Quantum computers are still a dream, but the era of quantum communications has arrived. A new experiment, carried out in Paris, showed for the first time that quantum communication is superior to classical methods of transmitting information.
“We were the first to demonstrate quantum superiority in the transmission of information that two parties need to complete a task,” says Eleni Diamanti, an electrical engineer at the Sorbonne University and co-author of the study.
Quantum machines - which use the quantum properties of matter to encode information - are expected to revolutionize computing. But progress in this area has been extremely slow. While engineers are working to create rudimentary quantum computers, theoretical scientists have faced a more fundamental obstacle: they have failed to prove that classical computers can never complete the tasks for which quantum computers are designed. Last summer, for example, a guy from Texas proved that a problem that for a long time was considered solvable only on a quantum computer can be quickly solved on a classical computer.
Welcome to the quantum age
However, in the field of communications (not computing), the benefits of the quantum approach can be confirmed. More than a decade ago, scientists proved that, in theory at least, quantum communication is superior to classical ways of sending messages for specific tasks.
“People were mainly engaged in computing tasks. One of the big advantages is that in the case of communication tasks, the benefits are demonstrable."
In 2004, Jordanis Kerenidis, co-author of Diamanti's work, and two other scientists presented a scenario in which one person needed to send information to another so that a second person could answer a specific question. Researchers have proven that a quantum circuit can accomplish a task by transferring exponentially less information than a classical system. But the quantum circuit they presented was purely theoretical - and far beyond the technology of the day.
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“We were able to confirm this quantum advantage, but it was extremely difficult to implement the quantum protocol,” says Kerenidis.
The new work is a modified version of the script envisioned by Kerenidis and his colleagues. As usual, let's turn to two subjects, Alice and Bob. Alice has a set of numbered balls. Each ball is randomly colored red or blue. Bob wants to know if a particular pair of balls, chosen at random, have the same color or are different. Alice wants to send Bob as little information as possible, while ensuring that Bob can answer his question.
This problem is referred to as a "pattern matching problem". It is essential for cryptography and digital currencies, where users often want to exchange information without divulging everything they know. It also perfectly demonstrates the benefits of quantum communication.
You can't just say: I want to send you a movie or something the size of a gigabyte and encode it into a quantum state, expecting to find a quantum advantage, says Thomas Vidick, a computer scientist at the California Institute of Technology. "We need to consider more subtle tasks."
For the classic solution to the matching problem, Alice must send Bob an amount of information proportional to the square root of the number of balls. But the unusual nature of quantum information makes a more efficient solution possible.
In the laboratory circuit used in the new work, Alice and Bob communicate using laser pulses. Each impulse represents one ball. The pulses pass through a beam splitter, which sends half of each pulse to Alice and Bob. When the pulse reaches Alice, she can shift the phase of the laser pulse to encode information about each ball - depending on its color, red or blue.
Meanwhile, Bob encodes information about the pairs of balls that interest him into his half of the laser pulses. Then the pulses converge in another beam splitter, where they interfere with each other. The interference pattern produced by the pulses reflects differences in how the phases of each pulse were shifted. Bob can read the interference pattern on the nearest photon detector.
Until the moment when Bob "reads" Alice's laser message, Alice's quantum message is capable of answering any question about any pair. But the process of reading the quantum message destroys it and Bob receives information about only one pair of balls.
This property of quantum information - that it can be read in a variety of ways, but ultimately only one will read it - greatly reduces the amount of information that can be conveyed to solve the sample matching problem. If Alice needs to send 100 classical bits to Bob so that he can answer his question, she can do the same task with about 10 qubits, or quantum bits.
This is the proof of principle you need to create a true quantum network, says Graham Smith, a physicist at JILA in Boulder, Colorado.
The new experiment is a clear triumph over classical methods. The researchers started the experiment knowing exactly how much information needed to be transmitted in the classical way to solve the problem. Then they convincingly demonstrated that quantum tools can solve it in a more compact way.
This result also offers an alternative route to a longstanding goal in computer science: proving that quantum computers are superior to classical computers.
Ilya Khel