#continuouslydriven

IBM Researchers Show Tenfold Stability Increase with Dressed Singlet-triplet Qubit

IBM Research Europe has developed a new method for protecting quantum information from environmental noise without affecting performance, advancing semiconductor quantum computing. By “dressing” a singlet-triplet (ST) qubit of hole spins in germanium, researchers increased coherence time by tenfold, enabling more dependable and effective quantum processors.

The Germanium Advantage

Electron-free semiconductor crystals or germanium holes were once deemed optimal for qubits. Germanium is appreciated for its intrinsic spin-orbit coupling and lack of valley states, which allow researchers to operate qubits with electrical signals instead of magnetic components.

Also, electrical sensitivity has two downsides. It allows fast control, but it exposes qubits to "charge noise," random electrical fluctuations in the environment that dephase the qubit's quantum state. To combat this, researchers work at low magnetic fields to lengthen dephasing time, although gate speeds are much slower.

Breaking Speed-Stability Tradeoff

Patrick Harvey-Collard and Konstantinos Tsoukalas led the IBM group to use singlet-triplet (ST) qubits to break this impasse. Unlike single-spin qubits, singlet-triplet qubits are governed by the exchange interaction (J), a force between two spins that is strong even in a modest external magnetic field.

The researchers demonstrated a bare resonantly-driven singlet-triplet qubit with 99.68% gate fidelity at 20 mT and 20 mK. This bare qubit has 1.9 microseconds of dephasing (T2). Although impressive, the researchers wanted to prolong the qubit's “coherent” period—the key window of time a quantum computer can compute.

“Dressing” Qubit

Introducing a “continuously-driven” or “dressed” qubit state was revolutionary. By repeatedly applying a resonant electrical drive to the exchange interface, the scientists "dressed" the singlet-triplet qubit in energy. This steady drive shields the qubit from certain environmental noise frequencies.

The results were remarkable: the clothed singlet-triplet qubit had a tenfold increase in coherence time (T2ρ∗​) to 20.3 microseconds. Amazingly, this massive stability gain did not compromise accuracy. The team verified the clad qubit's 99.64% gate integrity using randomized benchmarking.

Frequency Modulation for Universal Control To use the clothed qubit in a quantum computer, the scientists required to demonstrate “universal control,” or the ability to rotate the qubit's state to any point on the Bloch sphere.

This was done using FM. By adjusting the driving signal frequency, the researchers rotated the dressed state exactly. This approach worked well because it used a larger frequency spectrum instead of raising the voltage, which might heat or crosstalk qubits.

A Scaling Path

The experiment used a Ge/SiGe heterostructure with six quantum dots. High-quality two-dimensional arrays made feasible by germanium hole spins make this design ideal for scaling up to larger computers.

The scientists also employed an improved “latched” Pauli spin barrier to read out the spin state as an electrical charge signal. Thus, they could clearly distinguish spin states and achieve a 94% initial state preparation and measurement (SPAM) fidelity.

Future View

The current study focused on a single dressed singlet-triplet qubit, but the researchers decided two-qubit gates were the next step. Entangle two clothed qubits to finish this quantum processor.

The prolonged coherence of dressed qubits will aid during "idle periods," when one quantum chip component waits for another to finish a readout or calculation, the researchers says. Maintaining the qubits "dressed" and stable throughout these delays could considerably reduce a huge quantum computer's error rate.