1,721,024 research outputs found
Time-energy uncertainty relation for quantum events
No full textTextbook quantum mechanics treats time as a classical parameter and not as a quantum observable with an associated Hermitian operator. For this reason, to make sense of usual time-energy uncertainty relations such as ΔtΔE≳ħ, the term Δt must be interpreted as a time interval and not as a time measurement uncertainty due to quantum noise. However, quantum clocks allow for a measurement of the "time at which an event happens"by conditioning the system's evolution on an additional quantum degree of freedom. Within this framework we derive here two uncertainty relations that relate the uncertainty in the quantum measurement of the time at which a quantum event happens on a system to its energy uncertainty
Quantum Radar
Full text linkWe propose a quantum metrology protocol for the localization of a noncooperative pointlike target in three-dimensional space, by illuminating it with electromagnetic waves. It employs all the spatial degrees of freedom of N entangled photons to achieve an uncertainty in localization that is N times smaller for each spatial direction than what could be achieved by N-independent photons or by classical light of the same average intensity
Quantum Measurements of Time
Full text linkWe propose a time-of-arrival operator in quantum mechanics by conditioning on a quantum clock. This allows us to bypass some of the problems of previous proposals, and to obtain a Hermitian time of arrival operator whose probability distribution arises from the Born rule and which has a clear physical interpretation. The same procedure can be employed to measure the "time at which some event happens" for arbitrary events (and not just specifically for the arrival time of a particle)
Squeezing metrology: A unified framework
No full textQuantum metrology theory has up to now focused on the resolution gains obtainable thanks to the entanglement among N probes. Typically, a quadratic gain in resolution is achievable, going from the 1/√N of the central limit theorem to the 1/N of the Heisenberg bound. Here we focus instead on quantum squeezing and provide a unified framework for metrology with squeezing, showing that, similarly, one can generally attain a quadratic gain when comparing the resolution achievable by a squeezed probe to the best N-probe classical strategy achievable with the same energy. Namely, here we give a quantification of the Heisenberg squeezing bound for arbitrary estimation strategies that employ squeezing. Our theory recovers known results (e.g. in quantum optics and spin squeezing), but it uses the general theory of squeezing and holds for arbitrary quantum systems
Method for Ensuring privacy while querying a database by using quantum superpositions and multiple responses
No full textIn a database query operation, a quantum private query (QPQ) protocol allows a user to determine whether the database provider has been trying to obtain information about their query by performing quantum superpositions of different queries in addition to performing normal queries. This means that, in addition to being able to request the jth or the kth records in the database, the user can also request both records in a quantum superposition. To find out whether the database provider is trying to discover her queries, the user sends proper superpositions of queries and then checks the answer provided by the database to determine whether the superposition has been preserved. If superposition has not been preserved, the user can be confident that the database provider has cheated, and has tried to obtain information on the query
Quantum metrology: Beauty and the noisy beast
No full textElegant but extremely delicate quantum procedures can increase the precision of measurements. Characterizing how they cope with the detrimental effects of noise is essential for deployment to the real world
Multiqubit noise deconvolution and characterization
No full textWe present a noise deconvolution technique for obtaining noiseless expectation values of noisy observables at the output of multiqubit quantum channels. For any number of qubits or in the presence of correlations, our protocol applies to any mathematically invertible noise model, even when its inverse map is not physically implementable, i.e., when it is neither completely positive nor trace preserving. For a generic observable affected by Pauli noise it provides a quadratic speedup, always producing a rescaling of its Pauli basis components. We show that it is still possible to achieve the deconvolution while experimentally estimating the noise parameters, whenever these are unknown (bypassing resource-heavy techniques such as quantum process tomography). We provide a simulation, with examples for both Pauli and non-Pauli channels
When does a particle arrive?
No full textWe compare the proposals that have appeared in the literature to describe a measurement of the time of arrival of a quantum particle at a detector. We show that there are multiple regimes where different proposals give inequivalent, experimentally discriminable, predictions. This analysis paves the way for future experimental tests
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