Passive filters.
Measurable gains.

The patented High-Energy Radiation Drain (HERD): a filter technology that removes Cooper-pair-breaking radiation from cryostat wiring, leaving microwave signals untouched.

Application note Original paper

How HERD works

HERD directs high-frequency radiation away from the main propagating path of the filter and into an absorptive region. Low-frequency radiation never enters that region and propagates undisturbed across the filter.

Leaky coaxial waveguide

Cooper-pair breaking radiation

Cooper-pair breaking takes place when the frequency of the incident radiation exceeds 2Δ/h, where Δ is the superconducting energy gap. In aluminium, the most common material used to realize Josephson junctions, the threshold is around 80 GHz. Rejecting radiation above it is a requirement common to all types of lines connected to the QPU.

Absorptive filters and their drawbacks

A common approach is to fill coaxial lines with an attenuating dielectric such as Eccosorb, or copper-powder-filled epoxy. These are highly attenuating at high frequencies, but they also attenuate in band, and the attenuation increases with frequency, which distorts fast pulses. On charge lines the in-band loss also increases the active load on the coldest stage, which in turn limits the number of qubits that can be controlled with a finite cooling power. Because the fill is a mixture of absorptive particles and epoxy, performance varies from unit to unit, and magnetically loaded epoxies are sensitive to high magnetic fields.

The leaky coaxial waveguide

The coaxial line is surrounded by hollow waveguide sections that open onto it. A hollow waveguide carries no propagating mode below its cutoff frequency, so signals in the passband pass the openings undisturbed. Above cutoff, radiation couples into the waveguides and is absorbed at their terminations, outside the signal path.

Stopband extent

Thanks to its arrangement, the filter presents no parasitic leakage paths, so that, unlike conventional resonant filters, the stopband extends to very high frequencies. Because the absorptive material is separated from the main waveguiding path, variations in its properties do not affect in-band performance, and no magnetic materials are used.