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Unique method enables simulation of error-correctable quantum computers

Quantum computers still face a major hurdle on their pathway to practical use cases: their limited ability to correct the arising computational errors. To develop truly reliable quantum computers, researchers must be able to simulate quantum computations using conventional computers to verify their correctness—a vital yet extraordinarily difficult task.

Now, in a world-first, researchers from Chalmers University of Technology in Sweden, the University of Milan, the University of Granada, and the University of Tokyo have unveiled a method for simulating specific types of error-corrected quantum computations—a significant leap forward in the quest for robust quantum technologies.

Quantum computers have the potential to solve complex problems that no supercomputer today can handle. In the foreseeable future, ’s computing power is expected to revolutionize fundamental ways of solving problems in medicine, energy, encryption, AI, and logistics.

Quantum Dots For Reliable Quantum Key Distribution

Making the exchange of a message invulnerable to eavesdropping doesn’t strictly require quantum resources. All you need to do is to encrypt the message using a one-use-only random key that is at least as long as the message itself. What quantum physics offers is a way to protect the sharing of such a key by revealing whether anyone other than sender and recipient has accessed it.

Imagine that a sender (Alice) wants to send a message to a recipient (Bob) in the presence of an eavesdropper (Eve). First, Alice creates a string of random bits. According to one of the most popular quantum communication protocols, known as BB84, Alice then encodes each bit in the polarization state of an individual photon. This encoding can be performed in either of two orientations, or “bases,” which are also chosen at random. Alice sends these photons one at a time to Bob, who measures their polarization states. If Bob chooses to measure a given photon in the basis in which Alice encoded its bit, Bob’s readout of the bit will match that of Alice’s. If he chooses the alternative basis, Bob will measure a random polarization state. Crucially, until Alice and Bob compare their sequence of measurement bases (but not their results) over a public channel, Bob doesn’t know which measurements reflect the bits encoded by Alice. Only after they have made this comparison—and excluded the measurements made in nonmatching bases—can Alice and Bob rule out that eavesdropping took place and agree on the sequence of bits that constitutes their key.

The efficiency and security of this process depend on Alice’s ability to generate single photons on demand. If that photon-generation method is not reliable—for example, if it sometimes fails to generate a photon when one is scheduled—the key will take longer to share. If, on the other hand, the method sometimes generates multiple photons simultaneously, Alice and Bob run the risk of having their privacy compromised, since Eve will occasionally be able to intercept one of those extra photons, which might reveal part of the key. Techniques for detecting such eavesdropping are available, but they involve the sending of additional photons in “decoy states” with randomly chosen intensities. Adding these decoy states, however, increases the complexity of the key-sharing process.

Smart amplifier cuts power consumption, paving way for more qubits and less decoherence

Quantum computers can solve extraordinarily complex problems, unlocking new possibilities in fields such as drug development, encryption, AI, and logistics. Now, researchers at Chalmers University of Technology in Sweden have developed a highly efficient amplifier that activates only when reading information from qubits. The study was published in the journal IEEE Transactions on Microwave Theory and Techniques.

Thanks to its smart design, it consumes just one-tenth of the power consumed by the best amplifiers available today. This reduces decoherence and lays the foundation for more with significantly more qubits and enhanced performance.

Bits, which are the building blocks of a conventional computer, can only ever have the value of 1 or 0. By contrast, the common building blocks of a quantum computer, quantum bits or qubits, can exist in states having the value 1 and 0 simultaneously, as well as all states in between in any combination.

True single-photon source boosts secure key rates in quantum key distribution systems

Quantum key distribution (QKD), a cryptographic technique rooted in quantum physics principles, has shown significant potential for enhancing the security of communications. This technique enables the transmission of encryption keys using quantum states of photons or other particles, which cannot be copied or measured without altering them, making it significantly harder for malicious parties to intercept conversations between two parties while avoiding detection.

As true single-photon sources (SPS) are difficult to produce, most QKD systems developed to date rely on attenuated light sources that mimic single photons, such as low-intensity . As these laser pulses can also contain no photons or more than one photon, only approximately 37% of pulses employed by the systems can be used to generate secure keys.

Researchers at the University of Science and Technology of China (USTC) were recently able to overcome this limitation of previously proposed QKD systems, using a true SPS (i.e., a system that can emit only one photon on demand). Their newly proposed QKD system, outlined in a paper published in Physical Review Letters, was found to outperform techniques introduced in the past, achieving a substantially higher secure key rate (SKR).

A new problem that only quantum computing can solve

As quantum computing develops, scientists are working to identify tasks for which quantum computers have a clear advantage over classical computers. So far, researchers have only pinpointed a handful of these problems, but in a new paper published in Physical Review Letters, scientists at Los Alamos National Laboratory have added one more problem to this very short list.

“One of the central questions that faces is what classes of problems they can most efficiently solve but cannot,” says Marco Cerezo, the Los Alamos team’s lead scientist. “At the moment, this is the Holy Grail of quantum computing, because you can count on two hands such problems. In this paper, we’ve just added another.”

Quantum computing harnesses the unique laws of quantum physics, such as superposition, entanglement and interference, which allow for information processing capabilities beyond those of classical devices. When fully realized, quantum computing promises to make advancements in cryptography, simulations of quantum systems and data analysis, among many other fields. But before this can happen, researchers still need to develop the foundational science of quantum computing.

Quantum computers may crack RSA encryption with fewer qubits than expected

A team of researchers at AI Google Quantum AI, led by Craig Gidney, has outlined advances in quantum computer algorithms and error correction methods that could allow such computers to crack Rivest–Shamir–Adleman (RSA) encryption keys with far fewer resources than previously thought. The development, the team notes, suggests encryption experts need to begin work toward developing next-generation encryption techniques. The paper is published on the arXiv preprint server.

RSA is an encryption technique developed in the late 1970s that involves generating public and private keys; the former is used for encryption and the latter decryption. Current standards call for using a 2,048-bit encryption key. Over the past several years, research has suggested that quantum computers would one day be able to crack RSA encryption, but because quantum development has been slow, researchers believed that it would be many years before it came to pass.

Some in the field have accepted a theory that a quantum computer capable of cracking such codes in a reasonable amount of time would have to have at least 20 million qubits. In this new work, the team at Google suggests it could theoretically be done with as few as a million qubits—and it could be done in a week.