In the fast-evolving field of quantum computing, qubits are the building blocks of all quantum algorithms. These qubits, much like classical bits in a traditional computer, carry information, but unlike classical bits, they can exist in multiple states simultaneously, thanks to quantum principles like superposition and entanglement. This remarkable ability makes them an essential component of the future quantum technology that could revolutionize fields such as medicine, cryptography, artificial intelligence, and finance.
For scientists worldwide, creating qubits that are stable, controllable, and reliable enough for practical quantum computing has been an ambitious and challenging pursuit. In particular, the quest to develop long-lived qubits — capable of carrying quantum data over extended periods — has garnered a lot of attention, as short-lived qubits pose a significant barrier to scaling quantum computers. Researchers at Stanford University have made significant strides in this area by advancing the use of tin vacancy centers in diamond as qubits, a new path that could pave the way for scalable, efficient, and less resource-intensive quantum technologies.
What Are Tin Vacancy Qubits?
At first glance, the concept of using diamonds for quantum computing might seem counterintuitive. However, diamonds, the hardest known natural material, have properties that make them highly valuable for quantum information processing. Specifically, diamonds can be used to host vacancy centers – defects where one or more carbon atoms are missing from the crystal lattice, and the vacant site is typically filled by another atom.
For tin vacancy centers, scientists replace two carbon atoms in the diamond with a single tin atom, creating a tin vacancy qubit. These specialized qubits have several attractive characteristics, notably their potential for long lifetimes and the ability to operate at higher temperatures compared to other qubit systems, such as superconducting qubits. Longer-lived qubits are critical because they provide the time needed to perform quantum operations without decoherence, which is the process through which quantum information is lost.
However, one major challenge with tin vacancy qubits, until now, was the difficulty of effectively reading their spin states, which define the quantum information they carry. Qubits typically rely on their “spin” state, which can be in one of two positions, often referred to as “spin-up” or “spin-down.” This state can be detected through various forms of measurement, but the spin states of tin vacancy qubits are notably weak and prone to signal interference. This fuzziness made it challenging to accurately determine whether a tin vacancy qubit was in the spin-up or spin-down state.
Overcoming the Dim-Signal Challenge: Stanford’s Breakthrough
The recent breakthrough made by researchers at Stanford University, in collaboration with several research institutions, has turned the fortunes of tin vacancy qubits. The team managed to enhance the signal of the qubit, enabling precise reading of its spin state with remarkable accuracy. In their paper published in Physical Review X, the Stanford researchers revealed their success in boosting the signal of a tin vacancy qubit to an impressive 87% accuracy — a notable achievement in the quantum computing world.
Before this advance, scientists would have to average hundreds or thousands of readings to get a reliable measurement of a qubit’s state. This process was time-consuming and less efficient. The Stanford team, however, achieved this high level of accuracy in a single shot, meaning they could measure the qubit’s spin state from just a single measurement without the need for extensive averaging.
The project was led by Jelena Vuckovic, the Jensen Huang Professor of Global Leadership at Stanford, as well as a professor of electrical engineering and applied physics. Her group’s efforts represent a crucial advancement not only in the specific technology of tin vacancy centers but also for the broader field of quantum engineering. This collaboration was supported by Q-NEXT, a national quantum information science research center led by Argonne National Laboratory, with critical contributions from Sandia National Laboratories, which helped with the implantation of tin atoms into the diamond.
The Power of Tin Vacancy Qubits
One of the standout features of tin vacancy qubits is their long coherence times, meaning that they can hold quantum information for longer periods without degradation from their surroundings. This feature makes them far more stable than some other types of qubits, such as those based on superconducting circuits, which are prone to losing information due to noise from the surrounding environment. Another key advantage is the ability to operate at higher temperatures compared to other qubit systems, reducing the need for expensive and complicated cooling systems. Currently, many quantum systems operate at temperatures close to absolute zero to ensure qubit stability. Tin vacancy qubits’ ability to perform at elevated temperatures offers significant cost and energy savings in future quantum devices.
The breakthrough that the Stanford group achieved in reading tin vacancy qubits with single-shot accuracy also lays the groundwork for one of the most exciting potential applications of quantum technology: the quantum internet. A quantum internet would enable secure communication by using quantum properties — such as entanglement — to transmit information across vast distances. For this to work, however, quantum bits (qubits) need to be highly reliable, with precise control over their states, such as the ability to confidently determine whether a qubit is spin-up or spin-down, and consistency in this state over time. The research at Stanford has proven that tin vacancy qubits possess exactly the type of stability and accuracy required for this ambitious project.
Tuning the Magnetic Field for Better Performance
A critical aspect of the team’s breakthrough came in the form of modifying the magnetic field surrounding the tin vacancy qubit. When it comes to quantum computing, precision is paramount, and in this case, the qubit’s signal was still too weak for proper detection. To address this, the researchers adjusted the magnetic field by “tuning” it to achieve optimal conditions, allowing the quantum signal of the qubit to become significantly more pronounced.
As Souvik Biswas, a postdoctoral scholar working in Vuckovic’s group, explained, it was like “tilting a mirror just the right way to maximize the reflection of light.” By fine-tuning the orientation of the magnetic field, the group could boost the qubit’s signal, leading to more accurate and reliable measurements. This improvement was key to increasing the precision of the spin-state readout, significantly enhancing the performance of the tin vacancy qubit.
The breakthrough did not stop there. The research team also developed new techniques for improving their experimental setup, particularly focusing on the optical instruments required to excite and detect the tin vacancy qubits’ spin states. By improving the way the system interacts with light, they made it easier to measure even the faintest signals coming from the qubits.
Weak Measurements: A New Approach to Quantum Sensing
Another interesting aspect of the work from the Stanford team was their exploration of weak measurements. A weak measurement involves gently observing a quantum system, such as a tin vacancy qubit, without collapsing its wave function entirely — the wave function is the probability distribution that determines the outcome of a quantum event. Unlike a strong measurement, which forces the qubit’s spin to collapse fully into one of two states (spin-up or spin-down), a weak measurement leaves the system in a superposition of states, which allows scientists to gain additional insights into how qubits behave in response to external stimuli.
These weak measurements are more than just useful tools for measuring qubit spin. They also contribute to an important area of quantum mechanics called quantum sensing, a technique that can be used in a variety of applications, such as detecting electromagnetic fields, probing magnetic materials, and even exploring biological systems at a molecular level. This approach opens the door for novel experiments and applications beyond traditional computing.
The Road Ahead: Expanding Quantum Technologies
With these technical advancements, the Stanford team’s work is poised to advance not only the field of quantum computing but also future applications like the quantum internet and new materials for quantum devices. The properties of tin vacancy centers in diamond hold a lot of promise: they are reliable, they offer long coherence times, and they have the potential to operate at higher temperatures — making them far easier to work with compared to other, more traditional qubits.
But there’s still a great deal to be done. The team emphasized that there is ample opportunity for further optimizing the diamond devices in their setup. Fine-tuning the system for even better light collection could potentially improve its performance further.
As Rosenthal said, “The fact that it works that well with this unoptimized structure really speaks well of tin as a qubit.” This means that as scientists continue to perfect and develop these qubits further, they could unlock even greater potential in future quantum computers and quantum communication systems.
Conclusion
The breakthrough achieved by Stanford researchers in improving the performance of tin vacancy qubits marks a significant advancement in quantum computing. By enhancing the spin-state readout with 87% accuracy and achieving single-shot measurements, they have unlocked the full potential of tin vacancy centers in diamond as reliable and efficient qubits. These advancements not only make tin vacancy qubits promising for quantum computing but also position them as key players in the development of a future quantum internet. The ability to operate at higher temperatures and with long coherence times reduces the need for costly cooling systems, making them more practical for large-scale quantum applications. As further optimization is pursued, the future of diamond-based quantum technologies looks incredibly promising, offering new possibilities for quantum communication, sensing, and computation. Stanford’s success is a critical step toward realizing the full potential of quantum information science.
Reference: Eric I. Rosenthal et al, Microwave Spin Control of a Tin-Vacancy Qubit in Diamond, Physical Review X (2023). DOI: 10.1103/PhysRevX.13.031022