Building a quantum computer in reverse
By Nick Farina
Scaling has long been recognized as a major hurdle for quantum processors, along with a need for advances in quantum error correction and the control of quantum gates.
However, while rapid progress has been made in the latter two, far less progress has been made in the development of a CMOS-based scalable system, where the devices and qubits are sufficiently identical that the number of external control signals increases slowly with the number of qubits.
Therefore the development, and taping-out, of a CMOS-based scaling architecture has taken on new significance, as scaling has become the most critical remaining task for building a commercially viable quantum computer.
At EeroQ, we have made a key advance towards this goal, achieving tape-out, at a major US semiconductor foundry, of a 2,432 future qubit system with only ~30 control lines, which we’re calling Wonder Lake. This scaling architecture has passed the rigorous design checks required for compatibility with today’s standard chip manufacturing process (CMOS).
The architecture of a quantum processor requires multiple layers, all of which work in concert. In this post, we will go into some of the details about the layers of our newly taped-out chip. This chip will form the infrastructure needed for future devices that can hold the single electrons, which we are working to develop as a leading qubit platform.
With our announcement today, we offer a credible path to allow our systems, which are based on the isolated electron spins trapped above the surface of liquid helium (eHe), to scale from single qubits to 10,000 and beyond… by starting from scale, and building a quantum computer in reverse.
Why build a quantum computer in reverse, starting from scale?
This strategy puts EeroQ’s approach to quantum computing in a position to become a leapfrog technology. To-date, there have been many demonstrations of high-quality qubits, even more than 100 in single processors, but there has yet to be a practical and achievable way to scale to several thousand, or more, on a single chip.
Backstory: using isolated electron spins on helium as a qubit
EeroQ’s approach to quantum computing is different from any other company. At the heart of any quantum processor is a qubit, and EeroQ’s is the spin of the electron. In a 1999 paper in Science a collaboration between researchers at Bell Labs and Michigan State University proposed that an electron floating above the surface of liquid helium would make an exceptional quantum computer using the vertical motion of the electron above the helium surface, so-called “Rydberg states” of the electron motion. Shortly thereafter, in 2006, EeroQ CTO Stephen Lyon, proposed in Physical Review A that the spin state of the electron offers many of the advantages of Rydberg states, but with the added benefit of vastly enhanced quantum coherence in excess of 10 seconds.
Based on these initial ideas, and subsequent technological breakthroughs, at EeroQ we will ultimately fabricate the majority of our future processors on single chips manufactured in a commercial CMOS foundry. Once the wafers arrive from the foundry, we’ll add a thin layer of liquid helium, deposit electrons into on-chip reservoirs, initialize their spin states, and begin a computation.
The electron qubit will rest about 10 nanometers above the helium surface, where it is trapped above electrodes located beneath the helium by control voltages.
At EeroQ we are building next generation quantum devices by combining the tiny size of electrons and superfluid helium which is the cleanest environment in nature with CMOS infrastructure and the lack of any need for modular interconnects. These efforts along with an efficient fabless production model put us in position to lead the industry.
Click on the images below to scan through the layers of the chip
What’s next?
After six years of stealth work, we now have an architecture to scale this system.
The next step is demonstration of a two-qubit gate based on the extremely well-understood physics of the magnetic dipole-dipole interaction, which can be “drag and dropped” onto the foundry chip.
Our first two-qubit gates will be produced by the 2 small magnetic spins of the electrons. Each electron has a magnetic field, and that field is one of the most accurately known quantities in physics; the magnitude being known to at least 12 digits of precision.
In this scheme the main source of imprecision in the entangling gate comes from the positioning of the 2 electrons, which will be controlled by engineering the microstructures on the CMOS chip that hold the electrons. The precision of the CMOS process will reduce fabrication related quantum gate errors to about 0.01%. We will then add our quantum gates to pre-designated locations on the chip, as shown below.
EeroQ’s Advantage
The work we have accomplished at EeroQ is a significant step on the road to building a commercially viable quantum computer and has allowed us to pursue our next near-term goals.
● 10+ second qubit coherence
● High qubit connectivity
● Identical qubits, controllable in parallel with only a few voltages on a CMOS chip
● Mobile qubits on the helium surface (providing up to a 50x reduction in overhead needed for error correction)
● 99.9% gate fidelities
● A system without modular interconnects … so that all the quantum computing power you’ll need will be in a device the size of your thumbnail!
There are two particularly challenging parts to making a useful quantum computer: high-quality quantum gates, and a path to scale.
With our latest work, we are proud to join the leadership ranks on scalability. Together with recent advances in error mitigation and more efficient algorithms, we can see the commercial quantum future coming together sooner than expected – led by the ability to leverage our architectural advantage to scale rapidly.
