Why quantum efficiency matters for quantum networks 

The silicon T centre is a colour-centre that emits light in the telecommunications band and hosts long-lived electron and nuclear spin qubits. This makes it a promising candidate for spin–photon interfaces: quantum information can be encoded in the electron spin, transferred to a long-lived nuclear spin for storage, and mapped onto telecom photons that distribute entanglement across metropolitan-scale fibre networks or between processors in a quantum computing data centre.

For such architectures to scale, photon generation must be both probable and repeatable. Photons must be emitted with high probability when the T centre is excited, and the same spin must be read out many times, via repeated photon emission, without resetting the entire entanglement process. This requirement is captured by a single key metric: quantum efficiency.

A giant isotope effect 

This work shows that the quantum efficiency of the T centre can be improved considerably through isotopic engineering. Replacing the T centre’s protium hydrogen with deuterium increases the observed excited-state lifetime by more than a factor of five. This dramatic change indicates a strong suppression of nonradiative recombination, long recognized as the dominant loss mechanism in protium T centres.

Using first-principles calculations, performed in collaboration with colleagues at the US Naval Research Laboratory, the origin of this effect was identified: the dominant nonradiative decay pathway involves energy transfer to vibrations of the silicon lattice. Substituting protium with the heavier isotope deuterium reduces the vibrational energy of the carbon–hydrogen bond in the defect. As a result, nonradiative decay from the excited state requires the simultaneous excitation of many more vibrational quanta. Therefore, the excited energy is much more likely to be emitted as photons, yielding an emission efficiency approaching 100%.

What this enables 

This discovery places the deuterium T centre among the most efficient quantum emitters in silicon. Near-unity emission efficiency directly enhances single-photon sources, spin–photon interfaces, and entanglement distribution protocols, significantly strengthening the case for silicon-based quantum networking and scalable quantum computing architectures.

Key Takeaways

1. The giant isotope effect improves T centre lifetime and efficiency
2. Vibrational loss is identified as the main limitation
3. Near-unity emission efficiency is enabled by deuteration

We gratefully acknowledge the contributions of all collaborators involved in this research.  Read the published paper on Physical Review Letters or the open-access version on arXiv.

Related content: To discover how T centres in silicon work as spin‑photon qubits, their telecom‑band emission advantage, and how they enable highly scalable quantum systems, read our blog: What Is a T Centre?

The post Investigating Deuterium to Boost T Centre Performance first appeared on Photonic.

This work determines the hyperfine tensor that governs the energy level splittings in the ground state of the T centre, involving the electron spin ‘communications’ qubit and the hydrogen nuclear spin ‘memory’ qubit. These splittings differ depending on magnetic field direction and magnitude. To determine the hyperfine tensor, optically-detected magnetic resonance measurements were taken for magnetic fields of various directions and magnitudes using an isotopically-purified bulk Silicon-28 sample.  

In the excited state, the nuclear spin is much more weakly coupled. As a result, the nuclear spin can experience spin flips or phase errors during optical cycles due to the change in hyperfine coupling between the ground and excited states. The T centre hydrogen hyperfine possesses opposite-sign principal values, which allows for elimination of both spin flips and phase errors through magnetic field selection, a trait not common amongst spin-photon interfaces. This work determines the magnetic field such that these errors are asymptotically eliminated for an operational sequence.  

Related content: To discover how T centres in silicon work as spin‑photon qubits, their telecom‑band emission advantage, and how they enable highly scalable quantum systems, read our blog: What Is a T Centre?

The post Exploring Magnetic Field Control for More Robust Entanglement Between T Centres first appeared on Photonic.

T centres are a powerful qubit modality based on color centres in silicon. T centres are unique in matching many of the best characteristics of other modalities (coherence times, gate speed, photonic interconnects, etc.), with additional advantages of their own. They are also highly practical—T centres are fabricated in a silicon matrix, making it much easier to implement the vast array of control and support systems needed to stabilize and operate a qubit. Most importantly, T centres have a built-in ability to connect with one another using light, which is the key to efficient error-correction codes and networking between quantum processors.

What is the structure of a T centre?

A T centre is a specific type of silicon colour centre—an atomic-level implanted structure manufactured in a silicon substrate. They exhibit special optical and spin properties that make them work well as qubits, including the ability to absorb and emit light at specific wavelengths. Colour centres in silicon are especially useful, as they interface seamlessly with standard photonic integrated circuit technology.

Each T centre is composed of a hydrogen atom and two carbon atoms with an associated electron, all squeezed into the place a one silicon atom would normally occupy. This compact structure gives a T centre a powerful range of qubit capabilities, all housed in a single, easily addressable silicon host.

Due to the T centre’s special structure, the carbon and hydrogen atoms can each function as nuclear spin qubits, with long coherence times ideal for memory and long-term quantum information storage (a benefit associated with diamond NV centres and other semiconductor spin qubits).

Another key part of this structure is the electron – it has its own spin which can be manipulated and coupled to other qubits on fast timescales (a property for which quantum dot qubits are known). The electron in a silicon T centre has the added advantage that it absorbs and emits light at a low-loss telecom wavelength, making it perfect for communicating directly with other T centres over fibre without the additional conversion steps required by some other modalities.  For this reason, they are often referred to as photonically-linked silicon spin qubits.

The surrounding silicon also plays a key role, making it easier to stabilize, control, and connect the T centres. Silicon is the most widely used material in semiconductor manufacturing, so the techniques needed to control qubits like microwaves and lasers are already well established and mass-producible at high densities. T centres are also naturally isolated and stabilized by their silicon host, which makes them less susceptible to thermal interference. As a result, they require less cooling than many other qubits, simplifying system design and reducing cost.

What makes a T centre special?

Taken together, a T centre has an ideal set of attributes for quantum computing: a total of four spins that can be used as qubits for computation and memory, efficient communication with other qubits at telecom wavelengths over standard fibre networks, control and connection via existing semiconductor PIC technology, high qubit densities in a single chip, and relatively low cooling requirements.

These properties, in turn, allow easier and more robust entanglement between qubits—key to quantum computing—and facilitate modular, distributed quantum computing, making it easier to scale up and scale out to commercial utility.

When compared to other qubit modalities, T centres offer a strong hybrid of the best qualities, and benefit additionally from the manufacturability of silicon. As a result, considerable research is being done on T centres and similar colour centres globally, and new startups are emerging in industry.

Photonic Inc: Leading distributed quantum computing with T centres

Photonic Inc. was founded to leverage the unique properties of T centres for quantum computing and intends to be the first to create a distributed quantum computing platform at commercial scale.

The company is scaling up and scaling out each aspect needed to create a full-stack quantum computer – qubit, control, coding, and connectivity – taking a highly practical approach to delivering on commercial-scale quantum computing. One that is more manufacturable, with less implementation hurdles (engineering vs science), and lower final cost of ownership. One that can be scaled and networked much easier than other qubit modalities. At Photonic, we believe T centres will enable the quantum networks of the future.

To learn more about how T centres enable commercial-scale distributed quantum computing, read our white paper: What could networks of quantum supercomputers look like? 

The Mission to Design the Ideal Qubit
There are several types of silicon colour centres, and many more are being actively explored for quantum computing potential. Photonic’s founders, Dr. Stephanie Simmons and Dr. Michael Thewalt, searched extensively for a colour centre with the right qubit properties and entanglement capabilities needed to enable quantum networking.

The post What is a T Centre? first appeared on Photonic.

Related content: To discover how T centers in silicon work as spin‑photon qubits, their telecom‑band emission advantage, and how they enable highly scalable quantum systems, read our blog: What Is a T Centre?

The post The T Centre Tango: Understanding how laser light couples to both the T centre and its environment first appeared on Photonic.

The timeline to useful quantum computing just got shorter. Introducing SHYPS: the first QLDPC code family that can efficiently perform error-corrected quantum computation. The patent pending SHYPS codes use materially fewer qubits per logical qubit than traditional surface codes.

Qubits—as quantum bits, the foundational building blocks for quantum computers are known—are notoriously susceptible to “noise” from their environment. Left alone, this noise makes the entire system unreliable. To reach the point where quantum systems can run commercial-scale applications, quantum “error correction” is required to address the high sensitivity to that noise.

The traditional method for quantum error correction has been surface codes, due to their impressive time efficiency and low connectivity requirements. However, the downside to surface codes is the huge overhead: thousands of physical qubits are needed for each application-grade logical qubit. This large ratio of physical to logical qubits has contributed to projections that put commercial-scale quantum computing decades in the future due to the sheer number and size of systems that would be required to run useful applications.

Another class of codes, Quantum Low-Density Parity Check (QLDPC) codes, emerged about 20 years ago as a promising means to lower overheads. However, researchers had been unable to discover an implementation of efficient quantum computing in these codes, leaving them useful for memory (i.e., storage for quantum states) but lacking the ability to perform the applications that will make quantum computing so impactful. Researchers spent over a decade on this challenge, and today Photonic’s new paper, “Computing Efficiently in QLDPC Codes,” is the first to demonstrate how to compute using SHYPS QLDPC codes. Unlocking efficient logic in a QLDPC code moves the goalposts for commercial-scale quantum computing 5x, 10x, even 20x closer as more efficient quantum error correction enables quantum computers to reach the computing capability that unlocks exponential algorithms with quantum advantage using vastly fewer physical resources.

Thank you to our collaborators at Microsoft for their work on this animated overview of how the SHYPS code is making waves:

On each of the requirements for efficient fault-tolerant quantum computing, SHYPS provides significant advancements or matches leading-edge performance of surface codes.

Efficient Logical Operations: Competitive with the current quantum standard set by surface codes
Physical to Logical Qubit Ratio: A 20x reduction in physical overhead
Single-Shot Capabilities: A 30x reduction in runtime
Fault-Tolerant Operations: Meets all formal requirements for all needed operations, necessary for real-world applicability
Good Error Suppression: Competitive performance, if matched on physical qubit count.

This fast and lean QLDPC code family has specific hardware requirements for implementation that not every approach to quantum computing can deliver. Photonic’s Entanglement Firstarchitecture provides the high levels of connectivity needed to realize the benefits of QLDPC codes.

Taking these out of the theoretical, Photonic’s SHYPS codes have been stress tested in the most complete simulations known to date, demonstrating that the logic works in practice, not just in theory. Better yet, this approach is implementable on distributed systems, working both within and between modules. This breakthrough simultaneously delivers the efficiency gains promised by QLDPC codes and removes a key barrier to commercially useful quantum applications.

When informed about this advancement, David Shaw, Lead Analyst at Global Quantum Intelligence, said “this is a truly major milestone. The quantum field must now be divided into those whose hardware can run these new codes, and those who can’t. We’re going to see a race between players that invest in the scarce skills required for in-house code innovation, and those that seek to be fast followers. Implementing logic always looked like the hard part of standing-up better codes. This new work has knocked it out of the park.”

Quantum applications are now 10x closer than previously thought.

Learn more about the SHYPS QLDPC code family and Photonic’s advances in efficient quantum error correction:

• For more discussion of these results and their impact, read “Launching SHYPS: QLDPC is the New Error Correction”
• For a deep dive, read the scientific paper “Computing Efficiently in QLDPC Codes”

The post Introducing SHYPS: Error Correction Codes to Accelerate the Timeline to Useful Quantum first appeared on Photonic.

In this research we show how T centres can be combined with diodes to create light-emitting diodes (LEDs) in the telecommunications O-band. By placing a single T centre in one of these LEDs, we show that we can produce single photons on demand with electrical pulses and couple them to an optical fibre. We also show that many T centres can be simultaneously excited in waveguides for use as an on-chip light source, producing hundreds of thousands of photons per second.

Single photons are desired for many other quantum technologies including all-optical quantum computers and quantum networks. Such a wide range of applications is made possible by the T centre’s unique advantages: combining telecommunications-band optical emission, long-lived spin qubits and silicon integration. In silicon it is possible to combine the photonic and electronic elements required for these devices. A silicon single-photon source can be networked on-chip with a wide range of well-established silicon photonic devices including detectors, switches and high-quality optical resonators. Previous electrically-triggered photon sources for silicon photonics relied on challenging and costly hybrid material processing.

What does this mean for future quantum technologies? We are always looking for new and innovative ways to increase the number of qubits on a chip, and their performance. Rather than routing laser pulses to every T centre, these new devices could be used to trigger entanglement electrically between many T centres simultaneously. In the future, this technology could also be used to entangle spin qubits. This approach may reduce the resources required for large-scale quantum computers. While optical controls will still be needed for some tasks, this parallel electrical control has the potential to increase the number of qubits that can fit on a chip and entangle them faster.

 

Key takeaways:

1. Single silicon T centres can be electrically excited, for single-photon sources directly integrated with silicon photonics.
2. Electrical initialization of the T centre spin qubit enables a new parallelizable control mechanism.
3. Future schemes that leverage electrical control may allow more qubits to fit on a chip, as well as faster entanglement.

 

We would like to acknowledge the contributions of all involved in this research.

The post Exciting Qubits: Electrically exciting T centres may allow for higher qubit density and faster entanglement first appeared on Photonic.

There are many examples of technological development from theory to commercial reality—of which air travel is one. Prior to 1903, the goal of sustained human flight was theoretical. From watching birds, bats and bugs, humans had long known flight was possible, but machines modelled on their wings (most famously done by da Vinci) proved unsuccessful for centuries. A shift to a fixed wing approach brought with it physical models that demonstrated aircraft flight was possible, but these models lacked a power source. The Wright brothers’ first 12 second flight proved that human flight via heavier-than-air vehicles was possible and, years later, improvements were made to aircraft that allowed sustained transatlantic passenger flight, initiating a global inevitability. However, it wasn’t until the development of jet engines, pressurized fuselages, and a network of international airports, that long distance, reliable, profitable transport of people and cargo by air became a reality.

Quantum computing, by commercializing a branch of physics, is following a similar, phased, development trajectory. What is especially exciting in the field now is the building momentum. In the past decades, quantum computing has moved from theory to possibility, and at Photonic, we’re focused on making distributed, fault-tolerant, quantum computing a global inevitability. 

Photonic’s approach is a combined technological platform for both quantum computing and networking; our architecture is optimized for entanglement distribution and leverages colour centre spins in silicon (T centres). Using low-overhead quantum error correction, and with a native optical interface that enables high performance non-local operations, silicon T centres can drastically accelerate the timeline for realizing modular, scalable, fault-tolerant quantum processors and repeaters. Photonic has been relentlessly pursuing (and achieving!) the technological milestones required for this end goal.

Photonic’s Quantum Approach: The Path to Possible 

Why do we place such importance on having a platform based on spins in silicon with a native optical interface, that can serve as a combined platform for both fault-tolerant quantum computing and quantum networking? Because we know that without the ability to reliably scale and distribute entanglement, the full value of quantum computing can’t be realized. A silicon-based platform allows us to leverage years of development to enable scalability of high-performance chips, and the optical interface allows us to plug into a system of optical interconnects to distribute entanglement between chips, leading to a fully integrated system based on research and tested technologies with years of optimization and engineering from other industries. 

Those familiar with quantum computing will know the path to scale is typically broken out into three phases, and that there exists variation in both naming (phases, levels, stages) and the specifics of each step. Photonic’s focus is, as it has always been, on quantum computing for commercial relevance, which we see unfolding with the availability of large numbers of logical qubits in the third phase. The Photonic approach to scale is distributed, where thousands of logical qubits can be networked across modules. Along the way to this point are these important developmental steps:

• NISQ (“noisy intermediate-scale quantum”): Dominated by the production of small numbers of qubits out of a variety of physical phenomena, NISQ prototypes were single module devices containing qubits that were too few or too noisy to implement quantum error correction effectively.
• Small-Scale Logical Qubits: In this phase, the introduction of error correction and resultant fault tolerance has brought us to our current state, with increases in the number and quality of logical qubits within a single computing module.
• Large-Scale Networked Logical Qubits: Complex quantum computations that require many logical qubits will be possible as systems will have efficient means of utilizing large numbers of identical, manufacturable, and high-quality logical qubits.

NISQ machines do not have enough high-quality qubits to reliably run algorithms that provide sufficient advantages over classical supercomputers. Specifically, they do not have the capacity to provide the required number or quality of qubits for quantum error correction, and without that capability, lack a path to delivering a system that is appreciably better than what can be achieved with classical systems. While the work done has been instrumental in exploring the potential of quantum computing, it hasn’t yielded any significant commercially relevant use cases to date.  

Presently we are just seeing the start of very active error corrected quantum demonstrations. Small-scale fault tolerant qubits have relatively recently become a reality. We are in the ‘pre-dominant design explosion’ of methods as demonstrated by the diversity of approaches in the quantum computing ecosystem. We will see single quantum modules demonstrate quantum error correction (QEC) protocols such as surface code or QLPDC codes. There may be useful scientific results to emerge from these computers with small numbers of logical qubits, however, the known high-value use cases of commercial relevance (e.g. Shor’s algorithm) require more qubits.   

It is with the arrival of large-scale quantum computing that the industry will see systems capable of implementing algorithms that provide exponential speedups for tackling complex computational challenges and market demand for high-value, high-impact use cases such as drug discovery, materials science, and catalyst development. One approach to effective performance at this scale is to design quantum platforms to work in a distributed, networked manner, with operations acting within and between modules. Operations that act on more than one module consume distributed entanglement. Commercially viable modular quantum platforms will need to be able to distribute entanglement at a rate that meets or exceeds the needed amount to avoid bottlenecks in computation time. High connectivity allows for the distribution of entanglement directly to where it’s needed rather than taking a winding route through multiple connections (akin to flying direct rather than having multiple stopovers). So, a critical technical consideration for distributed quantum computing is the total entanglement distribution bandwidth between modules.  

Inevitable Progress to Scale and Beyond

At Photonic, we’ve been working from a quantum systems engineering perspective, starting with what a scalable, distributed, quantum computing network would require. We’ve focused on these requirements (e.g., telecom connectivity, data centre compatibility) to build a platform to accommodate both performance of the individual computing module and the scalability of the network.  

Our architecture is designed to enable horizontal scalability while optimizing for entanglement distribution, harnessing the potential of silicon colour centres. The telecom, silicon-integrated approach we have taken at Photonic has intrinsic capabilities in this regard, as colour centres (photons and spins) are extremely promising for high-fidelity operation, while high connectivity brings fault tolerance within reach with low overheads.  

Our native optical interface produces entangled photons that can be used to distribute entanglement at telecom wavelength so that no transduction is required to use optical interconnects between modules. The additional benefit of operating at telecom wavelengths is the ability to integrate with existing infrastructure – namely optical switch networks that have been developed for telecommunications. As such, Photonic’s architecture is well suited to quickly and reliably execute large-scale algorithms across multiple modules using a fast, reliable, quantum network.

Photonic’s Strategy on Scale

Photonic’s strategy lies in taking the most direct path to realizing the true potential of large-scale, distributed, fault-tolerant systems for commercially relevant quantum computing applications.   

• As the interest in NISQ-based tech declines, we’re seeing a highly competitive race to deliver commercial value, especially in the form of logical qubit development. There will increasingly be expectations for fault-tolerant qubits, and the appetite for them will grow exponentially. 
• Many companies and researchers are making great progress in small scale fault tolerant prototypes. Each milestone reached benefits the industry as a whole. We’re all able to learn from innovations in materials, qubits, and better understand the strengths and limits of various approaches. 
• There is ever increasing desire to get to a stage where the key, known, commercially valuable algorithms are implementable; on our path, this necessitates a functional, modular, system. At this stage, we anticipate platforms capable of distributed quantum computing will thrive. We are encouraged to see the industry also acknowledging the necessity of interconnected quantum modules. A recent Global Quantum Intelligence Outlook Report on Scalable Quantum Hardware released the following conclusion:

The report highlights the necessity for a modular approach to scaling in nearly all proposed quantum computing architectures. This modular approach, which emphasizes distributed rather than monolithic quantum computing stacks, offers not only scalability but also flexibility, maintainability, and redundancy. It also emphasizes how most architectures will ultimately need to leverage interconnects, and how performant optical photonic interconnects hold the promise of synergies in quantum communications and networking.

We cannot overstate how exciting it is to be on the cusp of technological advancements that will have impacts that we can’t yet fully fathom. Just as the Wright brothers and others at the earliest frontiers of flight were likely not able to consider the range of applications from specialized water bombers to fighter jets to planes capable of carrying 250 tons of cargo or conducting edge of space flights, our understanding of the potential of quantum computing is just beginning. From catalyst discovery for new fuels, materials science implication for solar energy capture and battery development, and pharmaceutical design for medical advancement, there are seemingly endless possibilities. We are dedicated to developing quantum computing and networking capacity to unleash the maximum potential for doing good in the world.

The post From Quantum Theory to Quantum Practice first appeared on Photonic.

However, this answer may raise other questions for those less familiar with the inner workings of quantum technologies: What is entanglement, how is it established, and how is it consumed? This video, created with our partners at Microsoft, illustrates the three steps that Photonic took to establish, distribute and consume entanglement in our demonstration.  

The post Distributed Quantum Entanglement first appeared on Photonic.

The following white paper opens with the discussion of the three distinct phases of quantum technology development towards commercially-relevant use cases. Then it introduces distributed quantum entanglement and the role it plays in enabling large scale fault-tolerant quantum computing and networking. Finally, the white paper provides an easy to follow description of Photonic’s recent demonstration of distributed entanglement generation and consumption for executing distributed quantum computing gates—the foundation of distributed quantum computing.

The white paper PDF is accessible using the following link: Photonic Distributed Entanglement Whitepaper

The post Distributed Quantum Computing in Silicon first appeared on Photonic.

Quantum computing is undergoing a period of rapid evolution. The era of noisy, intermediate scale quantum (NISQ) is coming to an end, and new systems demonstrating small-scale logical qubit architectures are attracting the interest of early adopters. We are entering the era of quantum error correction where the fidelity of qubits is improved by applying quantum error correction, which combines multiple noisy qubits into a single logical qubit with improved fidelity. For single-module systems, scalability of logical quantum computing systems is limited by how many qubits and associated control lines can be packed on a chip or a trap, and by the ratio of physical qubits to logical qubits. While these single module error-corrected systems solve one of the biggest problems constraining the usefulness of quantum computers—low qubit fidelity—they don’t have a solution for scaling beyond the limitations of a single quantum processor.

In this interview, Photonic’s Founder and Chief Quantum Officer shares her perspective of how quantum computers will ultimately scale to millions of qubits required for commercially relevant algorithms in chemistry, material design, drug discovery, and cybersecurity. To reach this scale, quantum computers must break free from the boundaries of a single quantum processor and unlock horizontal scalability by leveraging modular architecture linked by high bandwidth quantum networking.

Learn more about Dr. Simmons’ view of the three phases in the evolution of quantum computing and the role of distributed entanglement in unlocking the horizontal scalability of quantum computing.

The post The Path to Distributed Quantum Computing first appeared on Photonic.