Detecting extremely weak radio waves is at the heart of many modern technologies, including satellite navigation, long-distance communications, radio telescopes and magnetic resonance imaging (MRI) systems. In some detectors, weak radio signals are converted into optical signals that can then be transported long distances via optical fibres. In addition to requiring expensive modulators to convert the electronic signals into optical signals, these converters must be cooled to cryogenic temperatures, making them expensive and inconvenient to operate.

The new device was created by Eugene Polzik and colleagues at the University of Copenhagen, along with researchers at the Technical University of Denmark and the Joint Quantum Institute at the University of Maryland. The team says that its device can detect extremely weak radio waves by converting them directly into light signals. These signals can then be transmitted and analysed using standard optical tools and the device uses much less energy than conventional modulators.

High performance and efficiency

The detector works at room temperature and Polzik says that it "promises performance comparable to the best cryogenically cooled electronics". "Moreover, the radio signals in our method are efficiently converted into optical signals, which can be transmitted via optical cables with much lower loss than electrical signals can be transmitted by metal wires," he says.

At the heart of the device is an antenna that is connected to a capacitor. One of the two capacitor plates is an extremely high-quality silicon-nitride membrane that is about 500 μm across and about 200 nm thick, after being coated with a reflective layer of aluminium.

When the capacitor encounters radio waves at its resonant frequency, the nanomembrane vibrates. "The radio waves detected by the antenna induce charge fluctuations in the capacitor," says Polzik. "By applying an external bias voltage to the capacitor, we can convert these fluctuations into mechanical vibrations of the membrane." A laser beam is bounced off the membrane, which produces an optical phase shift that can be measured using standard optical techniques. "We have thus converted a radio signal detected by the antenna into an optical signal," says Polzik.

Running hot

When traditional radio receivers pick up faint radio waves, heat-related noise can distort the signal. But when radio signals are converted into a resonant mechanical vibration, the random effect of heat becomes negligible. The reflected light picks out the radio wave with little of the noise that affects standard radio receivers.

The new device has a room-temperature sensitivity of 100 pV Hz–1/2 for radio waves at 1 MHz. The team expects that this could be improved by a factor of 20, which would put the receiver on a par with the best devices using cryogenics.

The next steps for the team are to use microfabrication techniques to further miniaturize the device so that it fits on a chip and to improve its sensitivity. "We also plan to extend the frequency range of the devices from a megahertz domain to the hundred megahertz to the gigahertz domain, which is most relevant for applications in communication and sensing," says Polzik.

A quantum internet

Potential applications of the detector include those that currently use cooled preamplifiers. These include high-resolution nuclear-magnetic-resonance systems and radio telescopes – both of which rely on liquid-helium-cooled detectors. Chip-sized devices could lead to smaller and more energy-efficient communication devices and navigation systems.

In the long term, the technology could make it possible to convert quantum states of microwave radiation into optical quantum states, claims Polzik. "Such a conversion will be an important step towards distributed quantum networks. It may help researchers to use optical photons – ideal carriers of quantum information – to connect distant superconducting qubits," he says.

Clear technological potential

Physicist Mika Sillanpää at Aalto University in Finland, who was not involved in the study, says that the research "has clear technological potential" to become reality in the future. "From the basic-research standpoint, the work creates a hybrid physical system, which has potential to function in the quantum-mechanical limit," he says.

Sillanpää adds that the technology could be used as a "router" or node to connect quantum computers. "At the moment this is mostly hype, but might become reality one day," he says.

The detector is described in Nature.

Further reading>

Entangling photons on demand (Jun 2010)
Electrons surf between qubits (Sep 2011)