Scientists at IBM Research (#ibmresearch) have achieved major advances in quantum computing device performance that may accelerate the realization of a practical, full-scale quantum computer. For specific applications, quantum computing, which exploits the underlying quantum mechanical behavior of matter, has the potential to deliver computational power that is unrivaled by any supercomputer today.
Using a variety of techniques in the IBM labs, scientists have established three new records for reducing errors in elementary computations and retaining the integrity of quantum mechanical properties in quantum bits (qubits) – the basic units that carry information within quantum computing. IBM has chosen to employ superconducting qubits, which use established microfabrication techniques developed for silicon technology, providing the potential to one day scale up to and manufacture thousands or millions of qubits.
IBM has recently been experimenting with a unique “three dimensional” superconducting qubit (3D qubit), an approach that was initiated at Yale University. Among the results, the IBM team has used a 3D qubit to extend the amount of time that the qubits retain their quantum states up to 100 microseconds – an improvement of 2 to 4 times upon previously reported records. This value reaches just past the minimum threshold to enable effective error correction schemes and suggests that scientists can begin to focus on broader engineering aspects for scalability.
A picture of IBM’s “3D” superconducting qubit device where a qubit (about 1mm in length) is suspended in the center of the cavity on a small Sapphire chip. The cavity is formed by closing the two halves, and measurements are done by passing microwave signals to the connectors. Despite the apparent large feature size (the cavity is about 1.5 inches wide) for this single qubit demonstration, the team believes it is possible to scale such a system to hundreds or thousands of qubits.
A picture of the Silicon chip housing a total of three qubits. The chip is back-mounted on a PC board and connects to I/O coaxial lines via wire bonds (scale: 8mm x 4mm). A larger assembly of such qubits and resonators are envisioned to be used for a scalable architecture.
NY Times – Mark B. Ketchen, manager of the physics of information group at I.B.M.’s Thomas J. Watson Research Center in Yorktown Heights, N.Y. “I used to think it was 50 years [for a scalable quantum computer]. Now I’m thinking like it’s 15 or a little more. It’s within reach. It’s within our lifetime. It’s going to happen.”
Note – the IBM quantum computer researcher is not counting the DWave Adiabatic quantum computer system.
In separate experiments, the group at IBM also demonstrated a more traditional “two-dimensional” qubit (2D qubit) device and implemented a two-qubit logic operation – a controlled-NOT (CNOT) operation, which is a fundamental building block of a larger quantum computing system. Their operation showed a 95 percent success rate, enabled in part due to the long coherence time of nearly 10 microseconds. These numbers are on the cusp of effective error correction schemes and greatly facilitate future multi-qubit experiments.
“The superconducting qubit research led by the IBM team has been progressing in a very focused way on the road to a reliable, scalable quantum computer. The device performance that they have now reported brings them nearly to the tipping point; we can now see the building blocks that will be used to prove that error correction can be effective, and that reliable logical qubits can be realized,” observes David DiVincenzo, professor at the Institute of Quantum Information, Aachen University and Forschungszentrum Juelich.
Based on this progress, optimism about superconducting qubits and the possibilities for a future quantum computer are rapidly growing. While most of the work in the field to date has focused on improvements in device performance, efforts in the community now must now include systems integration aspects, such as assessing the classical information processing demands for error correction, I/O issues, feasibility, and costs with scaling.
IBM envisions a practical quantum computing system as including a classical system intimately connected to the quantum computing hardware. Expertise in communications and packaging technology will be essential at and beyond the level presently practiced in the development of today’s most sophisticated digital computers.
The three breakthroughs described by IBM include nearly .1 millisecond (95 microseconds) coherence time for a q-bit isolated from its environment inside a 3-D copper waveguide cavity. The second demonstration was of a nearly identical q-bit, but mounted on a 2-D planar substrate, which was able to achieve a 10 microsecond coherence time. And the third breakthrough was demonstration of a 95-to-98 percent success rate for a two q-bit logical operation called a controlled-NOT. The significance here is that a C-NOT gate, together with single q-bit gates, can be configured to perform any quantum computation (in a manner similar to how the NAND gate can be configured to perform any classical computation.)
The basic q-bit repository demonstrated by IBM consisted of a super-cooled Josephson junction consisting of two superconducting electrodes separated by an insulator. A super-cooled capacitor connected the two superconducting electrodes in order to lower the frequency of its operation into a regime that standard measurement equipment can handle today—upwards of 20 GHz—necessitating the use of microwave-caliber test electronics.
The construction of the q-bit memories and gates were all performed with micro-fabrication techniques already in common usage for standard silicon chips, making IBM optimistic that it will be able to scale its system architecture up to thousands or even millions of q-bits per chip. As a result, calculations that were once considered impossible to perform can now at least be envisioned.
IBM’s solid-state structure stores a single superconducting quantum q-bit, the building block of future quantum computers.
Coherence times for superconducting qubits in a planar geometry have increased drastically over the past 10 years with improvements exceeding a factor of 1000. However, recently these appeared to have reached a plateau around 1-2 microseconds, the limits of which were not well understood. Here, we present experimental data showing that one limit is due to infra-red radiation, confirming observations from other groups. We observe increased coherence times after appropriate IR shielding. Further improvements are shown to be possible by increasing the feature size of the interdigitated shunting capacitor, strongly indicating that surface losses at the metal/substrate interface are limiting qubit coherence times. In our experiments we kept the ratio of line width to gap size constant, but increased the overall feature size. We will discuss this and other similar design approaches towards better coherence in superconducting qubits.
We experimentally explore the implementation of high-fidelity gates on multiple superconducting qubits coupled to multiple resonators. Having demonstrated all-microwave single and two qubit gates with fidelities over 90% on multi-qubit single-resonator systems, we expand the application to qubits across two resonators and investigate qubit coupling in this circuit. The coupled qubit-resonators are building blocks towards two-dimensional lattice networks for the application of surface code quantum error correction algorithms.