‘Squeezed light’ technology could accelerate path to quantum networking

Imagine a future where networks of entangled photons, harnessing the unique properties of quantum physics, enable computing and technologies beyond anything we know today.

Researchers at the U.S. Department of Energy’s Fermi National Accelerator Laboratory and the California Institute of Technology are using a special kind of light — called “squeezed light” — they believe can overcome key challenges in building scalable quantum networks.

The cutting-edge research represents essential progress toward building a quantum network that could transform scientific research by connecting powerful quantum computers.

These networks rely on entangled qubits — pairs of quantum bits that share information across distances. In quantum physics, entanglement occurs when two or more particles become linked in such a way that what happens to one affects the other, even if they are far apart.

Researchers are working to generate and distribute entangled qubits across longer distances to build larger quantum networks. However, quantum networks over fiber optic cable face challenges such as signal loss, memory decoherence, and delays inherent to communication technologies widely used today.

A new study led by Fermilab shows the potential of a quantum network protocol that can overcome these challenges by using squeezed light, a special state of light with reduced noise and enhanced sensitivity, to pick up faint signals. This research marks a first step toward increasing the entanglement generation rate, which is a key requirement for large-scale quantum networks.

Scientists use various methods to encode qubits with quantum information. One approach uses photons — packets of light energy that have both wave and particle properties — that scientists prepare in special ways to manipulate and control qubits.

The new method described in the study uses two optical encoding types. The combination helps overcome the weaknesses of each and significantly increases the rate of entangled pair generation over long distances.

To transfer entanglement from one pair of qubits to another, scientists use a process called entanglement swapping. Unlike traditional methods that produce only one entangled pair per swap, squeezed light allows many qubits to become entangled.

“The greater number of entangled pairs per light signal, the greater the entanglement distribution efficiency,” said Alexandru Macridin, a scientist at Fermilab who led this research. “But since entangled qubits readily decay, the entanglement generation rate is very important when you build this kind of thing. The more you create, the better.”

Using the new technique, scientists generate entangled pairs by preparing light at two distant locations. Both light sources are sent to a central site equidistant between them and routed through a beam splitter that separates them into two beams of light, one transmitted and one reflected. The light beams return to the central location, where they recombine and are measured. The laws of quantum mechanics dictate that measurement destroys the light but leaves multiple pairs of long-distance entangled qubits.

The method’s effectiveness depends on the strength of the squeezing, but current technology limits how much can be squeezed.

“I calculated that producing one extra entangled pair requires three decibels of squeezing,” explained Macridin. “This means that no more than three or four entangled qubit pairs can be produced using current technology because it only allows for squeezing up to 15 decibels of light.”

Going forward, the team will explore ways to reduce light loss and other effects from fiber optic cables. They will also work to improve the technology and compare light squeezing with other methods for generating multiple qubits. This research is part of a larger collaborative project led by Fermilab called the Advanced Quantum Network, or AQNET. Funded by the Department of Energy Office of Science Advanced Scientific Computing Research program, AQNET aims to connect a local quantum network at Fermilab with quantum nodes at Argonne National Laboratory, Northwestern University, and the University of Illinois at Urbana-Champaign using optical fiber. The ultimate goal is to build a nationwide quantum network.

“With AQNET, we are now at the stage of our development where our network connections can reach metropolitan-scale distances with fiber optics,” said Fermilab scientist Cristián Peña, who leads the project. “This new protocol is another step toward that goal.”

So far, Macridin and his colleagues have confirmed that the new protocol can improve entanglement distribution efficiency in ideal laboratory settings.

The protocol is also significant because it builds on existing entanglement swapping hardware developed at Fermilab. The hardware can be readily integrated into a wide range of applications, including quantum repeaters — devices that extend quantum entanglement over longer distances and support the development of diverse quantum network designs.

Fermi National Accelerator Laboratory is America’s premier national laboratory for particle physics and accelerator research. Fermi Forward Discovery Group manages Fermilab for the U.S. Department of Energy Office of Science. Visit Fermilab’s website at www.fnal.gov and follow us on social media.

What can neutrinos — particles that are imperceptible to all but the most sensitive devices ever built — tell us about how matter triumphed over antimatter in the early universe? As head-in-the-clouds as this question sounds, real people working on the Deep Underground Neutrino Experiment are developing down-to-earth detectors, infrastructure and processes to run this very ambitious experiment to find out.

The liquid argon-based technology chosen for the DUNE neutrino detectors promises to deliver stunning scientific insights and to be complementary to experiments pursuing similar goals that use the more traditional water-based technology. The U.S. Department of Energy’s Fermi National Accelerator Laboratory is the host for DUNE, in partnership with funding agencies and more than 1,400 scientists and engineers from around the world.

Atoms are mostly empty space with a nucleus in the middle and point-like electrons orbiting around it — like infinitesimal solar systems. The nucleus is held together by a force aptly named the “strong force” and it keeps its electrons in tow by the “electromagnetic force,” the familiar opposites-attract force. These are the two strongest of the four known fundamental forces of nature, and neutrinos are immune to both. They are, however, subject to what is called the “weak force,” which comes into play only if they get very close to a nucleus or an electron, and to gravity, which is negligible for this tiny particle.

Neutrinos — travelling at nearly the speed of light and basically blind to and unaffected by everything around them — therefore fly right through matter as though it’s not even there. Except that every once in a while a neutrino gets within weak force range of an atomic particle and crashes right into it. As in any collision, stuff sprays out — subatomic particles, in this case. DUNE wants to capture this once-in-a-while event in its detectors as many times as it can.

To improve the chances for neutrino interactions to both occur and then be detected, DUNE needs: (a) lots of neutrinos, (b) shielding from cosmic rays that would otherwise drown out the neutrino signals, and (c) lots of target material – the denser the better.

The Long-Baseline Neutrino Facility at Fermilab is building a beamline to send a prodigious flow of neutrinos (that’s part “a”) to DUNE’s two neutrino detectors, a smaller near detector at Fermilab, and a gigantic, modular far detector 800 miles downstream. DUNE’s far detector modules will be constructed and installed in South Dakota in a laboratory that is a mile underground (that’s “b,” the earth above the detector will absorb the cosmic rays). Finally, each detector module will be filled with kilotons of a quite well-endowed material, liquid argon (“c”).

In a detector known as a LArTPC, shorthand for liquid-argon time projection chamber, a bath of ultra-pure cryogenic liquid argon is subjected to a strong electric field created between a cathode and an anode, which are like the negative and positive terminals on a battery. Charged particles that emerge from neutrino-nucleus collisions strip electrons from argon atoms in the surrounding volume. These freed electrons, called ionization electrons, drift in the enormous electrified volume of argon, an inert element that won’t gobble them up enroute, to a multi-layered anode, which enables the time-projection aspect of the experiment. From the resulting 3D images, physicists can see how the event evolved and work back to understand the nature of the originating neutrino interaction.

“The ionization electrons carry the imprint of the neutrino interaction, said Hilary Utaegbulam, a graduate student at the University of Rochester. “They tell us where the neutrino interacted, how much energy it deposited, and depending on how the ionization patterns cluster, what type of neutrino it was. The electrons act as messengers that tell us a great deal about the interaction itself.”

The very fine-grained imaging from a liquid-argon time projection chamber makes it a desirable choice for neutrino experiments. Water Cherenkov technology, which relies on detecting photons generated when a charged particle moves faster than the speed of light in the water, has strengths that are complementary to those of LArTPCs, but lacks some of the latter’s capabilities. With an impressive history of discovery including that of neutrino oscillation about 25 years ago by the experiments SuperKamiokande in Japan and SNO in Canada, water Cherenkov is the technology choice of the other leading next-generation neutrino detector, HyperKamiokande.

The LArTPC technology, together with DUNE’s longer 800-mile baseline and its companion neutrino beam that spans a wide range of energies, will uniquely enable DUNE to measure all the sought-after neutrino oscillation properties.

“The DUNE LArTPCs offer millimeter spatial resolution on a timescale of milliseconds,” said Afroditi Papadopoulou, an Oppenheimer postdoctoral fellow at Los Alamos National Laboratory. “They also provide excellent particle identification, energy precision, and low particle-detection thresholds. All these properties make LArTPCs the ideal detectors for performing the high-impact measurements essential for world-leading discoveries.”

In addition, liquid argon is a tremendous scintillator. This means that when energy from an interaction bumps a neighboring argon atom up to an unstable excited state, the atom emits a packet of light energy called a photon as it returns to its ground state. LArTPCs are therefore supplemented with photon detectors. The photons, naturally traveling at the speed of light in argon, are detected nearly instantaneously, providing precise timing information. Light detection enhances the detector’s capabilities for all of DUNE’s planned measurements and opens up prospects for further physics explorations.

Finally, as liquid argon is a byproduct of the large industrial production of liquid oxygen and nitrogen, and is itself used in industrial applications such as welding, it is abundantly available and inexpensive. The only other liquids that could offer similar performance are, like argon, found in the far-right column of the periodic table, and are more challenging to acquire.

“The LArTPC design for DUNE gives us the benefits of large and scalable detectors without sacrificing high-resolution energy measurement over a wide range of neutrino energies,” said Fermilab scientist Anne Norrick. “We will have some healthy competition from HyperK, but when all is said and done, for neutrino detection, DUNE’s LArTPC technology is simply unmatched.”

Andrzej Szelc, a professor at the University of Edinburgh, has been elected as co-spokesperson for the Short-Baseline Near Detector experiment. SBND plays an essential role in the Short-Baseline Neutrino Program at the U.S. Department of Energy’s Fermi National Accelerator Laboratory.

“I’ve seen SBND’s voyage from an idea to becoming a reality. It’s a great honor to be chosen as the first international spokesperson to co-lead this collaboration into this next stage of data analysis,” said Szelc.

Spokespeople for physics experiments are principal leaders of the collaborations conducting the research. They ensure experiments operate effectively to meet scientific goals and serve as the primary representatives in communications.

The SBND collaboration brings together 210 scientists from 40 institutions in Brazil, Spain, Switzerland, the U.K. and the U.S.

Szelc, who previously served as the experiment’s physics coordinator, will succeed Ornella Palamara — a senior Fermilab scientist recently appointed director of user facilities and experiments — who has co-led the collaboration since 2014.

“Ornella has been instrumental in building the fantastic SBND collaboration, and her scientific vision and contributions to the experiment go back to the very beginning when we just had a notion of building a near detector along the neutrino beamline,” said David Schmitz, professor at the University of Chicago and co-spokesperson for the SBND collaboration. “Andrzej’s experience with SBND and other experiments of this kind is very broad, and I look forward to working with him as we continue our first physics run.”

As the near detector in the Short-Baseline Neutrino Program, SBND observes the neutrinos as they are produced in the Fermilab beam. This enables the SBN Program to definitively know the composition of the neutrino beam before it has a chance to change, through a process called oscillation, giving the collaboration a better handle on testing for the existence of a new type of neutrino.

Since seeing their first neutrinos last year, the SBND collaboration started their first official physics run in December. “We have this detector that works fantastically well, and we can reconstruct the neutrino interactions very, very precisely,” said Szelc. “Already, we’re seeing about 7,000 neutrinos per day. That adds up to the largest sample of neutrino interactions on argon in the world.”

SBND’s large data sample will enable physicists to study neutrino interactions in unprecedented detail. The physics of these interactions is crucial for other neutrino experiments, such as the long-baseline Deep Underground Neutrino Experiment.

“Our live time for capturing neutrinos has been 98.6%, which isn’t something every experiment can say for its first run,” said Schmitz. “And the quality of the data is extremely high, thanks to the international team of amazing scientists working on SBND, strong support from Fermilab for the experiment, and an incredibly stable beam delivery from the Fermilab Accelerator Complex, making this year the best yet for the Booster Neutrino Beam.”

With so many neutrino interactions, SBND is also advancing techniques for the analysis of scientific data, including machine learning methods, which can be applied at nearly every stage of the data analysis. The progress made with this kind of pattern recognition software can be used in other applications like medical physics — including analysis of images from X-rays, CT-scans and MRIs.

The Short-Baseline Near Detector international collaboration is hosted by the U.S. Department of Energy’s Fermi National Accelerator Laboratory. The collaboration consists of 40 partner institutions, including national labs and universities from five countries. SBND is one of the particle detectors in the Short-Baseline Neutrino Program that provides information on a beam of neutrinos created by Fermilab’s particle accelerators.