For years, solid-state quantum technology that operates at room temperature seemed remote. While the application of transparent crystals with optical nonlinearities had emerged as the most likely route to this milestone, the plausibility of such a system always remained in question. Scientists have now demonstrated the feasibility of a quantum logic gate comprised of photonic circuits and optical crystals.
In order to accomplish any kind of task, traditional classical computers work with information that is fully determined. The information is stored in many bits, each of which can be on or off. A classical computer, when given an input specified by a number of bits, can process this input to produce an answer, which is also given as a number of bits. A classical computer processes one input at a time.
In contrast, quantum computers store information in qubits that can be in a strange state where they are both on and off at the same time. This allows a quantum computer to explore the answers to many inputs at the same time. While it cannot output all the answers at once, it can output relationships between these answers, which allows it to solve some problems much faster than a classical computer.
Unfortunately, one of the major drawbacks of quantum systems is the fragility of the strange states of the qubits. Most prospective hardware for quantum technology must be kept at extremely cold temperatures — close to zero kelvin— to prevent the special states being destroyed by interacting with the computer’s environment.
Researchers have directed various efforts to resolve this issue but a definite solution is yet to be found. At the moment, photonic circuits that incorporate nonlinear optical crystals have presently emerged as the sole feasible route to quantum computing with solid-state systems at room temperatures. Unlike quantum systems that use ions or atoms to store information, quantum systems that use photons can bypass the cold temperature limitation; however, the photons must still interact with other photons to perform logic operations. This is where the nonlinear optical crystals come into play.
Researchers can engineer cavities in the crystals that temporarily trap photons inside. Through this method, the quantum system can establish two different possible states that a qubit can hold: a cavity with a photon (on) and a cavity without a photon (off). These qubits can then form quantum logic gates, which create the framework for the strange states.
In other words, researchers can use the indeterminate state of whether or not a photon is in a crystal cavity to represent a qubit. The logic gates act on two qubits together and can create “quantum entanglement” between them. This entanglement is automatically generated in a quantum computer and is required for quantum approaches to applications in sensing.
The application of nonlinear optical crystals remained in question until researchers presented a way to realize a quantum logic gate with this approach using established photonic circuit components.
Once they designed the quantum logic gate, the researchers performed numerous computer simulations of the operation of the gate to demonstrate that it could, in theory, function appropriately. Actual construction of a quantum logic gate with this method will first require significant improvements in the quality of certain photonic components.