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Photons from distant astronomical sources can be used as a classical source of randomness to improve fundamental tests of quantum nonlocality, wave-particle duality, and local realism through Bell’s inequality and delayed-choice quantum eraser tests inspired by Wheeler’s cosmic-scale Mach-Zehnder interferometer gedanken experiment. Such sources of random numbers may also be useful for information-theoretic applications such as key distribution for quantum cryptography. Building on the design of an astronomical random number generator developed for the recent cosmic Bell experiment [Handsteiner et al. Phys. Rev. Lett. 118, 060401 (2017)], in this paper we report on the design and characterization of a device that, with 20-nanosecond latency, outputs a bit based on whether the wavelength of an incoming photon is greater than or less than ≈700 nm. Using the one-meter telescope at the Jet Propulsion Laboratory Table Mountain Observatory, we generated random bits from astronomical photons in both color channels from 50 stars of varying color and magnitude, and from 12 quasars with redshifts up to \( z = 3.9 \). With stars, we achieved bit rates of \( \sim1 \times 10^6\,\mathrm{Hz/m^2} \), limited by saturation of our single-photon detectors, and with quasars of magnitudes between 12.9 and 16, we achieved rates between \( \sim 10^2 \) and \( 2 \times 10^3 \,\mathrm{Hz/m^2} \). For bright quasars, the resulting bitstreams exhibit sufficiently low amounts of statistical predictability as quantified by the mutual information. In addition, a sufficiently high fraction of bits generated are of true astronomical origin in order to address both the locality and freedom-of-choice loopholes when used to set the measurement settings in a test of the Bell-CHSH inequality.


In this Letter, we present a cosmic Bell experiment with polarization-entangled photons, in which measurement settings were determined based on real-time measurements of the wavelength of photons from high-redshift quasars, whose light was emitted billions of years ago; the experiment simultaneously ensures locality. Assuming fair sampling for all detected photons and that the wavelength of the quasar photons had not been selectively altered or previewed between emission and detection, we observe statistically significant violation of Bells inequality by 9.3 standard deviations, corresponding to an estimated \( p \) value of \( \lsim 7.4 \times 10^{−21} \). This experiment pushes back to at least ∼7.8 Gyr ago the most recent time by which any local-realist influences could have exploited the freedom-of-choiceloophole to engineer the observed Bell violation, excluding any such mechanism from 96% of the space-time volume of the past light cone of our experiment, extending from the big bang to today.



We propose a practical scheme to use photons from causally disconnected cosmic sources to set the detectors in an experimental test of Bells inequality. In current experiments, with settings determined by quantum random number generators, only a small amount of correlation between detector settings and local hidden variables, established less than a millisecond before each experiment, would suffice to mimic the predictions of quantum mechanics. By setting the detectors using pairs of quasars or patches of the cosmic microwave background, observed violations of Bells inequality would require any such coordination to have existed for billions of yearsan improvement of 20 orders of magnitude.


We derive criteria for whether two cosmological events can have a shared causal past or a shared causal future, assuming a Friedmann-Lemaitre-Robertson-Walker (FLRW) universe with best-fit cosmological parameters from the Planck satellite. We further derive criteria for whether either cosmic event could have been in past causal contact with our own worldline since the time of the hot “big bang,” which we take to be the end of early-universe inflation. We find that pairs of objects such as quasars on opposite sides of the sky with redshifts \( z \ge 3.65 \) have no shared causal past with each other or with our past worldline. More complicated constraints apply if the objects are at different redshifts from each other or appear at some relative angle less than 180°, as seen from Earth. We present examples of observed quasar pairs that satisfy all, some, or none of the criteria for past causal independence. Given dark energy and the recent accelerated expansion, our observable Universe has a finite conformal lifetime, and hence a cosmic event horizon at current redshift \( z = 1.87 \). We thus constrain whether pairs of cosmic events can signal each other’s worldlines before the end of time. Lastly, we generalize the criteria for shared past and future causal domains for FLRW universes with nonzero spatial curvature.


Being able to distinguish light-quark jets from gluon jets on an event-by-event basis could significantly enhance the reach for many new physics searches at the Large Hadron Collider. Through an exhaustive search of existing and novel jet substructure observables, we find that a multivariate approach can filter out over 95% of the gluon jets while keeping more than half of the light-quark jets. Moreover, a combination of two simple variables, the charge track multiplicity and the \( p_T \)-weighted linear radial moment (girth), can achieve similar results. Our study is only Monte Carlo based, so other observables constructed using different jet sizes and parameters are used to highlight areas that deserve further theoretical and experimental scrutiny. Additional information, including distributions of around 10 000 variables, can be found at


A new class of observables is introduced which aims to characterize the superstructure of an event, that is, features, such as color flow, which are not determined by the jet four-momenta alone. Traditionally, an event is described as having jets which are independent objects; each jet has some energy, size, and possible substructure such as subjets or heavy flavor content. This description discards information connecting the jets to each other, which can be used to determine if the jets came from decay of a color- singlet object, or if they were initiated by quarks or gluons. An example superstructure variable, pull, is presented as a simple handle on color flow. It can be used on an event-by-event basis as a tool for distinguishing previously irreducible backgrounds at the Tevatron and the LHC.

Recent Publications by

Jason Gallicchio

Student author • Faculty author


Calvin Leung, Amy Frances Brown, Hien Nguyen, Andrew S. Friedman, David I. Kaiser, and Jason Gallicchio

Astronomical Random Numbers for Quantum Foundations Experiments

Physical Review A 97 (2018) 042120.

Dominik Rauch, Johannes Handsteiner, Armin Hochrainer, Jason Gallicchio, Andrew S. Friedman, Calvin Leung, Bo Liu, Lukas Bulla, Sebastian Ecker, Fabian Steinlechner, Rupert Ursin, Beili Hu, David Leon, Chris Benn, Adriano Ghedina, Massimo Cecconi, Alan H. Guth, David I. Kaiser, Thomas Scheidl, and Anton Zeilinger

Cosmic Bell Test Using Random Measurement Settings from High-Redshift Quasars

Physical Review Letters 121 (2018) 080403.

Jason Gallicchio, Andrew S. Friedman, and David I. Kaiser

Testing Bell’s Inequality with Cosmic Photons: Closing the Setting-Independence Loophole

Physical Review Letters 112 (2014) 195.

Andrew S Friedman, David I Kaiser, and Jason Gallicchio

The shared causal pasts and futures of cosmological events

Physical Review D 88 (2013) .

Jason Gallicchio and Matthew D Schwartz

Quark and gluon jet substructure

Journal of High Energy Physics 2013 (2013) .

Yang Bai, Hsin-Chia Cheng, Jason Gallicchio, and Jiayin Gu

A toolkit of the stop search via the chargino decay

Journal of High Energy Physics 2013 (2013) .

Jason Gallicchio and Matthew D Schwartz

Pure samples of quark and gluon jets at the LHC

Journal of High Energy Physics 2011 (2011) .

Jason Gallicchio and Matthew D Schwartz

Quark and Gluon Tagging at the LHC

Physical Review Letters 107 (2011) 172001.

Jason Gallicchio and Matthew D. Schwartz

Seeing in Color: Jet Superstructure

Physical Review Letters 105 (2010) 022001.