## Goalkeepers

Like quantum computers, they have their own versions of bits called qubits, so they also need their own versions of logic gates that control the flow of information in a computer system.

Now a team from Purdue University has published a study in the journal *npj Quantum Information* detailing their creation of one of the first known quantum gates containing qudity as opposed to qubits – and can help initiate the era of quantum computing.

## Bits and Qudits

Bits are basically elements of today's classic computers. Each bit can store 1 or 0, and if you put enough bits, you can process quite a bit of information – but not as much as you could have processed using qubits.

Qubits are a quantum version of bits, but instead of storing either 1 or 0, qubits can store either one or the other at the same time, which dramatically increases the potential processing power of the computer.

The problem with qubits is that they are very unstable, and that makes creating a system with many of them extremely difficult – the first commercial IBM quantum computer, which was presented in January, contains only 20 qubits, which gives much less computing power than classic already working computers that can have hundreds of millions of logic gates.

For several years, scientists have been studying the value of trading in qubits for another quantum building block: qudits. Instead of being limited to only two states, such as qubits, qudity can exist in more than two states – for example, 0 and 1 and 2 – and for each added state the processing capacity of a single qudit increases.

In other words, less qudits than qubits are required to encode and process the same amount of information.

## Photon Fun

To create a quantum gate with built-in qudit, the Purdue team began by coding four qudites in two tangled photons in two domains – frequency and time. The team chose photons because they are not easily disturbed by their environment, and the use of multiple domains allowed them to get more entanglement using fewer photons.

"Photons are expensive in quantum sense because they are difficult to generate and control," said researcher Poolad Imany in a press release, "so it's great to pack as much information as possible for each photon."

The finished gate had an equivalent processing power of 20 qubits, although it only required the use of four qudites, for the benefit of additional stability through the use of photons – making it a promising system for future quantum computers.

**READ MORE:** Scientists are building a transistor gate for quantum information processing – using qudits [Purdue University]

**More about qudits:** *A new breakthrough in work reveals quantum "fingerprints"*

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