Qudits: The Face of Quantum Computing

Keeping the fast pace of development in the world of quantum computation, scientists could include another abbreviation to this field, which is named as Qudits. Qudits define the new future of quantum computing which is quite different than Qubits. The community could develop a microchip that can generate qudits, each assuming 10 or more states which is different from a qubit. Qubits operate on the two states, 0 and 1. Qudits are the high dimensional states i.e. D- level quantum states with D>2 (where D=2 represent qubits). These are formed from the entanglement of photons. “We have now achieved the compact and easy generation of high-dimensional quantum states,” says the co-lead author Michael Kues, a quantum optics researcher at Canada’s National Institute of Scientific Research, its French acronym, in Varennes, Quebec.

Classical Computers work on transistors and logic gates which operates on the basis of the binary system whereas quantum computers use qubits which have the bizarre nature of quantum physics of staying in 0 and 1 simultaneously. A single qubit helps to perform two calculations at a time whereas two entangled qubits can perform four calculations at a time and so on. It increases the speed of the computer many folds as compared to the modern day classical computers.

Classical bits are apples and oranges; one or zero. Don’t be intimidated by those mathematical expressions hovering around the qubit, however. All they mean is that the qubit could be one, zero, or any value in between. Maybe 0.23 or 0.99, and so on. (Credit: Research2Reality)

Since quantum computers with qubit are in the phase of development, the researchers have found qudits which can perform even faster calculations than qubit computers. Qudits hold more than two states simultaneously. In principle, a quantum computer with two 32 state qudits, for example, would be able to perform as many operations as 10 qubits.

The aforesaid microchip is basically a photonic chip that can generate two entangled qudits each with 10 states in 100 dimensions. They use the well-developed semiconductor industry to fabricate optical waveguides and functional devices on compact and mass- producible chips. Here, the photons are created in a coherent superposition state of multiple modes of high purity frequency. The number of entangled photons is increased along with their dimensionality. This will help to encounter the fundamental quantum investigations in many aspects including, increasing the sensitivity of quantum imaging schemes, improving the robustness and key rate of quantum communication protocols, enabling a richer variety of quantum simulations, and achieving efficient and error-tolerant quantum computation.

Researchers used the setup pictured above to create, manipulate, and detect qudits. The experiment starts when a laser fires pulses of light into a micro-ring resonator, which in turn emits entangled pairs of photons. Because the ring has multiple resonances, the photons have optical spectrums with a set of evenly spaced frequencies (red and blue peaks), a process known as spontaneous four-wave mixing (SFWM). The researchers were able to use each of the frequencies to encode information, which means the photons act as qudits. Each qudit is in a superposition of 10 possible states, extending the usual binary alphabet (0 and 1) of quantum bits. The researchers also showed they could perform basic gate operations on the qudits using optical filters and modulators, and then detect the results using single-photon counters. (Credit: Nature)

“The next big challenge we will have to solve is to use our system for quantum computation and quantum communication applications,” says Kues. He further adds, “While this will take some additional years, it is the final step required to achieve systems that can outperform classical computers and communications.”

Nonlinear Photonics team at INRS. (Credit: INRS)

In conclusion, one can achieve these high dimensional entangled photonic states in an integrated platform which helps in quantum computing by handling more complex and efficient gates in optical circuits of manageable experimental complexity.


  1. https://en.wikipedia.org/wiki/Qubit.
  2. Qudits: The Real Future of Quantum Computing?
  3. “On-chip generation of high-dimensional entangled quantum … – Nature.” 29 Jun. 2017


The author is a research fellow at the Indian Institute of Science Education and Research (IISER), Pune, Maharashtra. Presently, he is working in the field of quantum information processing at Quantum Optics and Atomic Physics laboratory. He is very enthusiastic in writing scientific articles and interested in developing scientific equipment.