First generation of world-first Qubit-Photon Interface (QPI) could entangle multiple quantum processors
Quantum computers can theoretically find answers to problems that regular computers would take eons to solve, but scientists currently face great challenges scaling them up for practical applications. Now, a startup in England says it has developed a quantum version of the network interface cards that connect servers together in data centers. The company’s goal is to help fuse today’s relatively small quantum processors into useful machines.
Classical computers switch transistors either on or off to symbolize data as ones or zeroes. In contrast, quantum computers use quantum bits, also known as qubits. The fuzzy nature of quantum physics lets qubits exist in a state called superposition, in which they are essentially both 1 and 0 at the same time. This phenomenon lets each qubit perform two calculations at once. It will also spit out both answers at once, jumbled together in an uninterpretable superposition, unless it’s used in a clever algorithm, such as Shor’s or Grover’s, or to model inherently quantum phenomena such as atoms or molecules. The more qubits are quantum mechanically linked, or entangled, within a quantum computer, the greater its computational power can grow, in an exponential fashion, for those problems where clever algorithms exist.
“The potential for quantum computing to really transform many industries is massive,” says Carmen Palacios-Berraquero, founder and CEO of quantum networking firm Nu Quantum in Cambridge, England. “But these computers need to be really quite large and powerful for that to happen.”
Currently quantum computers are noisy intermediate-scale quantum platforms, meaning their qubits number in the hundreds at most. Future quantum computers will likely need thousands or even millions of qubits to help compensate for errors and prove useful, Palacios-Berraquero says.
However, superposition and entanglement are infamously fragile phenomena, being susceptible to heat and other disturbances. This makes scaling up the number of qubits in quantum computers a huge technical challenge.
“It will be impossible to build a single quantum processing unit that has hundreds of thousands or millions of qubits,” Palacios-Berraquero says. “There are a lot of physical limitations because of yield and performance.”
The Quantum Network Interface Card
To overcome this obstacle, Nu Quantum has built a prototype qubit-photon interface to connect multiple quantum processors together. “We are there to develop the networking technology that can help the industry cross the scaling chasm it faces,” Palacios-Berraquero says. “Instead of trying to build bigger and bigger computers that are inefficient, that are one-off machines, we want to shift to modular, smaller quantum processing units that are more efficient and interconnected by a network.”
The new device is designed to entangle qubits with photons. These photons can then entangle with other qubits and so entangle all the qubits together.
“It’s the first time in the history of the quantum industry where a company has built the equivalent of a network interface card,” Palacios-Berraquero says.
Photons can transfer entanglement from one qubit to another at essentially the speed of light, which can prove especially important, given how quantum states are often short-lived. Photons also offer wireless entanglement, limiting physical connections that can disrupt fragile quantum states, says Claire Le Gall, Nu Quantum’s vice president of technology.
The heart of the device is a microscopic cavity, a spherical mirror “that is almost atomically smooth,” Palacios-Berraquero says. The way it confines photons in tiny spaces helps them interact with qubits in a stronger manner, Le Gall explains.
To boost qubit-photon entanglement, Nu Quantum wanted to control the length of these cavities to ensure they resonated with qubits. They stabilized the lengths of these cavities to 80 picometers, or less than the width of an atom. “Another way of thinking about it is how this is a millimeter-long cavity, so stabilizing it to 80 picometers is like knowing the height of the Empire State Building with the width of a human hair,” Le Gall says.
Although scientists have previously achieved such qubit-photon entanglement in the lab, “those are proof of concepts, often with cavities that are assembled by hand,” Le Gall says. Instead, Nu Quantum is working on devices that are “rugged, robust and repeatable, such that you can build the same thing again and again and it will work the same each time.”
How to Scale Up Quantum Computers
The best prior attempts at qubit-photon interfaces achieved entanglement rates of about 200 times per second, with a fidelity of about 97 percent, “or about three errors every 100 attempts,” Le Gall says. Nu Quantum hopes that its prototype device will offer major improvements, with entanglement rates of about 10,000 times per second or more. “We hope to reach 98 percent fidelity a year from now, and then quickly be in excess of 99 percent,” Palacios-Berraquero says.
Currently, a variety of qubit platforms exist, such as superconducting circuits, electromagnetically trapped ions and spins within silicon. To start, Nu Quantum aims to use their new device with qubits composed of neutrally charged rubidium atoms using near-infrared photons. The company also plans to experiment with trapped ion qubits. Palacios-Berraquero notes that scientists are currently investigating ways to help convert quantum microwave signals to quantum optical signals, which can open up their device for use with superconducting circuit and silicon spin qubits.
The researchers expect their new device will interact with just one qubit on a quantum processor. That qubit then goes on to interact with any number of the other qubits within its machine, Palacios-Berraquero says.
Nu Quantum has successfully integrated their prototype qubit-photon interface within an ultra-high vacuum cell. “The next step will be to test it with a qubit,” Palacios-Berraquero says.
In addition to helping quantum computing networks scale up, the new prototype could one day help scale up quantum sensor networks and quantum communication networks as well. “It’s a very kind of fundamental product prototype for a range of use cases,” Palacios-Berraquero says.
Read full article here: IEEE coverage