Scalable Quantum Computing

Devising a New Method for Scaling Up Quantum Computers

In 2019, Google announced the development of a quantum computer with 53-qubits. Google claimed to achieve quantum supremacy after performing a particular calculation significantly faster than the world’s fastest supercomputer. Like most of today’s most massive quantum computers, this system boasts tens of qubits — the quantum counterparts to bits encoded information in conventional computers.

To make more extensive quantum systems, most of today’s quantum prototypes will need to overcome scalability and stability challenges. Scalability requires increasing the density of signaling and wiring, which is very hard to achieve without depleting the system’s stability. RIKEN’s Superconducting Quantum Electronics Research Team developed a new circuit-wiring scheme over the last three years after collaborating with other institutes. The collaboration opens the door to scaling quantum systems up to 100 or more qubits within the next decade.

First Challenge: Scalability

Quantum computers compute information using complex and delicate interactions between qubits based on the principles of quantum mechanics. So that we understand this better, we must understand qubits. Individual qubits build a quantum computer, analogous to a conventional computer which uses binary bits. A qubit needs to survive in a very fragile quantum state instead of the zero or one binary states of a bit. Rather than just being zero or ones, qubits can also be in a state called a superposition — where they are partially in a state of both zero and one simultaneously. This allows quantum computers based to process data in parallel for each possible logical state, zero or one. They can thus perform more efficiently and, therefore, faster calculations than conventional computers based on bits for particular types of problems.

It is much harder to build a qubit than a conventional bit, and making it survive at room temperature is difficult too. We need full control over the quantum-mechanical behavior of a circuit. Scientists devised a few quantum mechanical ways to do this with some reliability. At RIKEN, a Josephson junction, a superconducting circuit with an element creates a powerful quantum-mechanical effect. Thus, the qubits can now be produced reliably and iteratively with nanofabrication techniques used widely in the industries designing semiconductor chips.

The challenge due to scalability arises because each qubit needs connections and wiring that produce controls and readouts with minimal crosstalk. RIKEN moved past tiny two-by-two or four-by-four arrays of qubits and realized how densely the associated wiring in a quantum circuit could be packed. They had to create better quantum systems and fabrication methods to avoid getting the wires cross each other.

RIKEN built a four-by-four array of qubits using their wiring methodology. Each qubit is connected vertically from the backside of a chip, rather than a separate ‘flip chip’ interface used by others. The wiring scheme used by others brings the wiring pads to lie in the edges of a quantum chip. This is where complexity arrives. Our wiring scheme involves sophisticated fabrication with a densely packed array of superconducting material vias (electrical connections) through a silicon chip.

Nonetheless, it should allow us to scale up our quantum circuits to much larger devices. RIKEN is extensively working towards a 64-qubit device, which they hope to build within the next three years. A 100-qubit device will follow this in another five years as part of a nationally-funded research program. This platform should ultimately allow up to 1,000 qubits to be integrated on a single chip.

Second Challenge: Stability

The other major challenge surrounding stability for quantum computers is to deal with the intrinsic vulnerability of the qubits to noise or fluctuations from external factors like temperature. A qubit needs a state of quantum superposition, or ‘quantum coherence’ to survive. Earlier we could make this state last for nanoseconds. We can maintain coherence qubits for up to 100 microseconds by cooling quantum computers to cryogenic temperatures and creating several other environmental controls. On average, before coherence is lost, a few hundred microseconds would allow us to perform a few thousand information processing operations.

One way we deal with instability is to use quantum error correction in theory. To build a single ‘logical qubit,’ we exploit several physical qubits and apply an error correction method to diagnose and fix errors to protect the logical qubits. Scalability is far from realizing this for many reasons.

Quantum circuits

In the 1990s, it wasn’t evident if electronic circuits as a whole could behave in a quantum mechanical manner. To create switch-on and -off states in the circuit, and realize a stable qubit in a circuit, the circuit needs to support a superposition state.

RIKEN eventually devised the idea of using a superconducting circuit. This superconducting state is free of all electrical resistance and losses. Thus, it can respond to even small quantum-mechanical effects. To test this circuit, RIKEN used a microscale superconducting island made of aluminum connected to a larger superconducting ground electrode via a Josephson junction — a junction separated by a nanometer-thick insulating barrier — and we trapped superconducting electron pairs that tunneled across the junction. The smallness of the aluminum island was able to accommodate at most one excess pair due to an effect called Coulomb blockade between negatively charged pairs. The states of zero or one excess pair on the island can be used as the state of a qubit. The quantum-mechanical tunneling maintains the coherence of qubits and allows to create a superposition of the qubit states. Microwave pulses control these states fully.

Hybrid Quantum Computers

It is highly unlikely that quantum systems will be home shortly because of their very delicate nature. Industrial giants such as IBM, Google, and Honeywell, as well as many start-up companies and academic institutes worldwide, are increasingly investing in research, having recognized the enormous benefits of research-oriented quantum computers.

A commercial quantum-computer with full error correction is probably still more than a decade away. Nevertheless, state-of-the-art technical developments are already bringing about the possibility of new science and applications. Quantum circuits perform useful tasks in the lab at a smaller-scale .

For example, RIKEN uses their superconducting quantum-circuit platform along with other quantum-mechanical systems. This hybrid quantum system allows them to measure a single quantum reaction within collective excitations — be it precessions of electron spins in a magnet, crystal lattice vibrations in a substrate, or electromagnetic fields in a circuit — with unprecedented sensitivity. The measurements that such a system does should advance our understanding of quantum physics and quantum computing. The quantum system at RIKEN is sensitive enough to measure a single photon at microwave frequencies. Its energy is about five orders of magnitude lower as compared to that of a visible-light photon without destroying or absorbing it. RIKEN hopes that this will serve as a building block for quantum networks in the future. It will connect distant qubit modules, among other things too.

Quantum Internet

Another major challenge for is to build a quantum internet in the future. Interfacing a superconducting quantum computer to an optical quantum communication network for RIKEN’s hybrid system. This would be developed in anticipation of a future that includes a quantum internet-connected by optical wiring reminiscent of today’s internet. A careful design of such an internet is a must. As even a single infrared light photon at a telecommunication wavelength cannot hit a superconducting qubit without disturbing the quantum information that is stored with the qubit. RIKEN is investigating hybrid quantum systems that transduce quantum signals from a superconducting qubit to an infrared photon, and vice versa, via other quantum systems, such as one that involves a tiny acoustic oscillator.

Quantum science is already in our hands every day. Laser diodes and transistors would have never been invented without a proper understanding of the properties of electrons in semiconductors, based on understanding quantum mechanics. So through internet and the smartphones, we are almost totally reliant on quantum mechanics. We will only become more so in the future.

Originally published at https://www.firstqbit.com on July 10, 2020.