Many groups are at the threshold of producing scalable quantum computers. Google In March 2018, unveiled the world’s largest quantum computer processor to date. Dubbed Bristlecone, it’s a 72-qubit gate-based superconducting system, beating IBM which had developed 50-qubit processor. The Mountain View company’s Research at Google team created the 72-qubit processor by scaling its previous 9-qubit system based on tiny, superconducting circuits. It’s estimated that a single 50-qubit quantum computer would outperform today’s most powerful mainframes. Researchers at D-Wave, IBM, MIT Lincoln Lab, and elsewhere have also developed superconducting qubits of high quality.
However, the most important challenge in development of scalable quantum computer with hundreds and thousands of bits. This will allow it to do something beyond the ken of a classical computer, such as simulate molecular structures in chemistry and materials science, or tackle certain problems in cryptography or machine learning. However, constructing a large-scale quantum processor is challenging because of the errors and noise that are inherent in real-world quantum systems.
One approach to addressing this challenge is to utilize modularity—a strategy used frequently in nature and engineering to build complex systems robustly. Such an approach manages complexity and uncertainty by assembling small, specialized components into a larger architecture. These considerations have motivated the development of a quantum modular architecture, in which separate quantum systems are connected into a quantum network via communication channels. Networks would open the door to several applications, including solving computations that are too large to be handled by a single quantum computer and establishing unbreakably secure communications using quantum cryptography.
In this architecture, an essential tool for universal quantum computation is the teleportation of an entangling quantum gate. Now a team of researchers from Yale University have successfully turned their idea into reality, demonstrating a practical approach to making this incredibly delicate form of technology scalable. These physicists have developed a practical method for teleporting a quantum operation – or gate – across a distance and measuring its effect. While this feat has been done before, it’s never been done in real time. This paves the way for developing a process that can make quantum computing modular, and therefore more reliable.
“Our work is the first time that this protocol has been demonstrated where the classical communication occurs in real-time, allowing us to implement a ‘deterministic’ operation that performs the desired operation every time,” says lead author Kevin Chou. The researchers used qubits in sapphire chips inside a cutting-edge setup to teleport a type of quantum operation called a controlled-NOT gate. Importantly, by applying error-correctable coding, the process was 79 percent reliable. “It is a milestone toward quantum information processing using error-correctable qubits,” says principal investigator Robert Schoelkopf. It’s a baby step on the road to making quantum modules, but this proof-of-concept shows modules could still be the way to go in growing quantum computers to the scale we need.
An international team, led by a scientist from the University of Sussex, are trying to develop scalable quantum computer with large number of qubits by networking individual quantum computing modules in order to obtain a fully modular large-scale machine . Physicists from the Institute of Theoretical Physics of the University of Innsbruck and Nicolai Friis, have found a technique to transfer quantum information between systems that are encoded differently which can be useful to implement
Entanglement lights the way to scalable quantum computers: Nov 2019
A technique for remotely entangling ions of strontium much more accurately and at far higher rates than previously possible has been unveiled by physicists in the UK. The team says that their scheme paves the way to scalable quantum computers made from multiple ion traps that are linked to one another via photonic interconnects.
Trapped ions offer a way of generating qubits with very low levels of noise, and therefore maintain the quantum coherence that is required to perform calculations. Indeed, the quantum states of ions have been made to persist for over 10::min. Each ion is held in a vacuum using electric fields and is suspended over a micro-fabricated chip. Manipulated by laser beams, the ions can then be placed in a superposition and entangled with their neighbours.
Although the coherence times of rival technologies based on bulk matter are often far shorter – superconducting qubits, for example, generally last for less than a thousandth of a second – ion traps are relatively slow and are limited in the numbers of qubits they can store. This is because it becomes increasingly difficult to accommodate the wiring and laser beams needed as more qubits are added.
As such, researchers are exploring ways of connecting ion qubits in different traps. In the latest work, Christopher Ballance and colleagues at Oxford University have shown how to link trapped ions by entangling them using the photons they emit when excited by a laser beam. This technique was first realized by Chris Monroe and colleagues at the University of Maryland in the US, and now the Oxford group has boosted both the rate and fidelity of the entanglement by collecting more of the photons given off by the ions and by limiting imperfections in the emission process.
Their experiment involves generating a sequence of very short laser pulses, splitting each pulse in two and then directing each half of that pulse to an ion of strontium-88. Each of the excited ions then decays to a superposition of two different energy levels, causing it to emit a photon whose polarization is entangled with that of the ion. The train of photons emerging from each half of the experiment is then focused by a lens and fed into a length of fibre-optic cable.
The ions are entangled by directing the photons that emerge from the fibres onto a beam splitter, with the output from that monitored by two detectors. It is when both detectors click that the ions become entangled. The quality, or “fidelity”, of the entangled state is obtained through a Bell-state measurement. With the ions separated by 5::m of optical fibre, Ballance says that the technique provides entanglement over a sufficiently long distance to network many quantum computers together.
As they report on the arXiv server, the researchers found that they could generate, on average, 182 entangled ion pairs per second, with a fidelity of 94%. This compares to a rate of just five entangled pairs per second that was achieved by Monroe’s group in 2014, and a mere 0.001 every second in 2007.
Monroe says the Oxford result “is a big deal, and the most recent demonstration of the fast-improving rate of off-chip quantum communication between ions”. He reckons it should be possible to push the rate well beyond 1000 entangled pairs per second, at which point, he says, “it is approaching the speed of local ion-ion operations and therefore useful for scaling.”
In fact, Ballance reckons that it might be possible to improve the latest rate by a factor of up to 100, in part by replacing the 5::cm-diameter lenses currently used to direct photons with reflective surfaces that can be placed much closer to the ions – and therefore collect more light. As with classical computers, he says that the aim is to “get to the point where the interconnect is not the bottleneck”.
Using Fiber Optics, ORNL Team Demonstrates Universal Quantum Computing: Dec 2018
Researchers at Oak Ridge National Laboratory (ORNL) have demonstrated a frequency-based approach to quantum computing. The researchers performed two distinct, independent operations simultaneously on two qubits encoded on photons of different frequencies. Qubits are the smallest unit of quantum information.
Quantum scientists working with frequency-encoded qubits have been able to perform a single operation on two qubits in parallel, but never two distinct operations, said the ORNL team. According to the researchers, coherent quantum frequency operations are challenging because it is difficult to mix frequencies arbitrarily and with low noise. “To realize universal quantum computing, you need to be able to do different operations on different qubits at the same time, and that’s what we’ve done here,” Pavel Lougovski, a research scientist, said.
For their experiment, the team used two entangled photons contained in a single strand of fiber optic cable. Because the photons were traveling through the same device, stability and control over the photons were maintained. “When the photons are taking different paths in the equipment, they experience different phase changes, and that leads to instability,” said Brian Williams, a researcher on the team.
The researchers implemented distinct quantum gates in parallel on two entangled frequency-bin qubits in the optical fiber. The team’s quantum frequency processor allowed it to manipulate the frequency of photons to bring about superposition, the state that allows quantum computers to perform operations concurrently. Through this quantum operation the researchers were able to control the spectral overlap between adjacent spectral bins, observe frequency-bin interference, and demonstrate 97 percent interference visibility (i.e., a measure of how alike two photons are). These results indicate that the photons’ quantum states were virtually identical. By integrating this tunability with frequency parallelization, the researchers were able to synthesize independent gates on entangled qubits.
The researchers applied Bayesian inference — a statistical method associated with machine learning — to confirm that the operations on the quantum processor were done with high fidelity and with absolute control. “A lot of researchers are talking about quantum information processing with photons, and even using frequency,” researcher Joseph Lukens said. “But no one had thought about sending multiple photons through the same fiber optic strand, in the same space, and operating on them differently.”
Lukens said the team’s results show that “we can control qubits’ quantum states, change their correlations, and modify them using standard telecommunications technology in ways that are applicable to advancing quantum computing.” Once the building blocks of quantum computers are in place, he said, “We can start connecting quantum devices to build the quantum internet, which is the next exciting step.”
The team believes that leveraging the existing fiber optic network infrastructure — which cost billions of dollars — is practical. Its realization of closed, user-defined gates on frequency-bin qubits in parallel could be used to develop fiber-compatible quantum information processing and quantum networks.
First ever blueprint unveiled to construct a large scale quantum computer: Feb 2017
An international team, led by a scientist from the University of Sussex, have unveiled the first practical blueprint for how to build a quantum computer, the most powerful computer on Earth. The work features a new invention permitting actual quantum bits to be transmitted between individual quantum computing modules in order to obtain a fully modular large-scale machine capable of reaching nearly arbitrary large computational processing powers.
Previously, scientists had proposed using fibre optic connections to connect individual computer modules. The new invention introduces connections created by electric fields that allow charged atoms (ions) to be transported from one module to another. This new approach allows 100,000 times faster connection speeds between individual quantum computing modules compared to current state-of-the-art fibre link technology.
The new blueprint is the work of an international team of scientists from the University of Sussex (UK), Google (USA), Aarhus University (Denmark), RIKEN (Japan) and Siegen University (Germany). Prof Winfried Hensinger, head of Ion Quantum Technology Group at the University of Sussex, who has been leading this research, said: “For many years, people said that it was completely impossible to construct an actual quantum computer. With our work we have not only shown that it can be done but now we are delivering a nuts and bolts construction plan to build an actual large-scale machine.”
The effort is part of the UK Government’s plan to develop quantum technologies towards industrial exploitation and makes use of a recent invention (4) by the Sussex team to replace billions of laser beams required for quantum computing operations within a large-scale quantum computer with the simple application of voltages to a microchip.
Researchers Develop Data Bus for Quantum Computer
Future quantum computers will be able to solve problems where conventional computers fail today. We are still far away from any large-scale implementation, however, because quantum systems are very sensitive to environmental noise. Although systems can be protected from noise in principle, researchers have been able to build only small prototypes of quantum computers experimentally.
One way to reduce the error rate is by encoding quantum information not in one single quantum particle but in several quantum objects. These logical quantum bits or qubits are more robust against noise. In the last few years, theoretical physicists have developed a whole range of error correction codes and optimized them for specific tasks.
Physicists Hendrik Poulsen Nautrup and Hans Briegel from the Institute of Theoretical Physics of the University of Innsbruck and Nicolai Friis, now at the Institute of Quantum Optics and Quantum Information in Vienna, have found a technique to transfer quantum information between systems that are encoded differently.
Interface between processor and memory
Similar to classical computers, future quantum computers might be built with different components. Scientists have already built small-scale quantum processors and memories experimentally, and they have used different protocols to encode logical qubits: For example, for quantum processors they use so-called color codes and for quantum memories surface codes.
“For the two systems to interact with each other quantum mechanically, we have to connect them,” says PhD student Hendrik Poulsen Nautrup. “We have developed a protocol that allows us to merge quantum systems that are encoded differently.” The scientists suggest to locally modify specific elements of the encoded quantum bits.
This process is also called lattice surgery, which is used to couple systems such as quantum processors and memories. Once the systems are temporarily “sewed” together, quantum information can be teleported from the processor to the memory and vice versa. “Similar to a data bus in a conventional computer, scientists can use this technique to connect the components of a quantum computer,” explains Poulsen Nautrup.
This new scheme is another step towards building a universal quantum computer and research for experimental realization is under way. The research was conducted within the framework of the doctoral program Atoms, Light, and Molecules offered at the University of Innsbruck and was funded by the Austrian Science Fund and the Templeton World Charity Foundation.