The effect of quantum entanglement on the emission time of photoelectrons has been calculated by physicists in China and Austria. Their result includes several counter-intuitive predictions that could be testable with improved free-electron lasers. The photoelectric effect involves quantum particles of light (photons) interacting with electrons in atoms, molecules and solids. This can result in the emission of an electron (called a photoelectron), but only if the photon energy is greater than the binding energy of the electron. “Typically when people calculate the photoelectric effect they assume it’s a very weak perturbation on an otherwise inert atom or solid surface and most of the time does not suffer anything from these other atoms or photons coming in,” explains Wei-Chao Jiang of Shenzhen University in China. In very intense radiation fields, however, the atom may simultaneously absorb multiple photons, and these can give rise to multiple emission pathways. Jiang and colleagues have done a theoretical study of the ionization of a helium atom from its ground state by intense pulses of extreme ultraviolet (XUV) light. At sufficient photon intensities, there are two possible pathways by which a photoelectron can be produced. In the first, called direct single ionization, the photon in the ground state simply absorbs an electron and escapes the potential well. The second is a two-photon pathway called excitation ionization, in which both of the helium electrons absorb a photon from the same light pulse. One of them subsequently escapes, while the other remains in a higher energy level in the residual ion. The two photoemission pathways are distinct, so making a measurement of the emitted electron reveals information about the state of the bound electron that was left behind. The light pulse therefore creates an entangled state in which the two electrons are described by the same quantum wavefunction. To better understand the system, the researchers modelled the emission time for an electron undergoing excitation ionization relative to an electron undergoing direct single ionization. “The naïve expectation is that, if I have a process that takes two photons, that process will take longer than one where one photon does the whole thing,” says team member Joachim Burgdörfer of the Vienna University of Technology. What the researchers calculated, however, is that photoelectrons emitted by excitation ionization were most likely to be detected about 200 as earlier than photons detected by direct single ionization. This can be explained semi-classically by assuming that the photoionization event must precede the creation of the helium ion (He + ) for the second excitation step to occur. Excitation ionization therefore requires earlier photoemission. The researchers believe that, in principle, it should be possible to test their model using attosecond streaking or RABBITT (reconstruction of attosecond beating by interference of two-photon transitions). These are special types of pump-probe spectroscopy that can observe interactions at ultrashort timescales. “Naïve thinking would say that, using a 500 as pulse as a pump and a 10 fs pulse as a probe, there is no way you can get time resolution down to say, 10 as,” says Burgdörfer. “This is where recently developed techniques such as streaking or RABBITT come in. You no longer try to keep the pump and probe pulses apart, instead you want overlap between the pump and probe and you extract the time information from the phase information.” The team also did numerical simulations of the expected streaking patterns at one energy and found that they were consistent with an analytical calculation based on their intuitive picture. “Within a theory paper, we can only check for mutual consistency,” says Burgdörfer. How long does the photoelectric effect take? The principal hurdle to actual experiments lies in generating the required XUV pulses. Pulses from high harmonic generation may not be sufficiently strong to excite the two-photon emission. Free electron laser pulses can be extremely high powered, but are prone to phase noise. However, the researchers note that entanglement between a photoelectron and an ion has been achieved recently at the FERMI free electron laser facility in Italy. “Testing these predictions employing experimentally realizable pulse shapes should certainly be the next important step.” Burgdörfer says. Beyond this, the researchers intend to study entanglement in more complex systems such as multi-electron atoms or simple molecules. Paul Corkum at Canada’s University of Ottawa is intrigued by the research. “If all we’re going to do with attosecond science is measure single electron processes, probably we understood them before, and it would be disappointing if we didn’t do something more,” he says. “It would be nice to learn about atoms, and this is beginning to go into an atom or at least its theory thereof.” He cautions, however, that “If you want to do an experiment this way, it is hard.” The research is described in Physical Review Letters . 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Texas Longhorns water bottle throwing incident: Investigation leads to ... no one being caught or punishedA wild first season of the expanded Big 12 is down to what should be a chaotic final weekend. Through all the upsets, unexpected rises and falls, there are nine teams still in the mix to play in the conference championship game. No. 14 Arizona State and No. 17 Iowa State have the best odds, yet a multitude of scenarios could play out — 256 to be exact. There's even the possibility of an eight-team tie. It may take a mathematician to figure out which teams are in the Dec. 7 game in Arlington, Texas — even for the ones who win. Travis Hunter, Colorado. The Buffaloes' two-way star has excelled on both sides of the field, making him one of the favorites to win the Heisman Trophy. Cam Skattebo, Arizona State. The senior running back can do a little of everything, but excels at punishing would-be tacklers. He's one of the nation's leaders in yards after contact and the focal point of the Sun Devils' offense. Shadeur Sanders, Colorado. If it weren't for Hunter, Sanders might be the Heisman favorite. The son of coach Deion Sanders, Shedeur is fifth nationally with 3,488 yards passing and has been a big part of the Buffaloes' turnaround. DJ Giddens, Kansas State. The Wildcats' running back is one of the nation's most versatile players. He is ninth nationally with 1,271 rushing yards and has added 21 receptions for 258 yards. Tetairoa McMillan, Arizona. The Wildcats have struggled this season, but McMillan has not. He is third nationally with 1,251 receiving yards with seven touchdowns on 78 catches. Jacob Rodriguez, Texas Tech. The Red Raiders' junior linebacker leads the Big 12 with 68 tackles, averaging 10.2 per game. He also has four sacks. Brendan Mott, Kansas State. He's a menace to opposing quarterbacks, leading the Big 12 with 8 1/2 sacks. The Big 12 has nine teams already bowl eligible and two more a win away. The winner of the Big 12 championship game will be in the mix for a College Football Playoff spot. Arizona State, Iowa State, No. 19 BYU, Colorado, Kansas State, Baylor, TCU, Texas Tech and West Virginia have already clinched bowl berths. Kansas and Cincinnati can get into the postseason with wins this weekend. Gus Malzahn, UCF. Despite successes in recruiting, the Knights are 10-14 in two seasons since moving to the Big 12. Maybe not enough to get shown the door this year, but another mediocre season could lead UCF to make a change. Kyle Whittingham, Utah. Whittingham was one of the Pac-12's best coaches, leading the Utes to consecutive conference titles. Utah was expected to contend for the Big 12 title its first year in the league, but enters the final weekend 1-7 in conference play, which could push Whittingham toward retirement since it's doubtful he'd be fired. Neal Brown, West Virginia. The Mountaineers' coach was in a precarious spot at the end of last season and West Virginia hasn't lived up to expectations this season. The Mountaineers are eligible to go to a bowl game for the second straight season, but Brown could be on the hot seat even after signing a contract extension before the season. Josiah Trotter, West Virginia. The redshirt freshman is the latest Trotter to have success at the linebacker position, following the footsteps of his father, former Philadelphia Eagles player Jeremiah Trotter, and brother Jeremiah Trotter Jr., a current Eagles linebacker. Sam Leavitt, Arizona State. The Michigan State transfer has been just what the Sun Devils' needed: an agile quarterback who extends plays with his legs and rarely makes bad decisions. Bryson Washington, Baylor. The Bears' running back has rushed for 812 yards — 196 against TCU — and 10 TDs. TCU has the Big 12's highest rated 2025 recruiting class with six four-star players among 26 commitments, according to the 247 Sports composite. Receiver Terry Shelton of Carrollton, Texas, is the highest-rated recruit at 71st nationally. Baylor is next with five five-star players among its 20 commitments, including running back Michael Turner, rated 13th at his position out of North Richland Hills, Texas. Texas Tech is ranked seventh in the Big 12, but has four four-star recruits. Get local news delivered to your inbox!
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