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New Two-Qudit Gate Helps Manipulate Quantum Information More Reliably

Quantum information processing assures to be much quicker and more secure when compared to what is achieved by the existing supercomputers, but it is not yet a reality as its building blocks—qubits—are extremely unstable.

A two-qudit gate, among the first of its kind, maximizes the entanglement of photons so that quantum information can be manipulated more predictably and reliably. (Image credit: Purdue University image/Allison Rice)

Scientists from Purdue University are among the first to construct a gate—what could be a quantum version of a transistor, employed in existing computers for processing information—with qudits. Qubits can exist only in superpositions of 0 and 1 states, while qudits exist in multiple states, like 0, 1, and 2. With more states, more data can be encoded and processed.

In addition to being intrinsically more efficient than qubit gates, the qudit gate is also more stable since the scientists packed the qudits into photons—particles of light that are not easily interrupted by their environment. The findings of the scientists have been reported in npj Quantum Information.

Additionally, the gate forms one of the largest entangled states of quantum particles so far— photons in this case. Entanglement is a quantum phenomenon that enables measurements on one particle to automatically influence measurements on another particle, introducing the potential to make communication between parties strong or to teleport quantum information from one point to another, for instance.

More entanglement in what is called the Hilbert space—the area where quantum information processing can occur—is better.

Earlier photonic methods were able to attain 18 qubits encoded in six entangled photons in the Hilbert space. Purdue scientists increased entanglement with a gate using four qudits—the equivalent of 20 qubits—encoded in just two photons.

In quantum communication, less is more.

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.

Poolad Imany, Postdoctoral Researcher, School of Electrical and Computer Engineering, Purdue University

The research group realized more entanglement with a lesser number of photons by encoding one qudit in the time domain and the other in the frequency domain of each of the two photons.

They constructed a gate with the two qudits encoded in each photon, for a total of four qudits in 32 dimensions, or probabilities, of both frequency and time. With more dimensions, the entanglement is also more.

Beginning from two photons entangled in the frequency domain and subsequently operating the gate to entangle the frequency and time domains of each photon produces four completely entangled qudits, which take up a Hilbert space of 1,048,576 dimensions, or 32 to the power of four.

Normally, gates constructed on photonic platforms to control quantum information encoded in separate photons function only for a part of the time since photons naturally do not interact with each other very well, rendering it highly complicated to control the state of one photon depending on the state of another.

Purdue scientists made the operation of the quantum gate deterministic rather than probabilistic by encoding quantum information in the frequency and time domains of photons.

The group put the gate into operation with a set of standard off-the-shelf equipment used in the optical communication industry on a daily basis.

This gate allows us to manipulate information in a predictable and deterministic way, which means that it could perform the operations necessary for certain quantum information processing tasks.

Andrew Weiner, Scifres Family Distinguished Professor of Electrical and Computer Engineering, Purdue University

Weiner’s lab specializes in ultrafast optics.

Next, the group intends to employ the gate in quantum communications tasks like high-dimensional quantum teleportation and also for running quantum algorithms in applications like simulating molecules or quantum machine learning.


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