Scientists have built a new kind of molecular qubit that could help connect quantum computers over existing telecommunications technology — laying the foundation for a future quantum internet.
The new qubit contains a rare-earth element called erbium, which has optical and magnetic properties that allow it to transmit quantum information using the same wavelengths as fiber-optic networks.
The team published their findings Oct. 2 in the journal Science. In a statement, they called the technology "a promising new building block for scalable quantum technologies," from ultra-secure communication links to long-distance networks of quantum computers — often referred to as the quantum internet.
Plenty of research has gone into building the technology required for a quantum internet, including a new chip built in September that helps beam quantum signals over real-world fiber-optic cables. In the new study, the researchers focused on building a new type of qubit that could help to transmit data.
"By demonstrating the versatility of these erbium molecular qubits, we're taking another step toward scalable quantum networks that can plug directly into today's optical infrastructure,” David Awschalom, the study's principal investigator and a professor of molecular engineering and physics at the University of Chicago, said in the statement.
A different type of qubit
Qubits are the most basic form of quantum information and serve as the quantum equivalent to bits in classical computing.
That's largely where the comparison ends, though. Whereas classical bits compute in binary 1s and 0s, qubits behave according to the weird rules of quantum physics, allowing them to exist in multiple states at once — a property known as superposition. A pair of qubits could, therefore, be 0-0, 0-1, 1-0 and 1-1 simultaneously.
Qubits typically come in three forms: superconducting qubits, which are made from tiny electrical circuits; trapped ion qubits, which store information in charged atoms held in place by electromagnetic fields; and photonic qubits, which encode quantum states in particles of light.
Molecular qubits use individual molecules, often built around rare-earth metals whose electron spin defines their quantum state. This spin gives the electron a tiny magnetic field, the direction of which defines the qubit's value. Like a regular bit, it can represent a 1, a 0, but it can also be a superposition of both states.
What makes the new, erbium-based qubit unique is that it behaves like both a spin qubit and a photonic qubit; it can store information magnetically while being read out using optical signals.
In an experiment, the researchers showed that the erbium atom's spin could be placed in a controlled superposition — a key requirement for a functioning qubit. Because the spin state influences the wavelength of light the atom emits, the team could read the qubit's quantum states using standard techniques like optical spectroscopy.
"These molecules can act as a nanoscale bridge between the world of magnetism and the world of optics," Leah Weiss, co-first author on the paper and postdoctoral scholar at the University of Chicago Pritzker School of Molecular Engineering, said in the statement. "Information could be encoded in the magnetic state of a molecule and then accessed with light at wavelengths compatible with well-developed technologies underlying optical fiber networks and silicon photonic circuits."
Long-distance quantum data
Operating at telecom wavelengths provides two key advantages, the first being that signals can travel long distances with minimal loss — vital for transmitting quantum data across fiber networks.
The second is that light at fiber-optic wavelengths passes easily through silicon. If it didn't, any data encoded in the optical signal would be absorbed and lost. Because the optical signal can pass through silicon to detectors or other photonic components embedded beneath, the erbium-based qubit is ideal for chip-based hardware, the researchers said.
"Telecommunications wavelengths offer the lowest loss rate for light traveling through optical fibers. This is critical if you want to reliably send information encoded in a single photon (a single particle of light) beyond the lab," Awschalom told Live Science in an email.
Scale is another benefit, Awschalom explained. Each qubit is built from a single molecule around 100,000 times smaller than a human hair. Because their structure can be tuned via synthetic chemistry, molecular qubits can be integrated into environments that others can't — including solid-state devices or even inside living cells.
This level of control could help tackle one of quantum computing's biggest engineering challenges: building quantum compatibility directly into existing technologies.
"Integration is a key step in scaling the technology and an outstanding challenge in the field," Awschalom said. "We are working on integrating these qubits in on-chip devices and believe that this will open new regimes in controlling, detecting, and coupling molecules."
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