Penn Engineering researchers have developed a chip that surpasses the security and robustness of existing quantum communication hardware. Their technology communicates in ‘qudits’, doubling the quantum information space of all previous on-chip lasers.

Liang Feng, Professor in the Faculty of Materials Science and Engineering (MSE) and Faculty of Electrical Systems and Engineering (ESE), along with MSE postdoc Zhifeng Zhang and ESE Ph.D. Student Haoqi Zhao debuted the technology in a recently published study *Nature**. *The group collaborated with scientists from the Polytechnic University of Milan, the Institute for Interdisciplinary Physics and Complex Systems, Duke University and the City University of New York (CUNY).

**Bits, qubits and qudits**

While non-quantum chips use bits to store, transmit, and compute data, advanced quantum devices use qubits. Bits can be 1s or 0s, while qubits are digital units of information that can be 1s and 0s at the same time. In quantum mechanics, this state of simultaneity is called “superposition”.

A quantum bit in a superposition state of more than two levels is denoted as qu*that* to signal those extra dimensions.

“In classical communication,” Feng says, “a laser can emit a pulse encoded as either a 1 or a 0. These pulses can easily be cloned by an interceptor trying to steal information and are therefore not very secure. Pulse can have any superposition state between 1 and 0. Superposition makes it so that a quantum pulse cannot be duplicated. Unlike algorithmic encryption, which blocks hackers with complex math, quantum cryptography is a physical system that keeps information secure.”

However, qubits are not perfect. With only two layers of overlay, qubits have limited storage space and low tolerance for interference.

The Feng Lab device’s four-level qudits enable significant advances in quantum cryptography, increasing the maximum secret key rate for information exchange from 1 bit per pulse to 2 bits per pulse. The device offers four levels of overlay, opening the door to further dimensional increases.

“The biggest challenge,” says Zhang, “was the complexity and non-scalability of the standard structure. We already knew how to generate these four-level systems, but it required a lab and many different optical tools to control all of the parameters associated with magnification. Our goal was to achieve this on a single chip. And that’s exactly what we did.”

**The physics of cybersecurity**

Quantum communication uses photons in tightly controlled superposition states. Properties such as location, momentum, polarization, and spin exist as multiplicities at the quantum level, each determined by probabilities. These probabilities describe the probability that a quantum system – an atom, a particle, a wave – assumes a single attribute when measured.

In other words, quantum systems are neither here nor there. They are both here and there. Only the act of observing – detecting, looking, measuring – causes a quantum system to assume a fixed property. Like a subatomic statue game, quantum overlays assume a single state as soon as they are observed, making them impossible to intercept or copy undetected.

The hyperdimensional spin-orbit microlaser builds on the team’s previous work with vortex microlasers that fine-tune the orbital angular momentum (OAM) of photons. The latest device improves on the previous laser’s capabilities by adding another layer of command via photonic spin.

This additional layer of control – the ability to manipulate and couple OAM and Spin – is the breakthrough that allowed them to achieve a four-layer system.

The difficulty of controlling all of these parameters simultaneously was what has hampered qudit generation in integrated photonics and represents the greatest experimental achievement of the team’s work.

“Think of the quantum states of our photon as two planets stacked on top of each other,” says Zhao. “Before, we only had information about the latitudes of these planets. This allowed us to create a maximum of two overlay layers. We didn’t have enough information to stack them in four. Now we also have the lines of longitude. This is the information we need to manipulate photons in a coupled way and achieve dimensional enhancement. We coordinate the rotation and rotation of each planet and keep the two planets in a strategic relationship.”

**Quantum cryptography with Alice, Bob and Eve**

Quantum cryptography relies on overlay as a tamper-proof seal. In a popular cryptographic protocol called Quantum Key Distribution (QKD), randomly generated quantum states are sent back and forth between the sender and receiver to test the security of a communication channel.

When the sender and receiver (always Alice and Bob in the fairytale world of cryptography) detect some discrepancy between their messages, they know that someone has tried to intercept their message. However, if the transmission remains largely intact, Alice and Bob understand the channel as secure and use the quantum transmission as a key for encrypted messages.

How does this improve the security of non-quantum communications? If we think of the photon as a sphere rotating upwards, we can roughly imagine how a photon might classically encode the binary digit 1. If we imagine it rotating down, we understand 0.

When Alice sends out classic photons encoded in bits, Eve the eavesdropper can steal, copy, and replace them without Alice or Bob noticing. Even if Eve is unable to decrypt the data she has stolen, she may hide it in the near future when advances in computer technology could allow her to break through.

Quantum communication adds a stronger layer of security. If we imagine the photon as a sphere rotating up and down simultaneously, encoding 1 and 0 at the same time, we can get an idea of how a qubit in its quantum state retains its dimension.

When Eve attempts to steal, copy, and replace the qubit, her ability to gather the information is compromised and her manipulation is revealed by the loss of the overlay. Alice and Bob will know the channel is not secure and will not use a security key until they can prove Eve hasn’t been listening to it. Only then do they send the intended encrypted data using an algorithm activated by the qubit key.

Although the laws of quantum physics may prevent Eve from copying the intercepted qubit, she may be able to jam the quantum channel. Alice and Bob must keep generating keys and sending them back and forth until she stops meddling. Random perturbations that collapse the superposition as the photon travels through space also contribute to interference patterns.

The information space of a qubit, which is limited to two levels, has a low tolerance for these errors.

To solve these problems, quantum communication needs additional dimensions. If we imagine a photon spinning in two different directions simultaneously (like the earth revolves around the sun) and rotating (like the earth rotates on its own axis), we get a sense of how the Feng Lab does it Qudits work.

If Eve attempts to steal, copy, and replace the Qudit, she will be unable to extract any information and her manipulation will be clear. The message sent has a much greater margin of error – not only for Eve’s tampering, but also for accidental errors introduced as the message travels through space. Alice and Bob can exchange information efficiently and securely.

“There’s a lot of concern,” says Feng, “that mathematical encryption, no matter how complex, is becoming less and less effective because we’re advancing so rapidly in computer technology. Quantum communication’s reliance on physical rather than mathematical barriers makes it immune to these future threats. It’s more important than ever that we continue to develop and refine quantum communication technologies.”

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