Purdue University researchers were able to build a gate (a quantum-version of a transistor) using two qudits, which represent a more stable form of qubits.
Qubits are notorious for being highly unstable, and the vast majority of quantum research so far has gone into finding ways to make qubits more stable. It’s also why there are so many types of universal quantum computers, from superconducting and trapped-ion quantum computers to the more exotic topological quantum computers.
Qubits need to be stable long enough so that the algorithm running on them can complete its task. However, the most stable qubits we’ve seen so far can last only a small fraction of a second.
The Purdue researchers created a more stable version of qubits by embedding them into photons (particles of light not easily disturbed by environmental noise). This new version is called a qudit, and unlike qubits that can exist only in a superposition of 0 and one states, qudits can exist in superpositions of 0, 1, and 2 states. More states mean that more data can be encoded and processed by a smaller amount of qubits, making the new solution much more efficient
Previous photonic approaches were able to encode 18 qubits within six entangled photons. Through the entanglement process, you can interconnect two particles so that when you change the state of one the other changes as well.
The Purdue researchers were able to use a gate to encode 20 qubits in only two photos. Poolad Imany, a postdoctoral researcher in Purdue’s School of Electrical and Computer Engineering, explained why it’s preferable to use as few photos as possible to encode information:
“Photons are expensive in the quantum sense because they’re hard to generate and control, so it’s ideal to pack as much information as possible into each photon.”
Typically, gates built on photonic platforms don’t work so well because photons don’t naturally interact with each other well, which makes it difficult to manipulate a state of one photon to affect another.
The Purdue researchers were able to encode the quantum information in the time and frequency domains of the four photons. This made operating the quantum gates deterministic as opposed to probabilistic, which means the operations could be performed as necessary. This generated four fully-entangled qudits, which occupy a Hilbert space of 1,048,576 dimensions, or possibilities.
The Purdue team intends to use its latest research to build algorithms for quantum machine learning applications and for simulating molecules.