01610nas a2200157 4500008004100000245005000041210005000091260001300141490000800154520116900162100002401331700001901355700001901374700002201393856003701415 2023 eng d00aShadow process tomography of quantum channels0 aShadow process tomography of quantum channels c4/4/20230 v1073 a

Quantum process tomography is a critical capability for building quantum computers, enabling quantum networks, and understanding quantum sensors. Like quantum state tomography, the process tomography of an arbitrary quantum channel requires a number of measurements that scale exponentially in the number of quantum bits affected. However, the recent field of shadow tomography, applied to quantum states, has demonstrated the ability to extract key information about a state with only polynomially many measurements. In this work, we apply the concepts of shadow state tomography to the challenge of characterizing quantum processes. We make use of the Choi isomorphism to directly apply rigorous bounds from shadow state tomography to shadow process tomography, and we find additional bounds on the number of measurements that are unique to process tomography. Our results, which include algorithms for implementing shadow process tomography enable new techniques including evaluation of channel concatenation and the application of channels to shadows of quantum states. This provides a dramatic improvement for understanding large-scale quantum systems.

1 aKunjummen, Jonathan1 aTran, Minh, C.1 aCarney, Daniel1 aTaylor, Jacob, M. uhttps://arxiv.org/abs/2110.0362901475nas a2200121 4500008004100000245003200041210003200073260001400105520115600119100001901275700002201294856003701316 2023 eng d00aStrongly incoherent gravity0 aStrongly incoherent gravity c1/20/20233 a

While most fundamental interactions in nature are known to be mediated by quantized fields, the possibility has been raised that gravity may behave differently. Making this concept precise enough to test requires consistent models. Here we construct an explicit example of a theory where a non-entangling version of an arbitrary two-body potential V(r) arises from local measurements and feedback forces. While a variety of such theories exist, our construction causes particularly strong decoherence compared to more subtle approaches. Regardless, expectation values of observables obey the usual classical dynamics, while the interaction generates no entanglement. Applied to the Newtonian potential, this produces a non-relativistic model of gravity with fundamental loss of unitarity. The model contains a pair of free parameters, a substantial range of which is not excluded by observations to date. As an alternative to testing entanglement properties, we show that the entire remaining parameter space can be tested by looking for loss of quantum coherence in small systems like atom interferometers coupled to oscillating source masses.

1 aCarney, Daniel1 aTaylor, Jacob, M. uhttps://arxiv.org/abs/2301.0837801697nas a2200181 4500008004100000245007000041210006900111260001400180490000800194520115900202100002301361700001501384700001901399700002001418700001901438700002101457856003701478 2020 eng d00aSearch for composite dark matter with optically levitated sensors0 aSearch for composite dark matter with optically levitated sensor c11/2/20200 v1253 a

Results are reported from a search for a class of composite dark matter models with feeble, long-range interactions with normal matter. We search for impulses arising from passing dark matter particles by monitoring the mechanical motion of an optically levitated nanogram mass over the course of several days. Assuming such particles constitute the dominant component of dark matter, this search places upper limits on their interaction with neutrons of αn≤1.2×10−7 at 95\% confidence for dark matter masses between 1--10 TeV and mediator masses mφ≤0.1 eV. Due to the large enhancement of the cross-section for dark matter to coherently scatter from a nanogram mass (∼1029 times that for a single neutron) and the ability to detect momentum transfers as small as ∼200 MeV/c, these results provide sensitivity to certain classes of composite dark matter models that substantially exceeds existing searches, including those employing kg-scale or ton-scale targets. Extensions of these techniques can enable directionally-sensitive searches for a broad class of previously inaccessible heavy dark matter candidates. 

1 aMonteiro, Fernando1 aAfek, Gadi1 aCarney, Daniel1 aKrnjaic, Gordan1 aWang, Jiaxiang1 aMoore, David, C. uhttps://arxiv.org/abs/2007.1206701396nas a2200157 4500008004100000245006600041210006600107260001500173300001100188490000600199520092600205100002601131700001901157700002501176856003701201 2018 eng d00aStructure of Correlated Worldline Theories of Quantum Gravity0 aStructure of Correlated Worldline Theories of Quantum Gravity c2018/06/21 a0840520 vD3 a

We consider the general form of "Correlated Worldline" (CWL) theories of quantum gravity. We show that one can have 2 different kinds of CWL theory, in which the generating functional is written as either a sum or a product over multiple copies of the coupled matter and gravitational fields. In both versions, the paths in a functional formulation are correlated via gravity itself, causing a breakdown of the superposition principle; however, the product form survives consistency tests not satisfied by the summed form. To better understand the structure of these two theories, we show how to perform diagrammatic expansions in the gravitational coupling for each version of CWL theory, using particle propagation and scalar fields as examples. We explicitly calculate contributions to 2-point and 4-point functions, again for each version of the theory, up to 2nd-order in the gravitational coupling.

1 aBarvinsky, Andrei, O.1 aCarney, Daniel1 aStamp, Philip, C. E. uhttps://arxiv.org/abs/1806.08043