56 research outputs found
Ignorance is bliss: General and robust cancellation of decoherence via no-knowledge quantum feedback
A "no-knowledge" measurement of an open quantum system yields no information
about any system observable; it only returns noise input from the environment.
Surprisingly, performing such a no-knowledge measurement can be advantageous.
We prove that a system undergoing no-knowledge monitoring has reversible noise,
which can be cancelled by directly feeding back the measurement signal. We show
how no-knowledge feedback control can be used to cancel decoherence in an
arbitrary quantum system coupled to a Markovian reservoir that is being
monitored. Since no-knowledge feedback does not depend on the system state or
Hamiltonian, such decoherence cancellation is guaranteed to be general, robust
and can operate in conjunction with any other quantum control protocol. As an
application, we show that no-knowledge feedback could be used to improve the
performance of dissipative quantum computers subjected to local loss.Comment: 6 pages + 2 pages supplemental material, 3 figure
Robustness of System-Filter Separation for the Feedback Control of a Quantum Harmonic Oscillator Undergoing Continuous Position Measurement
We consider the effects of experimental imperfections on the problem of
estimation-based feedback control of a trapped particle under continuous
position measurement. These limitations violate the assumption that the
estimator (i.e. filter) accurately models the underlying system, thus requiring
a separate analysis of the system and filter dynamics. We quantify the
parameter regimes for stable cooling, and show that the control scheme is
robust to detector inefficiency, time delay, technical noise, and miscalibrated
parameters. We apply these results to the specific context of a weakly
interacting Bose-Einstein condensate (BEC). Given that this system has
previously been shown to be less stable than a feedback-cooled BEC with strong
interatomic interactions, this result shows that reasonable experimental
imperfections do not limit the feasibility of cooling a BEC by continuous
measurement and feedback.Comment: 14 pages, 8 figure
Ultranarrow Optical Inhomogeneous Linewidth in a Stoichiometric Rare-Earth Crystal
We obtain a low optical inhomogeneous linewidth of 25 MHz in the stoichiometric rare-earth crystal EuCl3·6H2O by isotopically purifying the crystal in Cl35. With this linewidth, an important limit for stoichiometric rare-earth crystals is surpassed: the hyperfine structure of Eu153 is spectrally resolved, allowing the whole population of Eu1533+ ions to be prepared in the same hyperfine state using hole-burning techniques. This material also has a very high optical density, and can have long coherence times when deuterated. This combination of properties offers new prospects for quantum information applications. We consider two of these: quantum memories and quantum many-body studies. We detail the improvements in the performance of current memory protocols possible in these high optical depth crystals, and describe how certain memory protocols, such as off-resonant Raman memories, can be implemented for the first time in a solid-state system. We explain how the strong excitation-induced interactions observed in this material resemble those seen in Rydberg systems, and describe how these interactions can lead to quantum many-body states that could be observed using standard optical spectroscopy techniques
Single photon production by rephased amplified spontaneous emission
The production of single photons using rephased amplified spontaneous
emission is examined. This process produces single photons on demand with high
efficiency by detecting the spontaneous emission from an atomic ensemble, then
applying a population-inverting pulse to rephase the ensemble and produce a
photon echo of the spontaneous emission events. The theoretical limits on the
efficiency of the production are determined for several variants of the scheme.
For an ensemble of uniform optical density, generating the initial spontaneous
emission and its echo using transitions of different strengths is shown to
produce single photons at 70% efficiency, limited by reabsorption. Tailoring
the spatial and spectral density of the atomic ensemble is then shown to
prevent reabsorption of the rephased photon, resulting in emission efficiency
near unity
Approaching the adiabatic timescale with machine-learning
The control and manipulation of quantum systems without excitation is
challenging, due to the complexities in fully modeling such systems accurately
and the difficulties in controlling these inherently fragile systems
experimentally. For example, while protocols to decompress Bose-Einstein
condensates (BEC) faster than the adiabatic timescale (without excitation or
loss) have been well developed theoretically, experimental implementations of
these protocols have yet to reach speeds faster than the adiabatic timescale.
In this work, we experimentally demonstrate an alternative approach based on a
machine learning algorithm which makes progress towards this goal. The
algorithm is given control of the coupled decompression and transport of a
metastable helium condensate, with its performance determined after each
experimental iteration by measuring the excitations of the resultant BEC. After
each iteration the algorithm adjusts its internal model of the system to create
an improved control output for the next iteration. Given sufficient control
over the decompression, the algorithm converges to a novel solution that sets
the current speed record in relation to the adiabatic timescale, beating out
other experimental realizations based on theoretical approaches. This method
presents a feasible approach for implementing fast state preparations or
transformations in other quantum systems, without requiring a solution to a
theoretical model of the system. Implications for fundamental physics and
cooling are discussed.Comment: 7 pages main text, 2 pages supporting informatio
Gold surfaces and nanoparticles are protected by Au(0)-thiyl species and are destroyed when Au(I)-thiolates form
The synthetic chemistry and spectroscopy of sulfur-protected gold surfaces and nanoparticles is analyzed, indicating that the electronic structure of the interface is Au(0)–thiyl, with Au(I)–thiolates identified as high-energy excited surface states. Density-functional theory indicates that it is the noble character of gold and nanoparticle surfaces that destabilizes Au(I)–thiolates. Bonding results from large van der Waals forces, influenced by covalent bonding induced through s–d hybridization and charge polarization effects that perturbatively mix in some Au(I)–thiolate character. A simple method for quantifying these contributions is presented, revealing that a driving force for nanoparticle growth is nobleization, minimizing Au(I)–thiolate involvement. Predictions that Brust–Schiffrin reactions involve thiolate anion intermediates are verified spectroscopically, establishing a key feature needed to understand nanoparticle growth. Mixing of preprepared Au(I) and thiolate reactants always produces Au(I)–thiolate thin films or compounds rather than monolayers. Smooth links to O, Se, Te, C, and N linker chemistry are established
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