Single chip developed by Penn's Feng Liang's team successfully doubles quantum information space

Recently, researchers at the University of Pennsylvania (UPenn) have developed a chip that communicates with high-dimensional "qudits" (quantum information units with energy levels greater than 2), doubling the quantum information space compared to any previous on-chip laser, and exceeding the security and robustness of existing quantum communication hardware.

 

Liang Feng

 

Liang Feng, a professor in UPenn's Department of Materials Science and Engineering (MSE) and Department of Electrical Systems and Engineering (ESE), along with Zhifeng Zhang, an MSE postdoc and associate professor (associate appointment) in the School of Modern Engineering and Applied Science at Nanjing University, and Haoqi Zhao, a PhD student in ESE, published in Nature on Nov. 16, "Spin-orbiting microlaser emitted in four-dimensional Hilbert space The technology was first introduced [1].

 

 

The group collaborated with scientists from Politecnico di Milano, the Institute for Interdisciplinary Physics and Complex Systems, Duke University and the City University of New York (CUNY).

 

01Bits, quantum bits and qudit

 

While non-quantum chips use bits to store, transmit and compute data, state-of-the-art quantum devices use quantum bits. While a bit can be either a 1 or a 0, a quantum bit is a unit of digital information that can be both a 1 and a 0. In quantum mechanics, this state of simultaneity is called a "superposition". Quantum bits that are in superposition and larger than two energy levels are called qudit to represent more states.

 

"In classical communication," says Professor Feng [2], "a laser can emit a pulse encoded as a 1 or a 0. These pulses can be easily cloned by interceptors who want to steal information and are therefore not very secure. In quantum communication using quantum bits, pulses can have any superposition state between 1 and 0. The superposition makes the quantum pulse impossible to be copied. Unlike algorithmic encryption, which uses complex mathematics to stop hackers, quantum cryptography is a physical system that keeps information secure."

 

However, quantum bits are not perfect. With only two energy levels to superimpose, quantum bits have limited storage space and low tolerance for interference.

 

The four-energy-level qudit of Feng Liang's laboratory device has enabled a major advance in quantum cryptography, increasing the maximum confidential key rate for information exchange from one bit per pulse to two bits per pulse: the device provides a four-energy-level superposition, opening the door to further dimensionality.

 

The biggest challenge is the complexity and non-scalability of the standard setup," said Zhifeng Zhang. We already know how to generate these four-energy level systems, but it requires a lab and many different optical tools to control all the parameters associated with the increase in size. Our goal was to achieve this on a single chip, and that's exactly what we did."

 

02The physics of cybersecurity

 

Quantum communication uses superimposed state photons that are in tight control. Properties such as position, momentum, polarization, and spin exist at the quantum level as multiples, each of which is governed by probabilities: these probabilities describe the likelihood that a quantum system (an atom, a particle, a wave), when measured, has a single property.

 

In other words, quantum systems are neither here nor there; at the same time, they are both here and there. Only the act of observation: detection, observation, measurement, leads to quantum systems with fixed properties. Quantum superpositions assume a single state once they are observed, making it impossible to intercept them or replicate them without being detected.

 

The superdimensional spin-orbit microlaser builds on the team's earlier work on vortex microlasers [3], which sensitively tunes the orbital angular momentum (OAM) of photons. Recent devices have upgraded the capabilities of previous lasers by adding another level of control over photon spin.

 

Super-dimensional spin-orbit microlaser

 

Optical setup and spectral characterization of a miniature laser.

 

This additional control of energy levels enables manipulation and coupling of OAM and spin, and allows the team to achieve a breakthrough in quadruple-energy systems.

 

However, the difficulty of controlling all these parameters at once is what has been holding back qudit generation in integrated photonics; at the same time, represents the main experimental achievement of the team's work.

 

"Think of the quantum states of our photons as two planets stacked on top of each other," says Haoqi Zhao, "Previously, we only had information about the latitude of these particles. With this information, we can create up to two levels of superposition. We didn't have enough information to stack them in four layers. Now, we also have longitude. This is the information we need to manipulate the photons in a coupled fashion to achieve the increase in dimensionality. We are coordinating the rotation and spin of each particle and keeping the two particles in relation to each other."

 

Four-energy Bloch hyperspheres by finely controlled inter-ring coupling

 

03Not only quantum computing but also quantum cryptography requires four-energy level systems

 

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

 

If the sender and receiver (Alice and Bob) find some degree of discrepancy between their messages, they know that someone is trying to intercept their messages. However, if the transmission remains largely intact, Alice and Bob understand that the channel is secure and use the quantum transmission as a key to encrypt the message.

 

How does this improve the security of non-quantum communications? If we imagine the photon as a sphere spinning upward, we can roughly understand how the photon classically encodes the binary number 1. If we imagine it rotating downward, we can understand 0.

 

When Alice sends classical photons encoded in bits, Eve, the eavesdropper, can steal, copy, and replace them without Alice or Bob realizing it. Even if Eve cannot decrypt the data she steals, she may hide it and wait for the near future, when advances in computing technology may allow her to break through.

 

Quantum communication adds a stronger layer of security. If we imagine a photon as a sphere spinning up and down at the same time, encoding both 1s and 0s, we can understand how quantum bits maintain dimensionality in their quantum state.

 

When Eve tries to steal, copy, and replace the quantum bits, her ability to capture the information will be compromised and her tampering will show up in the loss of the superposition. alice and Bob will know that the channel is insecure and will not use the security key until they can prove that Eve has not intercepted it. Only then will they send the intended encrypted data using the algorithm enabled by the quantum bit key.

 

However, while the laws of quantum physics may prevent Eve from copying the intercepted quantum bits, she may interfere with the quantum channel. alice and Bob will need to continue generating keys and sending them back and forth until she stops interfering. The unexpected interference collapses the superposition effect as the photons travel through space, which is the reason for the interference pattern.

An information space limited to two energy levels of quantum bits has very little tolerance for these errors.

 

To solve these problems, quantum communication requires additional dimensions. If we imagine a photon spinning in two different directions (the way the Earth spins around the Sun) and rotating (the way the Earth spins on its own axis) at the same time, we can get a sense of how the qudit in Feng Liang's lab works.

 

If Eve tries to steal, copy and replace the qudit, she will not be able to extract any information and her tampering will be clear. The messages sent would be more fault-tolerant: not only to Eve's interference, but also to accidental defects introduced as the messages travel through space; Alice and Bob would be able to exchange information efficiently and securely.

 

"There is a lot of concern," Feng Liang said, "that mathematical encryption, no matter how sophisticated, will become less and less effective because we are advancing so rapidly in computing technology. Quantum communication's reliance on physical rather than mathematical barriers makes it immune to these future threats, and it is more important than ever that we continue to develop and refine quantum communication technology."

 

Reference link:

[1]https://www.nature.com/articles/s41586-022-05339-z

[2]https://phys.org/news/2022-11-microlaser-chip-dimensions-quantum-communication.html

[3]https://www.science.org/doi/10.1126/science.abb8091