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Kaushalya Jhuria in the lab testing the electronics from the experimental setup used to make qubits in silicon.

Credit: Thor Swift/Berkeley Lab

Quantum computers have the potential to solve complex problems in human health, drug discovery, and artificial intelligence millions of times faster than some of the worlds fastest supercomputers. A network of quantum computers could advance these discoveries even faster. But before that can happen, the computer industry will need a reliable way to string together billions of qubits or quantum bits with atomic precision.

Connecting qubits, however, has been challenging for the research community. Some methods form qubits by placing an entire silicon wafer in a rapid annealing oven at very high temperatures. With these methods, qubits randomly form from defects (also known as color centers or quantum emitters) in silicons crystal lattice. And without knowing exactly where qubits are located in a material, a quantum computer of connected qubits will be difficult to realize.

But now, getting qubits to connect may soon be possible. A research team led by Lawrence Berkeley National Laboratory (Berkeley Lab) says that they are the first to use a femtosecond laser to create and annihilate qubits on demand, and with precision, by doping silicon with hydrogen.

The advance could enable quantum computers that use programmable optical qubits or spin-photon qubits to connect quantum nodes across a remote network. It could also advance a quantum internet that is not only more secure but could also transmit more data than current optical-fiber information technologies.

To make a scalable quantum architecture or network, we need qubits that can reliably form on-demand, at desired locations, so that we know where the qubit is located in a material. And that's why our approach is critical, said Kaushalya Jhuria, a postdoctoral scholar in Berkeley Labs Accelerator Technology & Applied Physics (ATAP) Division. She is the first author on a new study that describes the technique in the journal Nature Communications. Because once we know where a specific qubit is sitting, we can determine how to connect this qubit with other components in the system and make a quantum network.

This could carve out a potential new pathway for industry to overcome challenges in qubit fabrication and quality control, said principal investigator Thomas Schenkel, head of the Fusion Science & Ion Beam Technology Program in Berkeley Labs ATAP Division. His group will host the first cohort of students from the University of Hawaii in June as part of a DOE Fusion Energy Sciences-funded RENEW project on workforce development where students will be immersed in color center/qubit science and technology.

Forming qubits in silicon with programmable control

The new method uses a gas environment to form programmable defects called color centers in silicon. These color centers are candidates for special telecommunications qubits or spin photon qubits. The method also uses an ultrafast femtosecond laser to anneal silicon with pinpoint precision where those qubits should precisely form. A femtosecond laser delivers very short pulses of energy within a quadrillionth of a second to a focused target the size of a speck of dust.

Spin photon qubits emit photons that can carry information encoded in electron spin across long distances ideal properties to support a secure quantum network. Qubits are the smallest components of a quantum information system that encodes data in three different states: 1, 0, or a superposition that is everything between 1 and 0.

With help from Boubacar Kant, a faculty scientist in Berkeley Labs Materials Sciences Division and professor of electrical engineering and computer sciences (EECS) at UC Berkeley, the team used a near-infrared detector to characterize the resulting color centers by probing their optical (photoluminescence) signals.

What they uncovered surprised them: a quantum emitter called the Ci center. Owing to its simple structure, stability at room temperature, and promising spin properties, the Ci center is an interesting spin photon qubit candidate that emits photons in the telecom band. We knew from the literature that Ci can be formed in silicon, but we didnt expect to actually make this new spin photon qubit candidate with our approach, Jhuria said.

The researchers learned that processing silicon with a low femtosecond laser intensity in the presence of hydrogen helped to create the Ci color centers. Further experiments showed that increasing the laser intensity can increase the mobility of hydrogen, which passivates undesirable color centers without damaging the silicon lattice, Schenkel explained.

A theoretical analysis performed by Liang Tan, staff scientist in Berkeley Labs Molecular Foundry, shows that the brightness of the Ci color center is boosted by several orders of magnitude in the presence of hydrogen, confirming their observations from laboratory experiments.

The femtosecond laser pulses can kick out hydrogen atoms or bring them back, allowing the programmable formation of desired optical qubits in precise locations, Jhuria said.

The team plans to use the technique to integrate optical qubits in quantum devices such as reflective cavities and waveguides, and to discover new spin photon qubit candidates with properties optimized for selected applications.

Now that we can reliably make color centers, we want to get different qubits to talk to each other which is an embodiment of quantum entanglement and see which ones perform the best. This is just the beginning, said Jhuria.

The ability to form qubits at programmable locations in a material like silicon that is available at scale is an exciting step towards practical quantum networking and computing, said Cameron Geddes, Director of the ATAP Division.

Theoretical analysis for the study was performed at the Department of EnergysNational Energy Research Scientific Computing Center (NERSC) at Berkeley Lab with support from the NERSC QIS@Perlmutterprogram.

The Molecular Foundry and NERSC are DOE Office of Science user facilities at Berkeley Lab.

This work was supported by the DOE Office of Fusion Energy Sciences.

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Lawrence Berkeley National Laboratory (Berkeley Lab) is committed to delivering solutions for humankind through research in clean energy, a healthy planet, and discovery science. Founded in 1931 on the belief that the biggest problems are best addressed by teams, Berkeley Lab and its scientists have been recognized with 16 Nobel Prizes. Researchers from around the world rely on the Labs world-class scientific facilities for their own pioneering research. Berkeley Lab is a multiprogram national laboratory managed by the University of California for the U.S. Department of Energys Office of Science.

DOEs Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visitenergy.gov/science.

Nature Communications

Experimental study

Not applicable

Programmable quantum emitter formation in silicon

27-May-2024

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New technique could help build quantum computers of the future - EurekAlert

Say quantum technologies and most people probably still imagine something decades into the future. But, as a new report released today demonstrates, quantum is already here especially as it relates to the telecom industry.

After years of incremental progress confined to research institutions, the emerging quantum technology sector has begun to gather commercial momentum. While most of the developments have been related to the quantum computing domain and its future promises, there are many other use cases for quantum tech applicable already today.

Quantum communications, including networks and forms of encryption, are currently being commercialised by a growing number of major telecom industry players and startups throughout the world. And Europe has a major part to play.

According to a report released today by Infinity, a startup and ecosystem support branch of Quantum Delta NL, 32% of the 100 quantum startups, scaleups, and SMEs servicing the telecom and telecom infrastructure sector are based in continental Europe. Germany, the Netherlands, France, Switzerland, and Spain are the strongest ecosystems. An additional 14% are in the UK and Ireland.

In addition, 50% of the enterprises that serve as consumers of the technology are located in continental Europe, with a further 11% in the UK and Ireland. Indeed, there are already more than 25 quantum networks being deployed in Europe today.

This includes a commercial quantum network in London, launched through a partnership between BT and Toshiba Europe, and an EU-wide quantum communications network being developed by Deutsche Telekom and two consortia called Petrus and Nostradamus.

Telecom companies are becoming a driving force for real-world adoption of quantum technology, said Teun van der Veen, Quantum Lead at the Netherlands Organisation for Applied Scientific Research (TNO). They are at the forefront of integrating quantum into existing infrastructures and for them it is all about addressing end-user needs.

Quantum networks utilise the unique properties of quantum mechanics such as superposition and entanglement to connect systems and transmit data securely. This is done through quantum channels, which can be implemented using optical fibres, free-space optics, or satellite links.

The promise of quantum networks and quantum encryption is that they would be near-impossible, if not entirely impossible, to hack, thus offering ultra-secure forms of communication.

As Infinitys report states, they can be used to establish quantum-secure links between data centres, Earth and spacecraft and satellites, military and governments, trains and rail network control centres, hospital and health care sites, etc.

Quantum networks can also form the backbone of a global quantum internet, connecting quantum computers in different locations. Furthermore, they can offer opportunities for blind cloud quantum computing, which keeps quantum operations a secret to everyone but the user.

With geopolitical tensions on the rise and looming cybersecurity threats, companies and governments are increasingly looking into ways of securing IT infrastructure and data.

Perhaps unsurprisingly then, Infinitys report finds that Quantum Key Distribution (QKD) is the most popular use of quantum technology in the telecom sector. QKD utilises quantum mechanics to allow parties to generate a key that is known only to them, and is used to encrypt and decrypt messages.

One startup that knows a lot about QKD technology is Q*Bird. The Delft-based communications security company just raised 2.5mn to further develop its QKD product Falqon, already in trial with the Port of Rotterdam (the largest port in Europe).

Quantum communications solutions see increased interest across digital infrastructure in the EU, said Ingrid Romijn, co-founder and CEO of Q*Bird. Together with partners like Cisco, Eurofiber, Intermax, Single Quantum, Portbase and InnovationQuarter, Q*Bird is already testing quantum secure communications in the Port of Rotterdam using our novel quantum cryptography (QKD) technology.

Romjin further stated that moving forward, more industries and companies will be able to implement scalable solutions protecting data communications, leveraging next-generation QKD technology.

Another technology garnering interest is post-quantum cryptography (PQC). Q-day (the day when a quantum computer breaks the internet) is, in all probability, still some way into the future.

However, most classical cryptography methods will be vulnerable to hacking from a sufficiently powerful quantum computer sooner. PQC algorithms are designed to be secure against both classical and quantum attacks.

Other technologies with potential applications for the telecom industry are quantum sensors, clocks, simulation, random number generation, and, naturally, quantum computing.

Meanwhile, despite the increasing market interest, the report also finds that Europes quantum technology startups require more support and investment to help achieve the technical and market breakthroughs to drive the field forward.

Currently, only 42% of the quantum tech for telecom startups worldwide have external funding, having raised a total of 1.9bn between them.And despite the relative forward-thinking approach of the EU as demonstrated by the Deutsche Telekom network project, the US still leads in terms of private sector activity and investment.

Other challenges include raising awareness among business leaders, increasing skilled workforce, overcoming technical limitations, and building a stronger business narrative.

These can be surmounted partially through more regulatory standardisation, more collaboration with industry, and more early-stage support and investment for startups, the report says.

The key market opportunities for the quantum communications sector going forward are in government bodies including military and security services, financial institutions, and critical infrastructure departments, as well as companies in the energy, defence, space, and technology sectors.

Growing collaboration between enterprises and startups in telecom signals the industrys commitment to integrating quantum solutions into commercial applications, said Pavel Kalinin, Operations and Platforms Lead at Infinity. Successful implementation of such technologies will depend on coordinated efforts to prepare the workforce, facilitate collaborations, and set industry benchmarks and standards.

You can read the report in its entirety here.

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European telecoms leading the way in quantum tech adoption, report finds - TNW

The electrical grid is very complicated. Nobody thinks about it ever until it doesn't work. But it is critical infrastructure that runs minute-to-minute energy being consumed now was generated milliseconds ago, somewhere far away, instantaneously shot through power lines and delivered.

This gets more complicated when locally generated sustainable energy joins the mix, pushing it beyond the capabilities of classical computing solutions. Home energy supplier E.ON is trialing quantum computer solutions to manage this future grid.

Speaking at the AI Summit London, E.ON chief quantum scientist Corey OMeara explained the challenges presented by future decentralized grids.

The way grids are changing now is, if buildings have solar panels on the roofs, you want to use that renewable energy yourself, or you might want to inject that back into the grid to power your neighbor's house, he said.

This decentralized energy production and peer-to-peer energy-sharing model presents a massive overhead for an aging grid that was never meant to be digital. E.ON is working on solving this renewable energy integration optimization problem using quantum computing.

E.ON also uses AI extensively and some functions could in the future be enhanced using quantum computing. An important example is AI-driven predictive maintenance for power plants.

Related:Unilever's Alberto Prado on Quantum Computing's Future, Impact on Emerging Tech

Power plants are complex objects that have thousands of sensors that measure and monitor factors such as temperatures and pressures and store the data in the cloud. We have AI solutions to analyze them to make sure that they're functioning correctly, said OMeara.

We published a paper where we invented a novel anomaly detection algorithm using quantum computing as a subroutine. We used it with our gas turbine data as well as academic benchmark data sets from the computer science field and found that the quantum-augmented solution did perform better but only for certain metrics.

E.ON plans to develop this trial into an integrated quantum software solution that could run on today's noisy, intermediate-scale quantum computers rather than waiting for next-generation fully error-corrected devices.

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Quantum, AI Combine to Transform Energy Generation, AI Summit London - AI Business

Last updated: June 13, 2024 09:00 EDT | 11 min read

Quantum computers might sound like another buzzword in the tech world, yet their threat to cryptocurrency is very real and approaching fast. Scientists may differ on the timeline, but they all agree: Q-day is not a matter of if, but when.

Weve spoken to quantum experts around the world to hear the latest estimates on when it will happen, what can be done to protect cryptocurrency, and whether these powerful machines could somehow benefit the crypto world.

Unlike traditional computers, which use bits as the smallest unit of data, each bit being a 1 or a 0, quantum computers use quantum bits, or qubits. These qubits can exist in 0 and 1 states or in multiple states at oncea property called superposition.

This allows quantum computers to perform calculations simultaneously and process large amounts of data much faster than standard computers.

As quantum computers can hold and process many possible outcomes at once, it reduces the time needed to solve problems that depend on trying many different solutions, such as factoring large numbers, which is the foundation of most cryptocurrency encryption.

Factoring large numbers, or integer factorization, is a mathematical process of breaking down a large number into smaller, simpler numbers called factors, which, when multiplied together, result in the original number. The process is called prime factorization if these integers are further restricted to prime numbers.

In cryptocurrency, security heavily relies on the mathematical relationship between private and public keys. A public key is a long string of characters associated with the wallet address. It can be shared openly. A private key, used to sign transactions, must remain confidential. This mathematical relationship is one-way, meaning that a public key can be derived from the private key but not the other way around.

Itan Barmes, who is the Global quantum cyber readiness capability lead at Deloitte, explained in a conversation with Cryptonews:

The quantum computer breaks this one-way relationship between the two. So, if you have someones public key, you can calculate their private key, impersonate them, transfer their funds elsewhere.

The task is currently nearly impossible for conventional computers. However, in 1999, mathematician Peter Shor showed that a quantum computer could solve the factoring problem much faster. Shors algorithm can also solve the Discrete Logarithm Problem, which is the basis for the security of most blockchains. This means if such a powerful quantum computer existed, it could break the cryptocurrency security model.

Not all cryptocurrencies would face the same level of risk from quantum attacks. In 2020, Itan Barmes and a team of Deloitte researchers examined the entire Bitcoin blockchain to determine how many coins were vulnerable. They discovered that about 25% of Bitcoins could be at risk.

Pay To Public Key (P2PK)

Pay to Pubkey Hash (P2PKH)

These addresses directly use the public key, making them visible and vulnerable to quantum attacks.

These addresses use a cryptographic hash of the public key. They dont expose the public key directly until coins are moved.

Vulnerable coins include those held in P2PK (Pay To Public Key) addresses, which directly expose the public key, making them easy targets for a quantum attack. Coins in reused P2PKH (Pay to Pubkey Hash) addresses are also at risk because these addresses display their public key when the owner moves the funds. This attack is called the storage attack, as it applies to coins residing in static addresses. Itan Barmes further explained:

A quantum attack only applies to specific coins, not everything. If we conducted the same research today, the percentage of vulnerable coins would be lower because the number of vulnerable addresses remains more or less the same, but due to mining, there are more coins in circulation.

Itan Barmes added that in addition to the storage attack, there is also an attack on active transactions, as the public key is exposed for the first time.

Such an attack must be performed within the mining time (for Bitcoin, around 10 minutes), which adds a requirement for the quantum computer to not only be powerful enough but also fast. This so-called transit attack is likely to be possible later than the storage attack due to this additional requirement.

Ideally, Bitcoin users must generate a new address for each transaction. Yet, recent research by Bitmex suggests that about 50% of transaction outputs still go to previously used addresses, which means the practice of address reuse is more common in Bitcoin transactions than we may think.

Are we nearing the point where quantum computers can pose a real threat? In 2017, a group of researchers, including Divesh Aggarwal and Gavin Brennen, published an article warning that the elliptic curve signature scheme used by Bitcoin could be completely broken by a quantum computer as early as 2027, by the most optimistic estimates.

Cryptonews reached out to the authors to ask whether their estimation has shifted. Gavin Brennen from Macquarie University in Australia replied that although a lot has changed in quantum computing space since then, the basic message is still the same:

Quantum computers pose a threat to blockchains, primarily by attacks on digital signatures, and cryptocurrencies should get started sooner rather than later to upgrade their systems to use post-quantum cryptography before their asset valuations are threatened.

To be able to break cryptocurrency security, quantum computers will likely need thousands, if not millions, of qubits. Currently, the most advanced machines have around 1000.

Another critical challenge is error reduction. Quantum bits are highly sensitive to their environment; even the slightest disturbance, like a change in temperature or vibration, can cause errors in computations, a problem known as quantum decoherence.

Dozens of companies, both public and private, are now actively advancing the development of large quantum computers. IBM has ambitious plans to build a 100,000-qubit chipset and 100 million gates by the end of this decade.

PsiQuantum aims to achieve 1 million photonic qubits within the same timeframe. Quantum gate fidelities and quantum error correction have also significantly advanced. Gavin Brennen continued:

What all this means is that estimates on the size of quantum computers needed to crack the 256-bit elliptic curve digital signatures used in Bitcoin have dropped from 10-20 million qubits to around a million. One article published by the French quantum startup Alice & Bob estimates that it could be cracked with 126,000 physical qubits, though that does assume a highly specialized error model for the quantum computer. In my opinion, a plausible timeline for cracking 256-bit digital signatures is by the mid-2030s.

Gavin Brennen added that substantial technological improvements would be required to reduce all types of gate errors, connect modules, and combine fast classical and quantum control, which is a challenging but surmountable problem.

Yet, if quantum technology becomes powerful enough to break cryptocurrency security, we may not even know about it, believes Marcos Allende, a quantum physicist and CTO of the LACChain Global Alliance. In an email conversation with Cryptonews, Allende wrote:

What is certain is that those who reach that power first will use it silently, making it impossible to guess that selected hackings are happening because of having quantum computers.

Many scientists remain skeptical about the quantum threat to cryptocurrency. Winfried Hensinger, a physicist at the University of Sussex in Brighton, UK, speaking to Nature magazine, described quantum computers as Theyre all terrible. They cant do anything useful.

Several challenges keep quantum computing from reaching its full potential. The delicate nature of qubits makes it difficult to maintain them in a quantum state for extended periods. Another challenge is cooling requirements. Many quantum processors must operate at temperatures close to absolute zero, which means they need complicated and costly refrigeration technology. Finally, the quantum systems would need to be integrated with the existing classical ones.

Just having 200 million qubits not connected to each other is not going to do anything. There are a lot of fundamental physics problems that need to be resolved before we get there. We are still very much at the beginning. But even in the past year, theres been tremendous improvement. The technology can accelerate in a way that all the timelines will be much shorter than we expect, Itan Barmes told Cryptonews.

Tommie van der Bosch, Partner at Deloitte and Blockchain & Digital Asset Leader of Deloitte North and South Europe, believes that the question is not if quantum computing will break cryptocurrency security but when: The fact that its a possibility is enough to start taking action. You should have a plan.

Indeed, this year several key crypto companies and the World Economic Forum (WEF) have shared concerns about the implications of quantum computing on cryptocurrency security.

The WEF, in its post published in May, warned that central bank digital currency (CBDC) could become a prime target for quantum attacks. Ripples recent report has also said that quantum computers could break the digital signatures that currently protect blockchain assets.

Earlier this year, Buterin, Ethereum founder, suggested the Ethereum blockchain would need to undergo a recovery fork to avoid the scenario when bad actors already have access to them and are able to use them to steal users funds.

To protect against these potential quantum attacks, blockchain systems will need to integrate post-quantum cryptographic algorithms. However, incorporating them into existing blockchain protocols is not easy.

New cryptographic methods must first be developed, tested, and standardized. This process can take years and requires the consensus of the cryptographic community to ensure the new methods are secure and efficient.

In 2016, the National Institute of Standards and Technology (NIST) started a project to set new standards for post-quantum cryptography. The project aims to finalize these standards later this year. In 2022, three digital signature methodsCRYSTALS-Dilithium, FALCON, and SPHINCS+were chosen for standardization.

Once standardized, these new cryptographic algorithms need to be implemented within the blockchains existing framework. After that, all network participants need to adopt the updated protocol.

Itan Barmes explained, Lets say someone could tell us exactly the date, three years from now, when we will have these kinds of quantum computers. How quickly do you think we can change the Bitcoin protocol to make it resilient to these attacks? The decentralized governance of Bitcoin can turn out to be a double-edged sword, by preventing timely action.

Quantum-resistant algorithms often require more processing power and larger key sizes, which could lead to performance issues on the blockchain. These include slower transaction times and increased computational requirements for mining and verification processes.

Tommie van der Bosch told Cryptonews that, ultimately, the rise of quantum computing could affect the entire economic model of cryptocurrencies.

Coins that upgrade to quantum-resistant protocols in time might gain a competitive advantage. Investors and users could prefer these quantum-safe cryptocurrencies, as they may see them as more secure long-term holdings. This shift could lead to an increase in demand for such cryptocurrencies, potentially enhancing their value and market share compared to those that are slower to adapt. Tommie van der Bosch told Cryptonews:

Lets draw a parallel with the banking system. Weve all seen the effects of a bank collapsing or even the rumor of one. Your money suddenly seems at risk. How quickly do people shift their assets? It can trigger a domino effect.

The development of quantum computing could also bring regulatory changes. Regulators could start enforcing stricter standards around trading and custody of cryptocurrencies that havent updated their cryptographic protocols. Such measures would aim to protect investors from sinking funds into potentially vulnerable assets.

Itan Barmes remarked, Not many people are aware that the cryptographic algorithm used in Bitcoin and essentially all cryptocurrencies is not part of the NIST recommendation (NIST SP800-186). The issue is already present if organizations require compliance to NIST standards. The issue becomes even more complex if algorithms need to be replaced; Whos responsibility is it to replace them?

Could quantum computing actually benefit the cryptocurrency industry? Gavin Brennen suggests it might. In an email exchange with Cryptonews, Brennen discussed the development of quantum-enabled blockchains.

Quantum computers could accelerate mining, although Brennen notes that the improvement over traditional mining rigs would be limited and require quantum computers with hundreds of millions of qubitsfar beyond current capabilities.

New computational problems have been suggested, like the boson sampling problem, that are slow for all types of classical computers but would be fast on a quantum device. Interestingly, the boson sampler is a small, specialized processor using photons of light, that is not as powerful as a full quantum computer, but much cheaper to build, and that solves a problem immune to ASIC speedups with an energy footprint that is orders of magnitude lower for reaching PoW consensus.

Currently, proof-of-work (PoW) requires vast amounts of electrical power for mining, raising concerns about sustainability and environmental impact. Boson sampling could become a greener alternative, significantly reducing the energy footprint of blockchain operations while maintaining security and efficiency.

The rest is here:
Quantum Computers May Break Bitcoin by 2030, But We Won't Know About It - Cryptonews

A novel vortex phenomenon involving photon interactions was identified by scientists, potentially enhancing quantum computing. Through experiments with dense rubidium gas, they observed unique phase shifts that mimic other vortices but are distinct in their quantum implications. Credit: SciTechDaily.com

Researchers at the Weizmann Institute of Science discovered a new type of vortex formed by photon interactions, which could advance quantum computing.

Vortices are a widespread natural phenomenon, observable in the swirling formations of galaxies, tornadoes, and hurricanes, as well as in simpler settings like a stirring cup of tea or the water spiraling down a bathtub drain. Typically, vortices arise when a rapidly moving substance such as air or water meets a slower-moving area, creating a circular motion around a fixed axis. Essentially, vortices serve to reconcile the differences in flow speeds between adjoining regions.

A vortex ring and lines created by the influence of three photons on one another. The color describes the phase of the electric field, which completes a 360-degree rotation around the vortex core. Credit: Weizmann Institute of Science

A previously unknown type of vortex has now been discovered in a study, published in Science, conducted by Dr. Lee Drori, Dr. Bankim Chandra Das, Tomer Danino Zohar, and Dr. Gal Winer from Prof. Ofer Firstenbergs laboratory at the Weizmann Institute of Sciences Physics of Complex Systems Department. The researchers set out to look for an efficient way of using photons to process data in quantum computers and found something unexpected: They realized that in the rare event that two photons interact, they create vortices. Not only does this discovery add to the fundamental understanding of vortices, it may ultimately contribute to the studys original goal of improving data processing in quantum computing.

The interaction between photons light particles that also behave like waves is only possible in the presence of matter that serves as an intermediary. In their experiment, the researchers forced photons to interact by creating a unique environment: a 10-centimeter glass cell that was completely empty, save for rubidium atoms that were so tightly packed in the center of the container that they formed a small, dense gas cloud about 1 millimeter long. The researchers fired more and more photons through this cloud, examined their state after they had passed through it, and looked to see if they had influenced one another in any way.

When the gas cloud was at its densest and the photons were close to each other, they exerted the highest level of mutual influence.

When the photons pass through the dense gas cloud, they send a number of atoms into electronically excited states known as Rydberg states, Firstenberg explains. In these states, one of the electrons in the atom starts moving in an orbit that is 1,000 times wider than the diameter of an unexcited atom. This electron creates an electric field that influences a huge number of adjacent atoms, turning them into a kind of imaginary glass ball.

The image of a glass ball reflects the fact that the second photon present in the area cannot ignore the environment the first photon has created and, in response, it alters its speed as if it has passed through glass. So, when two photons pass relatively close to each other, they move at a different speed than they would have if each had been traveling alone. And when the speed of the photon changes, so does the position of the peaks and valleys of the wave it carries. In the optimal case for the use of photons in quantum computing, the positions of the peaks and valleys become completely inverted relative to one another, owing to the influence the photons have on each other a phenomenon known as a 180-degree phase shift.

From bottom left, clockwise: Dr. Lee Drori, Tomer Danino Zohar, Dr. Alexander Poddubny, Prof. Ofer Firstenberg, Dr. Gal Winer, Dr. Eilon Poem and Dr. Bankim Chandra Das. Credit: Weizmann Institute of Science

The direction that the research took was as unique and extraordinary as the paths of the photons in the gas cloud. The study, which also included Dr. Eilon Poem and Dr. Alexander Poddubny, began eight years ago and has seen two generations of doctoral students pass through Firstenbergs laboratory.

Over time, the Weizmann scientists managed to create a dense, ultracold gas cloud, packed with atoms. As a result, they achieved something unprecedented: photons that underwent a phase shift of 180-degrees and sometimes more. When the gas cloud was at its densest and the photons were close to each other, they exerted the highest level of mutual influence. But when the photons moved away from each other or the atomic density around them dropped, the phase shift weakened and disappeared.

The prevalent assumption was that this weakening would be a gradual process, but researchers were in for a surprise: A pair of vortices developed when two photons were a certain distance apart. In each of these vortices, the photons completed a 360-degree phase shift and, at their center there were almost no photons at all just as in the dark center we know from other vortices.

The scientists found that the presence of a single photon affected 50,000 atoms, which in turn influenced the motion of a second photon.

To understand photon vortices, think of what happens when you drag a vertically held plate through the water. The rapid movement of the water pushed by the plate meets the slower movement around it. This creates two vortices that, when viewed from above, appear to be moving together along the waters surface, but in fact, they are part of a three-dimensional configuration known as a vortex ring: The submerged part of the plate creates half a ring, which connects the two vortices visible on the surface, forcing them to move together.

Another familiar instance of vortex rings is smoke rings. In the last stages of the study, the researchers observed this phenomenon when they introduced a third photon, which added an extra dimension to the findings: The scientists discovered that the two vortices observed when measuring two photons are part of a three-dimensional vortex ring generated by the mutual influence of the three photons. These findings demonstrate just how similar the newly discovered vortices are to those known from other environments.

The vortices may have stolen the show in this study, but the researchers are continuing to work toward their goal of quantum data processing. The next stage of the study will be to fire the photons into each other and measure the phase shift of each photon separately. Depending on the strength of the phase shifts, the photons could be used as qubits the basic units of information in quantum computing. Unlike the units of regular computer memory, which can either be 0 or 1, quantum bits can represent a range of values between 0 and 1 simultaneously.

Reference: Quantum vortices of strongly interacting photons by Lee Drori, Bankim Chandra Das, Tomer Danino Zohar, Gal Winer, Eilon Poem, Alexander Poddubny and Ofer Firstenberg, 13 July 2023,Science. DOI: 10.1126/science.adh5315

Prof. Ofer Firstenbergs research is supported by the Leona M. and Harry B. Helmsley Charitable Trust, the Shimon and Golde Picker Weizmann Annual Grant and the Laboratory in Memory of Leon and Blacky Broder, Switzerland.

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Vortex Power: The Swirl of Light Revolutionizing Quantum Computing - SciTechDaily