Researchers use quantum telepathy to win an ‘impossible’ game

To win bridge blackjack, which is played between two sets of partners, one player must somehow indicate to his teammate the strength of the hand he is holding. Telepathy will come in handy here. But telepathy isn’t real, is it?

This is correct. Despite this, physicists have for decades suspected that if bridge was played using cards governed by the rules of quantum mechanics, something as strange as telepathy would be possible. Now researchers are in China empirically proven This is called pseudoquantum morbidity – not in quantum bridge but in a two-player quantum competition called Mermin-Peres Magic Square (MPMS), where winning requires players to coordinate their actions without exchanging information with each other. Used wisely, quantum pseudoprocessing allows players to win every round of the game – a flawless performance that would otherwise have been impossible. The experiment was conducted using laser photons. Investigates the limits of what quantum mechanics allows for information to be shared between particles.

The work is “a beautiful and simple straightforward implementation of Mermin-Peres Magic Square,” says Arul Lakshminarayan of the Indian Institute of Technology in Madras, who was not involved in the demo. He adds that its beauty comes in part from its elegance in asserting that the state of a quantum system is not well defined prior to the actual measurement – something that is often considered the most perplexing feature of quantum mechanics. “These quantum games seriously undermine our common concept of things that have pre-existing properties that are revealed through observations,” he says.

Two quantum physicists, Asher Peres And the David Mermin, which independently created the MPMS in 1990. It involves two players (called Alice and Bob, as in quantum mechanical thought experiments) who have to fill in a “magic square” – a grid of three by three numbers – using the value +1 to be assigned or -1 for each element of the grid. In each round, the referee (Charlie) sends a random row to Alice and then a column to Bob (there are nine combinations of rows and columns). Players have to tell Charlie which values ​​of +1 or -1 to place in the three grid spaces. As with any magic square challenge (such as Sudoku), the sums for each row and column must meet certain constraints: here the product of all entries in the row must be +1, and the product of all columns must be -1. Bob and Alice win a round If they both assign the same value to the grid element where column and row overlap.

Classically, it is impossible to win all the rounds because even if Alice and Bob predict well each time, there is inevitably one round for each completed square as their tasks must collide. The best they can do is get eight wins out of nine.

The graph shows Mermin-Peres Magic Square (MPMS) and why players can't win all nine rounds in its classic scenario.
Credit: Lucy Reading-Ikanda

But now suppose that Alice and Bob can use this quantum strategy: Instead of assigning a value of +1 or -1 to each element in the network, they assign a pair of quantum bits (qubits), each with a value of +1 or -1 when measured. The value that each player gives to a particular network element is determined by measuring the two qubits and finding the product of the pair. Now the classic conflict can be avoided because Alice and Bob can derive different values ​​from the same two qubits depending on how their measurements are made. There is a certain scaling strategy that will ensure the winning criteria for any given round – that Alice’s and Bob’s three products are +1 and -1 respectively – are met for all nine permutations of the rows and columns.

However, there is a wrinkle in this strategy. To make the correct set of measurements, Bob and Alice must know which of the three grid elements is interfering with the other trigger – they need to be coordinated. But in MPMS, this is not a problem because they perform their measurements sequentially on the same three qubit pairs. This means that the pair that reaches Bob has a fingerprint of how Alice actually measures those quibits: they can pass the information on to each other.

The graphic shows a quantum MPMS game where players can win all nine rounds if they measure their qubit values ​​sequentially.
Credit: Lucy Reading-Ikanda

In 1993, Mermin show up MPMS can be used to demonstrate a quantitative phenomenon called context. First identified by Northern Irish physicist John Stuart Bell in 1966, the context refers to the fact that the outcome of a quantum measurement may depend on how the measurement was made. A set of classical measurements in a system will give the same results regardless of the sequence in which these measurements are performed. But for quantum measurements, this is not always the case. In MPMS, the context arises from the fact that measuring a particular qubit pair may give a different result depending on which other pair is also being measured.

But what if we block any communication in the MPMS by assigning different qubit pairs from Alice and Bob and saying they can’t consult on how to measure them? After that, only nine wins can be guaranteed to each player if he makes correct guesses about what the other player is doing. But in study Published in 2005, quantum theorist Gilles Brassard of the University of Montreal and his colleagues showed that players can use quantum principles to secure victory in all round Even without communicating using what they call quantum pseudopathology.

This strategy includes entangled One of each qubit pair sent to Alice or Bob with a corresponding qubit used by the other player. Entangled particles have correlated properties, so if Alice measures the value of her particle, this fixes the value of the Bob particle as well. Two entangled qubits can be bonded, for example: if Alice’s qubit is found to have the value +1, then Bob must be -1. There’s no way to say what value Alice qubits have before it’s scaled – it could be +1 or -1 – but Bob’s value will always be the opposite. Importantly, the entanglement property between pairs of particles is said to be “non-local,” meaning that it is not “local” to either particle but is common to the two. Even if the particles are separated by vast distances, the entangled pair should be considered as a single non-local object. It was the same basic idea to win a quantum game Suggestion In 2001 by quantum theorist Adán Cabello of the University of Seville in Spain in a game he called “all or nothing”, which was later shown to be equivalent to non-local (pseudotelepathic) MPMS.

The graphic shows how MPMS players can use entangled qubits to win all nine rounds without calling their measurements.
Credit: Lucy Reading-Ikanda

Some researchers consider entanglement to be a fundamental aspect of quantum mechanics. It means a kind of information exchange between particles. This is the key to taking advantage of entanglement in quantum pseudopoorness: Alice and Bob don’t have to exchange information to coordinate their actions because the necessary information is already shared in the same pairs of particles.

Both contextual and non-positional provide “quantitative resources” that can be used to gain some advantages over traditional methods of information processing. In quantum computing, for example, entanglement between quantum qubits is generally considered the resource that creates a shortcut to finding a solution to a problem unavailable to a classical computer.

Physicists have Show frequently Cabello All or Nothing in the real world Use Entangled photons. But while these experiments demonstrated how entanglement can impart a “quantum advantage” by overcoming the classical performance, Kai Chen of the University of Science and Technology of China, Shi Lin Wang of Nanjing University in China and their colleagues devised a new experiment that they say implement the full protocol. For a guaranteed win in every round – true and consistent quantitative pseudo-pseudoscience.

Ideally, Alice and Bob prepare several groups of four qubits before the game begins, and each quadruple consists of two interlocking pairs. Alice will get one of these pairs, and Bob will get the other. The researchers say that making entangled pairs of photons in each round of the game is a big challenge. For one thing, the production of a single entangled pair only occurs with low probability in their device, so making two at once would be highly unlikely. Also, detecting a couple simultaneously, as the pseudotelepathic MPMS requires, is somewhat impossible for this visual application.

Instead, Chen, Wang and their colleagues prepared pairs of single photons and entangled two of their properties independently: polarization state and a property called orbital angular momentum. The photons were contained in ultrashort laser pulses lasting only 150 femtoseconds and were entangled by passing them through so-called nonlinear optical crystals. A thin slice of barium borate first splits one photon into two low-energy photons with coherent angular moment. They were then entangled by their polarization as well, by sending them through a crystal of the yttrium-vanadium compound.

To demonstrate a near 100% success rate, the researchers needed to improve their detection efficiency so that almost none of the entangled photons escape the invisible. Even then, the theoretical limit cannot be precisely reached in the experiment – but the researchers were able to demonstrate that they could win each round with a probability of between 91.5 and 97 percent. This translates to reliably beating the limit of eight out of nine in 1,009,610 rounds out of a total of 1,075,930 plays.

The pseudotelepathic MPMS exploits the strongest degree of interparticle correlation that quantum mechanics can provide, Chen says. “Our experiment looks at how to generate extreme quantum correlations between particles,” he says. If these correlations are stronger, they would imply a faster-than-light exchange of information that a host of other independent experiments suggest is impossible.

Mermin says that while this success is empirically impressive, it reveals nothing new other than the fact that quantum mechanics works as we thought. Capello totally disagrees. In addition to being a test run, he says, the work shows a new wrinkle in what quantitative rules allow by simultaneously populating two sources of quantum advantage: one related to non-existence and one related to context. Investigating the two effects simultaneously should allow physicists to more precisely explore the links between them, Capello says.

Moreover, each of these resources can in principle be put to different uses in quantitative processing, enhancing their diversity. For example, nonlocality can be used for covert communication [using quantum cryptography] While contextuality can be used in quantitative computation,” says Capello. In this scenario, Bob could, for example, set up a secure connection with Alice while at the same time performing a calculation with Charlie faster than conventional methods would allow.

The use of co-entanglement in these experiments “leads to effects that seem classically magical,” says Lakshminarayan. But given how often quantum mechanics is misused as a pseudo-scientific justification for pseudoscientific claims, is it possible to question the problem of calling this phenomenon ‘pseudopathology’? It’s “a bad term that calls for meaningless explanations,” Mermin says. But while Cabello agrees, he realizes that evocative names can help publicize interest in the phenomenon. “Let’s not deceive ourselves,” he says. “It is probably thanks to the word pseudotelepathy [you and I] This conversation is going on.”

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