By combining two strange quantum effects – light that is entangled and atoms that act as if they have negative mass – scientists have built a new kind of sensor that cancels out quantum noise, even at room temperature. The result is a compact and practical tool that can detect incredibly faint signals – from brainwaves and heartbeats to distant earthquakes or ripples in space. It could one day be used in hospitals, space research or early-warning systems – anywhere we need to hear what was once thought impossible to detect.
You know the feeling: the closer you lean in to hear a whisper, the easier it is to miss it. Scientists have long faced a similar paradox: in quantum physics, the more precisely you try to measure something, the more you disturb it. That strange rule has made it incredibly hard to hear the quietest signals in nature – such as the flutter of a brainwave, the beat of a distant heart or a ripple in space from a faraway black hole.
Now, researchers at the Niels Bohr Institute of the University of Copenhagen, Denmark have found a way to turn this problem into a solution. Instead of fighting quantum noise, they have built a system in which the noise cancels itself – like using two perfectly tuned microphones to erase background hum and leave only the real signal behind.
“We have built something completely new,” explains Eugene Polzik, a professor who led the study. “One part is light made of two colours that are quantum linked. The other is a cloud of atoms – including one that acts as if it has negative mass. That sounds strange, but imagine a swing that moves the opposite way when pushed. When we bring these parts together using quantum-linked light, the noise cancels itself, and the signal comes through more clearly than ever before.”
“This is the first time such a hybrid quantum sensor has been constructed and tested – culminating more than two decades of work,” adds Polzik.
A sensor built from quantum opposites
At the heart of the experiment are two previously unconnected quantum systems: a cloud of atoms and two colours of entangled light. Together, they create a sensor that operates through a delicate balance of opposites.
“This is the first hybrid quantum sensor of its kind,” Polzik says. “It is like building a bridge between two worlds that had never touched before.”
What sets this system apart – beyond its elegance – is its practicality. Unlike many quantum technologies, it works at room temperature and does not require complex cooling systems.
“You do not need cryogenics,” Polzik notes. “That is a big step toward real-world use.”
The system uses twin laser beams that can link an atomic ensemble acting as a negative mass with, for example, an interferometer that uses light to detect gravitational waves. The mirrors that have a “normal” positive mass and the negative mass atoms interact in a way that cancels quantum noise, enabling extremely faint signals to be measured without disturbing them.
Rather than resisting quantum noise, they ride it – using nature’s own rules to cancel the disturbance and reveal the signal.
“It may sound exotic,” Polzik says, “but it works beautifully.”
How entangled light and atomic swings cancel noise
To pull it off, the team generated two entangled beams of different colours (852 nm and 1056 nm). They used special optical techniques to link the two colours of light at the quantum level – creating entangled beams that act as one. One of the entangled beams interacts with the negative mass atomic ensemble, and the other one can interact with the system being investigated: for example, the gravitational wave detector.
Creating strong entanglement of beams with two colours has been an outstanding challenge. The most difficult part was keeping the entangled beams intact and locked in phase as they propagate through the system.
“The most difficult part was keeping the two light beams in perfect sync – despite travelling through completely different paths,” explains Polzik. “That is the quiet hero of this experiment – the invisible coordination that keeps everything working in harmony.”
Inside the gas cell, the atoms acted like quantum oscillators – imagine tiny pendulums, each gently swinging at just the right frequency. In earlier experiments, one oscillated with positive mass and the other with negative mass. The two systems cancel each other’s quantum noise – so that disturbances in one are offset by those in the other. This was used for ultrasensitive magnetometry.
“Think about this swing, which is upside down,” Polzik says. “Normally you would not do it, right? It would fall out. But imagine for a minute that it does not. And the more you swing, the smaller your energy gets. This is the best kind of picture for a layperson of a negative mass oscillator.”
In the present work, the atoms served as a negative mass oscillator, and a positive mass oscillator was simulated electronically.
Measuring faint signals without disturbing them
The breakthrough was not just building the system – it was proving that it could work at the quantum limit. In quantum mechanics, the mere act of observing something can disturb it – a built-in side-effect known as backaction. Most sensors cannot avoid this. But by using atoms that act like negative mass, the team managed to cancel not only the usual imprecision noise but also the backaction itself.
“We do not have the gravitational wave detector in the laboratory, so we have to simulate,” explains Polzik. “And then we put them together electronically – and demonstrate that we can cancel both the imprecision noise and the backaction noise using our atoms, which work as a negative mass oscillator.”
The team also used a quantum strategy called conditional squeezing – in which two entangled systems help to sharpen each other’s measurements. It is like using one finely tuned microphone to filter out noise so that the other one can hear more clearly.
“Once the atoms are entangled, measuring one helps to predict the state of the other,” says Polzik. “It is like shining light on one eye and seeing more clearly out of the other.”
He continues: “By generating entanglement between the spins, we can make it squeezed – narrowing uncertainty in one direction even if it grows in the other. If you care only about, say, the magnetic fields that tilt the spin this way, you gain precision exactly where it matters.”
“To detect a signal in biomedical applications, we take two spins and entangle them so that they are always pointing in opposite directions. Then we bring one near the body and keep the other safely away. If their angles shift even slightly, it means something has changed inside.”
A decades-long journey in quantum sensing |
And the bigger picture?
“If your particles are perfect – but not connected – you still hit a wall of uncertainty,” Polzik says. “But when you entangle them, that wall can crack open. That is where the real magic begins.”
This was not just a technical breakthrough – it was a personal journey built on curiosity, creativity and close teamwork.
“Sometimes you do not start with a goal. You follow the physics,” adds Polzik. “We did not know that this would work when we started. But step by step, the pieces fit together. That is the beauty of following the physics.”
The idea behind the present experiment was proposed in articles published a few years ago. “There were two articles, one we wrote with my colleague from Russia and another with him and a Danish colleague. A close-knit team.”
“And sure, I could say now that it was my idea,” he smiles. “But in reality, I just proposed a path leading to this result, and it developed step by step – bit by bit. The origin of this research field – now called measurements in quantum reference frame or decoherence-free subspaces – can be traced to our first experimental article published in Nature in 2001.”
For most of his career, Polzik focused on solving fundamental problems in quantum mechanics. But now, he admits, the applications are becoming just as exciting.
Quantum sensing from brainwaves to black holes
These applications are no longer theoretical. The system currently performs in the frequency range between a few kilohertz to a hundred kilohertz. This is the frequency range for many of the most elusive signals, but pushing the frequency even further down to the tens or hundreds of hertz will be even more exciting. Further technical improvements should enable brainwaves, heartbeats and even early traces of gravitational waves to be detected.
“What is exciting is that this sensor works in the same frequency range as the human body,” notes Polzik. “Heartbeats. Brainwaves. Even the faint early signals of gravitational waves. That opens doors across many fields.”
And it is not just a sensor – it is a flexible platform.
“We are not just building one machine,” he says. “We are building a toolbox – pieces you can mix and match to solve different sensing problems.”
It is compact, robust and operates at room temperature – free from the cryogenic and vacuum requirements that limit many quantum systems.
Unlike kilometre-scale observatories such as the Laser Interferometer Gravitational-Wave Observatory (LIGO), which suppress quantum noise at narrow frequencies using massive optical resonators, this tabletop device achieves broadband suppression with elegant simplicity.
“The sensor and the spin system interact with two entangled beams of light,” Polzik explains. “After the interaction, we detect the beams and combine the signals. The result is broadband detection – across a wide range of frequencies – beyond the standard quantum limit.”
Listening to nature’s quietest signals
One of the most poetic insights in this work is that quantum technologies often aim to measure the unmeasurable. They work by shifting what we thought was a hard limit – inviting us to sense things we once believed were beyond reach.
For Polzik, this is not just a philosophical idea – it is a personal journey. He first made headlines two decades ago with groundbreaking work on quantum teleportation and sees his current research as part of the same quest.
“Surprisingly enough, quantum teleportation and quantum sensing have a lot in common,” he reflects. “In both cases, you want to measure something that you cannot measure at first glance.”
The sensor is not in use yet – but it shows the way forward. A future in which the quietest signals – from a human heartbeat to a ripple in space – can finally be heard.
Where mass can be negative, light can come in two colours, and noise – once thought inevitable – can cancel itself out.
It is not just about precision. It is about listening – really listening – to the universe as it whispers its secrets.
