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‘Unicorn’ Discovery Points to a New Population of Black Holes

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Almost a decade ago, Feryal Özel and her colleagues noticed something odd. While a variety of possible black holes had been found in our galaxy, none appeared to fall below a certain size. “There seemed to be a dearth of black holes below 5 solar masses,” she said. “Statistically, this was very significant.”

Since Özel, an astrophysicist at the University of Arizona, published a paper on the problem in 2010, this so-called mass gap has gone unexplained. Even after the LIGO and Virgo gravitational wave detectors started to identify dozens of hidden black holes — including a few surprises — the mass gap appeared to hold firm.

In time, astrophysicists such as Özel began to wonder: Are small black holes just hard to find, or might they not exist at all? “It’s important to establish observationally whether this gap is real, or whether it’s an observational artifact,” said Vicky Kalogera, an astrophysicist at Northwestern University and a leading member of the LIGO team.

Recent discoveries are beginning to suggest the latter might be the case. In the past two years, researchers have found several possible black holes in the mass gap. Then, earlier this month, astronomers presented evidence of what might be our best candidate yet — a 2.9-solar-mass object dubbed “the unicorn.”

Before LIGO, astronomers found black holes mostly by searching for the X-rays they produce as they suck in matter from a nearby star. They could also detect the gravitational effect a black hole would have on another star in a binary system. The unicorn researchers used this latter method, focusing on a system called V723 Monoceros, located about 1,000 light-years away. They studied the motion of a red giant star with a variety of telescopes, including the European Space Agency’s Gaia satellite, which is mapping the position of billions of stars in our galaxy, and NASA’s exoplanet-hunting Transiting Exoplanet Survey Satellite (TESS).

The researchers concluded that the red giant appears to be dancing with an unseen partner. “The simplest explanation for the dark companion is a single compact object, most likely a black hole, in the ‘mass gap,’” the team wrote.

The discovery, if confirmed, would help illuminate the fine distinction that nature makes at the end of a massive star’s life. When a giant star exhausts its fuel, the star’s mass flows inward and its core collapses. If the incoming mass can explode and overcome the star’s gravitational force, it bursts into a supernova. But if not — if there’s just too much mass — the star collapses in on itself and forms a black hole.

“It’s a race between the explosion happening and black hole formation,” said Todd Thompson, a theoretical astrophysicist at Ohio State University and a co-author of the recent paper. “This race has to be won within about a second. If it doesn’t explode in that one second, then it forms a black hole. If it does explode, it leaves behind a neutron star.”

Exactly what determines whether a star explodes as a supernova or collapses into a black hole isn’t clear. “The actual physics of the supernova explosion is a huge unknown,” said Thompson. The black hole mass gap “could be a vital clue to that process.”

There have been a few tentative discoveries in the mass gap so far. Benjamin Giesers from the University of Göttingen and colleagues discovered a possible 4.4-solar-mass black hole in 2018, while Thompson and his colleagues found a 3.3-solar-mass candidate in 2019.

Then last year, scientists from LIGO announced the detection of an object 2.6 times the mass of our sun, a very tantalizing candidate with a mass comfortably in the mass gap. “The best case for a mass-gap black hole is from LIGO,” said Thompson.

Yet at these lower masses, it’s hard to tell the difference between a black hole and a neutron star, since the latter can bulk up to a theoretical maximum of 3 solar masses. And depending on the conditions, neutron stars can appear dark as well. “If a neutron star is a pulsar with a beam pointed at you, it does some very obvious things,” said Tom Maccarone, an expert in black holes and neutron stars at Texas Tech University. “But if it’s not, it can be very difficult to separate” from a black hole.

As such, black hole discoveries such as the recent one from LIGO are still unconfirmed, as is the unicorn. “It looks plausible,” said Özel. “I think their methods are sound and the analysis is careful. There are still a couple of other possibilities as far as what it could be, but the conclusion that they come to — that it is most likely a single dark object with a mass of 2.9 solar masses — seems sound to me.”

Not everyone is so sure. For one thing, it’s possible the system is not a binary but a triple system, with two smaller objects accounting for the missing mass. Maccarone also said that if the unicorn is a black hole, it should be pulling matter away from its companion star and creating X-rays. “The X-ray luminosity is so faint for the black hole scenario,” he said.

There is also uncertainty about the mass of the object itself. While the best estimate is 2.9 solar masses, the authors say it could weigh as little as 2.6 and as much as 3.6 solar masses. “This range is too broad for us to be certain this is [not] a neutron star,” said Kalogera. “And there is nothing else in the observations that allows us to distinguish between a neutron star and a black hole.”

If this discovery turns out not to be a black hole — and especially if the other candidates don’t hold up to scrutiny either — then perhaps black holes simply do not form below 5 solar masses. Such a revelation would carry significant implications for our understanding of supernova physics. “If you really have a gap there, it suggests a very rapid explosion mechanism,” said Maccarone. “We don’t understand what drives supernova explosions well enough to have a strong theoretical bias for what to expect.”

But if the unicorn really is a black hole, it could be one of many in this gap waiting to be discovered and studied. “That’s what’s exciting about this paper,” says Özel. “If we come to a point where we know more than one or two of these objects, then we can understand what types of processes lead to these smaller-mass black holes.”

Source: https://www.quantamagazine.org/black-hole-mass-gap-gets-filled-by-new-discoveries-20210127/

Quantum

Self-testing with finite statistics enabling the certification of a quantum network link

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Quantum 5, 401 (2021).

https://doi.org/10.22331/q-2021-03-02-401

Self-testing is a method to certify devices from the result of a Bell test. Although examples of noise tolerant self-testing are known, it is not clear how to deal efficiently with a finite number of experimental trials to certify the average quality of a device without assuming that it behaves identically at each run. As a result, existing self-testing results with finite statistics have been limited to guarantee the proper working of a device in just one of all experimental trials, thereby limiting their practical applicability. We here derive a method to certify through self-testing that a device produces states on average close to a Bell state without assumption on the actual state at each run. Thus the method is free of the I.I.D. (independent and identically distributed) assumption. Applying this new analysis on the data from a recent loophole-free Bell experiment, we demonstrate the successful distribution of Bell states over 398 meters with an average fidelity of $geq$55.50% at a confidence level of 99%. Being based on a Bell test free of detection and locality loopholes, our certification is evidently device-independent, that is, it does not rely on trust in the devices or knowledge of how the devices work. This guarantees that our link can be integrated in a quantum network for performing long-distance quantum communications with security guarantees that are independent of the details of the actual implementation.

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Source: https://quantum-journal.org/papers/q-2021-03-02-401/

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Quantum

Project Ant-Man

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The craziest challenge I’ve undertaken hasn’t been skydiving; sailing the Amazon on a homemade raft; scaling Mt. Everest; or digging for artifacts atop a hill in a Middle Eastern desert, near midday, during high summer.1 The craziest challenge has been to study the possibility that quantum phenomena affect cognition significantly. 

Most physicists agree that quantum phenomena probably don’t affect cognition significantly. Cognition occurs in biological systems, which have high temperatures, many particles, and watery components. Such conditions quash entanglement (a relationship that quantum particles can share and that can produce correlations stronger than any produceable by classical particles). 

Yet Matthew Fisher, a condensed-matter physicist, proposed a mechanism by which entanglement might enhance coordinated neuron firing. Phosphorus nuclei have spins (quantum properties similar to angular momentum) that might store quantum information for long times when in Posner molecules. These molecules may protect the information from decoherence (leaking quantum information to the environment), via mechanisms that Fisher described.

I can’t check how correct Fisher’s proposal is; I’m not a biochemist. But I’m a quantum information theorist. So I can identify how Posners could process quantum information if Fisher were correct. I undertook this task with my colleague Elizabeth Crosson, during my PhD

Experimentalists have begun testing elements of Fisher’s proposal. What if, years down the road, they find that Posners exist in biofluids and protect quantum information for long times? We’ll need to test whether Posners can share entanglement. But detecting entanglement tends to require control finer than you can exert with a stirring rod. How could you check whether a beakerful of particles contains entanglement?

I asked that question of Adam Bene Watts, a PhD student at MIT, and John Wright, then an MIT postdoc and now an assistant professor in Texas. John gave our project its codename. At a meeting one day, he reported that he’d watched the film Avengers: Endgame. Had I seen it? he asked.

No, I replied. The only superhero movie I’d seen recently had been Ant-Man and the Wasp—and that because, according to the film’s scientific advisor, the movie riffed on research of mine. 

Go on, said John.

Spiros Michalakis, the Caltech mathematician in charge of this blog, served as the advisor. The film came out during my PhD; during a meeting of our research group, Spiros advised me to watch the movie. There was something in it “for you,” he said. “And you,” he added, turning to Elizabeth. I obeyed, to hear Laurence Fishburne’s character tell Ant-Man that another character had entangled with the Posner molecules in Ant-Man’s brain.2 

John insisted on calling our research Project Ant-Man.

John and Adam study Bell tests. Bell test sounds like a means of checking whether the collar worn by your cat still jingles. But the test owes its name to John Stewart Bell, a Northern Irish physicist who wrote a groundbreaking paper in 1964

Say you’d like to check whether two particles share entanglement. You can run an experiment, described by Bell, on them. The experiment ends with a measurement of the particles. You repeat this experiment in many trials, using identical copies of the particles in subsequent trials. You accumulate many measurement outcomes, whose statistics you calculate. You plug those statistics into a formula concocted by Bell. If the result exceeds some number that Bell calculated, the particles shared entanglement.

We needed a variation on Bell’s test. In our experiment, every trial would involve hordes of particles. The experimentalists—large, clumsy, classical beings that they are—couldn’t measure the particles individually. The experimentalists could record only aggregate properties, such as the intensity of the phosphorescence emitted by a test tube.

Adam, MIT physicist Aram Harrow, and I concocted such a Bell test, with help from John. Physical Review A published our paper this month—as a Letter and an Editor’s Suggestion, I’m delighted to report.

For experts: The trick was to make the Bell correlation function nonlinear in the state. We assumed that the particles shared mostly pairwise correlations, though our Bell inequality can accommodate small aberrations. Alas, no one can guarantee that particles share only mostly pairwise correlations. Violating our Bell inequality therefore doesn’t rule out hidden-variables theories. Under reasonable assumptions, though, a not-completely-paranoid experimentalist can check for entanglement using our test. 

One can run our macroscopic Bell test on photons, using present-day technology. But we’re more eager to use the test to characterize lesser-known entities. For instance, we sketched an application to Posner molecules. Detecting entanglement in chemical systems will require more thought, as well as many headaches for experimentalists. But our paper broaches the cask—which I hope to see flow in the next Ant-Man film. Due to debut in 2022, the movie has the subtitle Quantumania. Sounds almost as crazy as studying the possibility that quantum phenomena affect cognition.

1Of those options, I’ve undertaken only the last.

2In case of any confusion: We don’t know that anyone’s brain contains Posner molecules. The movie features speculative fiction.

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Source: https://quantumfrontiers.com/2021/02/28/project-ant-man/

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Stabilizer extent is not multiplicative

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Quantum 5, 400 (2021).

https://doi.org/10.22331/q-2021-02-24-400

The Gottesman-Knill theorem states that a Clifford circuit acting on stabilizer states can be simulated efficiently on a classical computer. Recently, this result has been generalized to cover inputs that are close to a coherent superposition of logarithmically many stabilizer states. The runtime of the classical simulation is governed by the $textit{stabilizer extent}$, which roughly measures how many stabilizer states are needed to approximate the state. An important open problem is to decide whether the extent is multiplicative under tensor products. An affirmative answer would yield an efficient algorithm for computing the extent of product inputs, while a negative result implies the existence of more efficient classical algorithms for simulating largescale quantum circuits. Here, we answer this question in the negative. Our result follows from very general properties of the set of stabilizer states, such as having a size that scales subexponentially in the dimension, and can thus be readily adapted to similar constructions for other resource theories.

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Source: https://quantum-journal.org/papers/q-2021-02-24-400/

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Quantum

A quantum algorithm for the direct estimation of the steady state of open quantum systems

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Quantum 5, 399 (2021).

https://doi.org/10.22331/q-2021-02-22-399

Simulating the dynamics and the non-equilibrium steady state of an open quantum system are hard computational tasks on conventional computers. For the simulation of the time evolution, several efficient quantum algorithms have recently been developed. However, computing the non-equilibrium steady state as the long-time limit of the system dynamics is often not a viable solution, because of exceedingly long transient features or strong quantum correlations in the dynamics. Here, we develop an efficient quantum algorithm for the direct estimation of averaged expectation values of observables on the non-equilibrium steady state, thus bypassing the time integration of the master equation. The algorithm encodes the vectorized representation of the density matrix on a quantum register, and makes use of quantum phase estimation to approximate the eigenvector associated to the zero eigenvalue of the generator of the system dynamics. We show that the output state of the algorithm allows to estimate expectation values of observables on the steady state. Away from critical points, where the Liouvillian gap scales as a power law of the system size, the quantum algorithm performs with exponential advantage compared to exact diagonalization.

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Source: https://quantum-journal.org/papers/q-2021-02-22-399/

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