According to scientists from the U.S. Army Research Laboratory, quantum computer circuits that will no longer require extremely cold temperatures to operate may soon become a reality in around 10 years.
For many years, room temperature-operating solid-state quantum technology appeared to be far-fetched. Although the use of transparent crystals with optical nonlinearities had evolved as the most potent way to achieve this milestone, the probability of such a system continues to be a mystery.
Now, the validity of this method has officially been confirmed by Army researchers. Dr Kurt Jacobs from the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory, working together with Dr Mikkel Heuck and Professor Dirk Englund from the Massachusetts Institute of Technology (MIT), became the first researcher to show the viability of a quantum logic gate containing optical crystals and photonic circuits.
If future devices that use quantum technologies will require cooling to very cold temperatures, then this will make them expensive, bulky, and power hungry. Our research is aimed at developing future photonic circuits that will be able to manipulate the entanglement required for quantum devices at room temperature.
Dr Mikkel Heuck, Massachusetts Institute of Technology
Quantum technology provides an array of upcoming developments in remote sensing, communications, and computing.
Conventional classical computers operate with fully determined data to achieve any kind of job. This data is preserved in several bits, with each bit being on or off. When an input specified by a number of bits is fed to a traditional computer, this input would be processed by the computer to generate an answer, which is also specified as a number of bits. One input will only be processed by a traditional computer at a given time.
On the other hand, quantum computers preserve the data in qubits that can exist in an unusual state, where they exist in both on and off states simultaneously. This enables a quantum computer to analyze the answers to several inputs concurrently.
While the quantum computer cannot produce output for all the answers at the same time, it can nevertheless generate output for the associations between these answers, thus enabling it to solve certain issues relatively faster than the traditional computer.
Regrettably, one of the main disadvantages of quantum systems is the delicateness of the qubits’ special states. A majority of the potential hardware meant for quantum technology should be maintained at very cold temperatures—near 0 K—so that the strange states are not damaged during the interaction with the computer’s setting.
Any interaction that a qubit has with anything else in its environment will start to distort its quantum state. For example, if the environment is a gas of particles, then keeping it very cold keeps the gas molecules moving slowly, so they don't crash into the quantum circuits as much.
Dr Kurt Jacobs, Army Research Laboratory, U.S. Army Combat Capabilities Development Command
While scientists have made numerous attempts to overcome this problem, a concrete solution has not been identified yet. For now, photonic circuits integrating nonlinear optical crystals have currently become the only viable option to quantum computing with room-temperature solid-state systems.
Photonic circuits are a bit like electrical circuits, except they manipulate light instead of electrical signals. For example, we can make channels in a transparent material that photons will travel down, a bit like electrical signals traveling along wires.
Dirk Englund, Professor, Massachusetts Institute of Technology
Quantum systems that utilize photons can bypass the limitation of cold temperatures, unlike quantum systems that utilize atoms or ions to preserve data. But to carry out logic operations, the photons still need to communicate with other photons. At this juncture, the nonlinear optical crystals come into play.
Scientists can design cavities in the crystals that momentarily retain the photons within. Using this technique, the quantum system can ascertain two different potential states that can be held by a qubit—that is, a cavity without a photon (off) and a cavity with a photon (on). Such qubits can subsequently form quantum logic gates that ultimately produce the framework for the unusual states.
This means scientists can employ the unknown state of whether or not a photon is present inside a crystal cavity to denote a qubit. The quantum logic gates act on a pair of qubits simultaneously, and can produce “quantum entanglement” between these qubits. An entanglement like this is automatically produced in a quantum computer, and is needed for quantum methods to sensing applications.
But researchers applied the concept to develop quantum logic gates by utilizing nonlinear optical crystals predominantly on speculation—up until this stage. Although it has demonstrated a huge potential, doubts still persist as to whether this technique could even result in feasible logic gates.
The use of nonlinear optical crystals had continued to be a mystery until the research team from MIT and the Army’s laboratory presented a method to achieve a quantum logic gate with this technique, using well-known photonic circuit parts.
“The problem was that if one has a photon travelling in a channel, the photon has a 'wave-packet' with a certain shape,” added Jacobs. “For a quantum gate, you need the photon wave-packets to remain the same after the operation of the gate.”
Jacobs continued, “Since nonlinearities distort wave-packets, the question was whether you could load the wave-packet into cavities, have them interact via a nonlinearity, and then emit the photons again so that they have the same wave-packets as they started with.”
As soon the quantum logic gate was developed, the scientists conducted various computer simulations of the gate’s operation to show that it could theoretically work properly. The real fabrication of a quantum logic gate with this technique will initially need considerable enhancements in the quality of specific photonic components, stated the scientists.
“Based on the progress made over the last decade, we expect that it will take about ten years for the necessary improvements to be realized. However, the process of loading and mitting a wave-packet without distortion is something that we should able to realize with current experimental technology, and so that is an experiment that we will be working on next,” concluded Heuck.
The researchers’ findings were published in a peer-reviewed journal, Physical Review Letters, on April 20th, 2020.
Heuck, M., et al. (2020) Controlled-Phase Gate Using Dynamically Coupled Cavities and Optical Nonlinearities. Physical Review Letters. doi.org/10.1103/PhysRevLett.124.160501.