Image via Microsoft Research
Quantum computers provide a promising new model of computation that enables exponential speedups over certain classical algorithms. But their Achilles' heel is a qubit's penchant for decoherence. That is, contemporary qubits are sensitive to changes in their environment and tend to lose their superposition because of it. Quantum superposition, as it turns out, is the central tenet of quantum computation and is vital for achieving the said exponential speedups.
Researchers have been working towards making these qubits more robust to changes in the environment without losing their controllability. A common solution is keeping these qubits in cryogenic environments where temperatures are tantalizingly close to absolute zero (0K), but this mechanical setup becomes a significant limitation in scaling up quantum computers for commercial use-cases. As a result, this remains an open research problem.
To this end, Microsoft in collaboration with a team from the University of Sydney has developed a cryogenic quantum control platform that uses specialized CMOS circuits to address the problem of qubit control and decoherence. In the paper "A Cryogenic Interface for Controlling Many Qubits", the researchers present Gooseberry, a CMOS chip that takes digital inputs and generates many parallel qubit control signals thereby allowing scaled-up support for thousands of qubits—a feat Microsoft deems a "leap ahead from previous technology".
Gooseberry enables this by operating at 100mK while dissipating sufficiently low power so that it does not heat up the qubits themselves. This means that the entire setup does not exceed the cooling capacity of commercially available quantum computing refrigerators. The team also used Gooseberry to create what it is calling the novel general-purpose cryo-compute core.
The proposed setup (shown above) uses a special breed of qubits called Topological Qubits. These qubits are more resilient to decoherence and have hardware-level error protection baked into them, reducing the overhead needed for software-level error correction and enabling meaningful computations to be done with fewer physical qubits. Taking a deeper look into the setup above, the Quantum-Classical interface layers are where the meat of the communication happens. Gooseberry sits abreast with the qubits in the lower stage due to its cryogenic requirements. It is thermally isolated from the qubits and its dissipated heat is drawn into a mixing chamber. Once ensconced near the qubits, Gooseberry converts classical instructions from the cryo-compute core into voltage signals which are then sent to the qubits.
(Left) A simplified version of the thermal conductance model of the Gooseberry chip. (Right) Gooseberry chip (red) sits close to the qubit test chip (blue) and resonator chip (purple).
Together the chips manage communication between various parts of a quantum computer. Essentially, they are used to send and receive information to and from every qubit, but in a way that maintains a stable cold environment, which is a significant challenge for a large-scale commercial system with tens of thousands of qubits or more. The stack itself operates at 2K, a temperature that is 20 times warmer than the temperature at which Gooseberry operates. This frees 400 times as much cooling power, allowing the stack itself to dissipate 400 times as much heat. Due to this, Microsoft believes that the stack is capable of general-purpose computing.
Putting Gooseberry to the test, the researchers connected with it a GaAs-based quantum dot (QD) device. Temperature of the components of the chip were measured as the control chip was powered up. As expected, the temperatures remained below 100mK, within the necessary range of frequencies and clock speeds. These results were extrapolated, showing the total system power needed for Gooseberry as a function of frequency and the number of output gates.
[Gooseberry] is able to operate within the acceptable limits while communicating with thousands of qubits. This CMOS-based control approach also appears feasible for qubit platforms based on electron spins or gatemons.
Though at present the proposed core can only handle some data and triggering manipulation, temperature freedom opens vital room for more technologies and ideas to work with.
This is a general-purpose CPU operating at cryogenic temperatures. At present, the core operates at approximately 2 K, and it handles some triggering manipulation and handling of data. With fewer limitations from temperature, it also deals with branching decision logic, which requires more digital circuit blocks and transistors than Gooseberry has
The team at Microsoft and the researchers from the University of Sydney believe that Gooseberry and the bundled cryo-compute core are big steps forward quantum computing. The cryo-compute core, acting as an interface between source code written by developers, Gooseberry, and qubits, shows that it’s possible to compile and run multiple types of code in a cryogenic environment, allowing for software-configurable communication between qubits and the outside world.