Warming up for speed computing

Quantum computers have held the popular imagination since the 1980s, when scientists Richard Feynman and Yuri Manin showed that the new device could simulate things that a traditional computer could not. For example, a quantum computer can simulate interactions among thousands of molecules to arrive at the right combination for a drug, something a conventional computer would never be able to do.

However, the need for ultra-cold environments — close to minus 273 K, colder than interstellar space — has hampered their adoption.

A quantum computer can store vast amounts of data and process them extremely fast. MIT Technology Review notes that 72 petabytes of data of a conventional computer can be stored in just 53 qubits in a Google Sycamore chip, a quantum computer. (A ‘bit’ in traditional computing can be in a state of ‘0’ or ‘1’. A qubit can be a combination of both, or even in an intermittent state, thus enabling computing capability of gigantic proportions.)

But qubits are stable — or ‘coherent’ — only at very low temperatures. As the surrounding temperature rises, qubits become useless for computing.

Now, that’s changing.

Research suggests scientists may be within reach of the ‘holy grail’ of quantum computing — enabling it at room temperature.

Alongside the cutting-edge research elsewhere in the world, Indian researchers too are pushing these frontiers — and their effort would receive a boost in a few months, when the ₹8,000-crore National Mission on Quantum Technologies is formally launched. Already, about 50 groups have been identified and ₹300 crore of initial funding has been given out, says Prof Ashutosh Sharma, Secretary, Department of Science and Technology.

At least three pathways are being independently attempted towards achieving quantum computing at room temperature. Nuclear magnetic resonance (NMR) — the technology used in MRI scans — is one. The idea is to have nuclei in atoms respond to microwave radiation in the presence of a magnetic field. It uses the spin states of nuclei as qubits. The spin is basically the orientation a subatomic particle assumes in the presence of a magnetic field.

Prof Arvind (who uses a single name) of IISER, Mohali, an NMR expert, says, “Nuclear spins using NMR was the first proposal to carry out quantum computation at room temperature.”

But scaling is an issue. “Only a tiny fraction of the spins — ten in a million — are in the ‘pure state’ that is needed to give a useful signal for NMR,” he says. So, we would need a large ensemble to maintain those populations.

The second pathway is the use of photons, which, according to Arvind, “shows the most promise for room temperature QC”. The use of photons, rather than ions, atoms or dilution refrigerators, allows for light signals to be read, instead of electric signals from atoms or ions.

However, to pass on information, particles in quantum systems need to interact. Photons don’t. A ray of light passes through another undisturbed. So, we need materials in which photons can talk to each other and exchange information. ‘Nonlinear crystals’ are one such material. The interaction efficiency currently is low, but it is still a possibility, Arvind says.

The US Army, in collaboration with MIT, recently proved the feasibility of a quantum logic gate (a building block for quantum circuits) using photonic circuits and optical crystals. One can engineer cavities in the crystals that temporarily trap photons inside. Thus, the quantum system can establish two different possible states a qubit can hold: a cavity with a photon (on) and without a photon (off). These qubits can then form quantum logic gates. However, improvements are needed in some photonic components for the construction of such gates. These could be a decade away.

For the third (Indian) pathway, we go to IIT-Bombay. Scientists, led by Prof Anshuman Kumar, placed a layer of 2D material below another 2D material and generated coherence (stability).

“You can manipulate coherence by applying an electric voltage. This actually changes the value of this coherence. This is good — you don’t need a light source, like a bulky laser, for execution,” says Kumar.

This means the device can be made compatible with existing microelectronics technology. Such compatibility is a strong point in favour of such 2D materials.

Kumar’s team used material made from a monolayer transition metal dichalcogenide (TMDC) because it is cheaper to synthesise. Another advantage compared with, say, NMR and trapped ion-based approaches, is the chip-scale integration. “These multiple qubits can be created in the same sheet of 2D material by applying another voltage — which are all very easy to do on a chip,” says Kumar. An experimental demonstration of the concept is about two years away, he adds.

While Indian researchers are chipping away enthusiastically at the solution, the convergence of global efforts could well place quantum computing in your company’s server rack, if not on your laptop, not too far in the future.

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