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Optical fibre for superconducting quantum computers

Fibre Optics and Quantum Computing

A universal quantum computer is expected to require some 1 million qubits (units of quantum information - the quantum mechanical analogue of the familiar ‘bit’). However, using multiple coaxial lines per qubit limits processor size to a few thousand qubits.

When using electrical wires, there’s a risk of overheating the qubits. The required superconducting circuits need to operate at cryogenic temperatures and connecting them to room-temperature electronics is complex. The National Institute of Standards and Technology (NIST) has measured and controlled a superconducting qubit using fiber instead - with promising results. (See the article in Nature).

Fibre offers low thermal conductivity and vast bandwidth, enabling efficient and massively multiplexed delivery of coherent microwave control pulses. A photonic link using an optical fibre to guide modulated laser light from room temperature to a cryogenic photodetector was employed. The researchers were able to demonstrate show that the photonic link was capable of meeting the requirements of superconducting quantum information processing.

Superconducting quantum computers use microwave pulses to store and process information, which meant the NIST team needed to very precisely convert light to microwaves. This was achieved using cryogenic photodetectors. System performance equalled that of metal cabling, while maintaining the qubit’s fragile quantum states. In order to make a scientific comparison, microwaves could be routed to the qubit through the photonic link or a coaxial line.

“I think this advance will have high impact because it combines two totally different technologies, photonics and superconducting qubits, to solve a very important problem. Optical fiber can also carry far more data in a much smaller volume than conventional cable.”

John Teufel

NIST physicist

In computing a bit (binary digit) is the smallest unit of data: a zero or a one. In simple terms, a qubit can also have a zero or one value in quantum computing (or a coherent superposition of both).

The NIST team conducted two types of experiments, using a photonic link to generate microwave pulses that measured or controlled the quantum state of the qubit.

To control the quantum state, electro-optic modulators converted microwaves to higher optical frequencies. These light signals streamed through optical fiber from room temperature to 4 kelvins (-269 C / -452 F) down to 20 millikelvins (thousandths of a kelvin). High-speed semiconductor photodetectors converted the light signals back to microwaves sent to the quantum circuit.

To measure the qubit’s state, researchers used an infrared laser to launch light at a specific power level through the modulators, fiber and photodetectors. The qubit’s state was accurately indicated 98% of the time - the same level of accuracy as obtained using the regular coaxial line.

The researchers envision a quantum processor in optical fibres can carry thousands of signals to and from the qubit.