Understanding Qubits – Teleportation and Controlled Interaction Breakthroughs

A lot is going on in the world of quantum computing. Chip giant Nvidia has launched an open-source CUDA-Q platform to accelerate quantum computing efforts, while China has created its largest quantum computing chip. Then there are scientists at The University of Manchester who have developed ultra-pure silicon that paves the way for next-generation computers.

All this excitement and development around quantum computers makes sense, given that the technology holds immense potential across various fields, including cryptography, drug discovery, solving complex optimization problems, enhancing machine learning algorithms, and much more.

Quantum computers can achieve all this by taking advantage of quantum theory, which is the behavior and nature of matter and energy on atomic and even smaller subatomic levels. Quantum computing utilizes subatomic particles such as photons and electronics. Qubits (quantum bits) then allow those particles to exist in multiple states simultaneously and are manipulated by control devices.

To handle exponentially faster speeds than your traditional computer while consuming less energy, quantum computers use superposition and entanglement. 

Superposition involves adding two or more quantum states to create yet another valid quantum state. The superposition of qubits allows quantum computers to process millions of operations simultaneously.

Entanglement occurs when two systems are linked so that knowing the state of one gives immediate knowledge about the other. This enables quantum computers to solve complex problems at a fast rate.

The problem here is decoherence, which is the loss of the quantum state in a qubit due to factors like radiation, vibration, or temperature change. This causes errors in computing. To protect Qubits from interference, they are placed in vacuum chambers, insulation, and supercooled refrigerators. 

As we saw, qubits play a critical role in making quantum computing, but not everything is known about them. However, two recent independent experiments have expanded our understanding of qubits, marking an important step toward building a functional quantum computer.

Quantum Teleportation Achieved

New research has successfully achieved quantum teleportation in spite of all the noise that typically disrupts the transfer of quantum state. In teleportation, a qubit is transferred from one location to another without sending the particle itself. 

In theory, the transfer of the quantum state can be done without any problem, but in the real world, disruptions and noises degrade the quality of quantum teleportation. So, the researchers in the latest study found that achieving perfect quantum teleportation despite the noise is a great feat.

Published in the journal Science Advances, the study talks about how entanglement and decoherence are counterforces of many quantum protocols and technologies. 

According to the research, quantum entanglement that occurs in correlations spanning arbitrarily long distances is very significant for the foundations of quantum mechanics. It has many applications in information processing and communication. However, the interactions between a quantum system and its environment are unavoidable, and decoherence can severely degrade these applications' performance. 

While there are many promising decoherence suppression protocols with recent works exploiting decoherence-free subspaces, dynamical decoupling, quantum error-correcting codes, delayed coherent quantum feedback, and reservoir engineering with auxiliary subsystems, avoiding decoherence is extremely demanding in practice.

As such, the study proposed an efficient protocol for quantum teleportation in absolute decoherence. 

The study conducted by researchers from the University of Science and Technology of China, Hefei, and the University of Turku, Finland, used multipartite hybrid entanglement between the auxiliary qubits and their local environments within the open–quantum system context, allowing it to achieve high accuracy. 

According to the researchers, linear optics is a particularly robust platform for performing different quantum information protocols and studying problems with decoherence. 

The work in this study, according to Jyrki Piilo, a Professor from the University of Turku, utilizes the notion of distributed entanglement. This entanglement distribution goes beyond used qubits and is done before operating the protocol. This means “exploiting the hybrid entanglement between different physical degrees of freedom,” Piilo said.

Traditionally, photon polarization has been used to entanglement qubits in teleportation. However, the new approach leverages the hybrid entanglement between photon polarization and frequency.

This brings a big change in how noise influences the protocol. The discovery, in fact, “reverses the role of the noise from being harmful to being beneficial to teleportation,” Piilo stated.

Traditionally, the teleportation protocol doesn't work when there is not just noise during qubit entanglement but also when hybrid entanglement is there at first without any noise. As opposed to this, when having hybrid entanglement and then adding noise, both the teleportation and quantum state transfer happen about perfectly.

This way, the latest discovery allows for almost ideal teleportation despite the noise associated with using photons.

The researchers call this a “significant proof-of-principle experiment,” with Dr. Zhao-Di Liu from the University of Science and Technology of China, Hefei noting:

“While we have done numerous experiments on different facets of quantum physics with photons in our laboratory, it was very thrilling and rewarding to see this very challenging teleportation experiment successfully completed.”

The study noted that in addition to fighting decoherence, hybrid entanglement has also helped them bring another layer of security. The study stated:

“It would be an interesting line of future research to investigate how deep the teleported information can be hidden.” 

It's just the beginning, with the study having fundamental importance in opening new pathways for future work in quantum protocols by having this one as a base research. One way the technique can be applied is in state transfer outside quantum teleportation and beyond decoherence-free subspaces.

The research also opens the possibility of seeing whether decoherence can be reversed in other physical platforms, including different sources of noise.

Click here to learn about the current state of quantum computing.

Realizing a Two Qubit Gate in a Conventional Silicon Transistor

The other study, which was conducted by researchers from Switzerland's oldest university, the University of Basel, in collaboration with those from The National Center of Competence in Research (NCCR) SPIN, made a breakthrough by getting a controllable interaction between two hole spin qubits in a traditional silicon transistor. 

Published in Nature, the study, which received open-access funding from the University of Basel, noted that semiconductor spin qubits offer the potential to employ industrial transistor technology to produce large-scale quantum computers. 

For a quantum computer to perform calculations, it needs “quantum gates,” which are operations that manipulate the qubits and couple them to each other. The researchers in the latest study were able to not only couple two qubits but also bring about a controlled flip of one of their spins, which depends on the other's spin state. The coupling depends on the exchange interaction of the two spin qubits.

“Hole spins allow us to create two-qubit gates that are both fast and high-fidelity. This principle now also makes it possible to couple a larger number of qubit pairs.” 

– Dr. Andreas Kuhlmann

Researchers have already shown a couple of years ago that the hole spins in an existing electronic device and can be trapped and used as qubits. Now, Kuhlmann led this team of Basel physicists to success in realizing an interaction between two qubits that can be controlled.

While the qubits in question benefit from being electrically controllable and having sweet spots to negate charge and noise, demonstrating a two-qubit interaction has been challenging.

A missing factor, as per the study, has been the understanding of exchange coupling during a strong spin-orbit interaction. To address this, the scientists researched two hole-spin qubits in a silicon “FinFETs” or fin field-effect transistor. Spin-orbit coupling means a hole's spin state is affected by its motion through space.

So, semiconductor quantum dot (QD) spin qubits are seen as most suitable for the future implementations of large-scale quantum circuits. However, even the most advanced spin-based quantum processor currently allows for universal control of six electron spin qubits in silicon (Si). This is closely followed by a four-qubit demonstration with holes in germanium. 

For the study, researchers used a qubit that uses the spin of an electron or a hole. Both electrons and holes spin and adopt either the up or down state. 

Hole spins, compared to electron spins, can be controlled completely electrically without the need for orbital degeneracy or additional components like on-chip micromagnets, which add complexity to the equation. This is due to their intrinsic spin-orbit interaction (SOI). Holes further benefit from reduced hyperfine interaction and the absence of a valley.

As such, the study demonstrates the ability to electrically control the exchange and perform a conditional spin-flip in 24 ns. The exchange Hamiltonian no longer has the Heisenberg form and can be engineered to enable two-qubit controlled rotation gates without sacrificing speed for accuracy or vice versa. According to the research:

“This ideal behavior applies over a wide range of magnetic field orientations, rendering the concept robust with respect to variations from qubit to qubit, indicating that it is a suitable approach for realizing a large-scale quantum computer.”

This study suggests the potential for arranging millions of hole spin qubits on just one chip. Its approach also shows great possibility for the development of a large-scale quantum computer.

Future improvements in device fabrication are required to reduce variability. When combined with robust controlled rotation (CROT) sweet spots, these advancements “will make two-qubit gate operations with anisotropic exchange highly attractive for large-scale qubit arrays.”

The research advances, if combined with fast readout and operation above 1 K, can allow FinFET to be used as a universal quantum processor arranged on a chip used in classical control electronics.

Companies Involved in the Development of Quantum Computers

Now, let's take a look at companies actively working on quantum computers:

#1. IBM

IBM has been leading quantum computing research for many years and developed IBM Q System One, the first circuit-based commercial quantum computer. The company provides access to its quantum systems through the IBM Quantum Experience platform. 

Earlier this month, IBM unveiled its 1,000+ qubit quantum processor Condor and its utility-scale processor IBM Quantum Heron with 133 qubits. It also announced the launch of a modular quantum computer, Quantum System Two. Meanwhile, through the software stack Qiskit, IBM aims to make the development of quantum computing widely accessible.

This year, Japanese national research laboratory RIKEN announced that it will deploy IBM's quantum processor and quantum computer architecture for integration with supercomputer Fugaku. 

The company's recent research in the field meanwhile includes:

• High-threshold and low-overhead fault-tolerant quantum memory.
• Encoding a magic state with beyond-break-even fidelity.
• Simulating large-size quantum spin chains on cloud-based superconducting quantum computers.

As of writing, the company stocks are trading at $167.36, up 2.33% YTD, while its market cap is $153.73 bln. IBM has reported revenue (TTM) of $62.07bln, EPS (TTM) of 9.19, and P/E (TTM) of 18.22. The dividend yield is 3.99%. 

During its recent Q1 2024 financial reporting, IBM saw its revenue increase by 1.5% YoY during the quarter to $14.5 bln and free cash flow of $1.9 billion. The company notes that its “solid revenue and free cash flow growth” reflects the strength of its cloud and AI strategy.

#2. Google 

In the world of quantum computing, Google has been making strides with its Quantum AI lab, which works on both hardware and software. A few years ago, the division launched Sycamore, a 53-qubit quantum processor. Currently, the tech giant's hardware is focused on superconducting qubits while its advanced software stack explores the power of quantum computing. 

A couple of months ago, Google launched a multi-year, global competition to find real-world use cases for quantum computing with a prize of $5 million, which will be split among finalists. Google noted in March:

“While there are many reasons to be optimistic about the potential of quantum computing, we're still somewhat in the dark about the full scope of how, when, and for which real-world problems this technology will prove most transformative.” 

The company's recent research in this field includes suppressing quantum errors by scaling a surface code logical qubit, phase transition in random circuit sampling, and measurement-induced entanglement and teleportation on a noisy quantum processor.

As of writing, the company stocks are trading at $107.48, up 21.94% YTD, while its market cap is $2.12 trillion. Google has reported a revenue (TTM) of $218.14bln while having an EPS (TTM) of 6.52 and a P/E (TTM) of 26/13. It pays a dividend yield of 0.47%. 

For its 1Q24 earnings, the company reported a 13% jump in revenue to $86.3 bln, net income of $20.28 bln, and the first-ever $20 per share dividend. In the spring of 2024, its market cap hit a new milestone of $2 trillion, making it the world's fourth-most valuable public company.

Conclusion

There has been a race to build a functional quantum computer, for which researchers are focused on understanding qubits and working with different qubit technologies. Qubits are the basis of the quantum computer as they handle all the processing, transfer, and storage of data. Hence, all the research has been happening around qubits, including the latest two covered here, which aim to help in the building of a practical quantum computer.

Click here for the list of the five best quantum computing companies.