllability-aware Skill Discovery (CSD), which actively seeks complex,
hard-to-control skills without supervision. The key component of CSD is a
controllability-aware distance function, which assigns larger values to state
transitions that are harder to achieve with the current skills. Combined with
distance-maximizing skill discovery, CSD progressively learns more challenging
skills over the course of training as our jointly trained distance function
reduces rewards for easy-to-achieve skills. Our experimental results in six
robotic manipulation and locomotion environments demonstrate that CSD can
discover diverse complex skills including object manipulation and locomotion
skills with no supervision, significantly outperforming prior unsupervised
skill discovery methods. Videos and code are available at
https://seohong.me/projects/csd/Comment: ICML 202
Optical estimation of unitary Gaussian processes without phase reference using Fock states
Since a general Gaussian process is phase-sensitive, a stable phase reference
is required to take advantage of this feature. When the reference is missing,
either due to the volatile nature of the measured sample or the measurement's
technical limitations, the resulting process appears as random in phase. Under
this condition, we consider two single-mode Gaussian processes, displacement
and squeezing. We show that these two can be efficiently estimated using photon
number states and photon number resolving detectors. For separate estimation of
displacement and squeezing, the practical estimation errors for hundreds of
probes' ensembles can saturate the Cram\'{e}r-Rao bound even for arbitrary
small values of the estimated parameters and under realistic losses. The
estimation of displacement with Fock states always outperforms estimation using
Gaussian states with equivalent energy and optimal measurement. For estimation
of squeezing, Fock states outperform Gaussian methods, but only when their
energy is large enough. Finally, we show that Fock states can also be used to
estimate the displacement and the squeezing simultaneously.Comment: 16 pages, 8 figure
Quantum Rabi interferometry of motion and radiation
The precise determination of a displacement of a mechanical oscillator or a
microwave field in a predetermined direction in phase space can be carried out
with trapped ions or superconducting circuits, respectively, by coupling the
oscillator with ancilla qubits. Through that coupling, the displacement
information is transferred to the qubits which are then subsequently read out.
However, unambiguous estimation of displacement in an unknown direction in the
phase space has not been attempted in such oscillator-qubit systems. Here, we
propose a hybrid oscillator-qubit interferometric setup for the unambiguous
estimation of phase space displacements in an arbitrary direction, based on
feasible Rabi interactions beyond the rotating-wave approximation. Using such a
hybrid Rabi interferometer for quantum sensing, we show that the performance is
superior to the ones attained by single-mode estimation schemes and a
conventional interferometer based on Jaynes-Cummings interactions. Moreover, we
find that the sensitivity of the Rabi interferometer is independent of the
thermal occupation of the oscillator mode, and thus cooling it to the ground
state before sensing is not required. We also perform a thorough investigation
of the effect of qubit dephasing and oscillator thermalization. We find the
interferometer to be fairly robust, outperforming different benchmark
estimation schemes even for large dephasing and thermalization
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