A classical computer is a collection of information storage units called bits. These physical devices have two states each, denoted 0 and 1. Any computation that a computer performs is essentially the result of the manipulation of the states of bits.
Similarly, a qubit is a physical system with two quantum states, and it is the fundamental physical component of a quantum computer. A qubit can exist in one of the two states or – unlike classical computers – a superposed state with contributions from both states.
This superposition is a quantum feature that the bits in conventional computers don’t exhibit. Superposed states, also known as coherent superpositions, are important in quantum information-processing protocols. However, superpositions are fragile. The fragility arises out of the interaction between the qubit and other systems. The more the number of interaction channels, the faster the superposition “decoheres” and the qubit ends up in one of the two states.
Many qubits, one problem
A collection of qubits is required to make a quantum device. For this, any group of qubits needs to satisfy a few basic requirements.
One: the qubits should be identical. The qubits can’t be guaranteed to be identical since they need to be manufactured, and some ‘imperfections’ will creep in.
Two: it should be relatively easy to integrate several qubits that can be operated controllably. Here, controllability refers to both the manipulation of individual qubits (a.k.a. “addressability”) and qubit-qubit interactions. An important, related aspect is the qubit system should be robust enough to function at room temperature without losing quantum features for reasonably long durations.
Many different physical systems are suitable for realising qubits. Some well-studied and practical options include superconducting junctions, trapped ions, and quantum dots. However, all these systems can operate as qubits only at very low temperatures or in a high vacuum or both.
In some cases, like superconducting junctions, a low temperature is in fact essential for them to work as qubits. In other cases, a low temperature is required for quantum features like superposition to survive for longer in the computer.
Temperature and commercial viability
As a result, quantum computers based on such technologies are expensive. Researchers are working on alternative, simpler technologies to reduce costs. Less expensive technologies will allow more participation in this research frontier.
If a technology is not economically viable, it is not easy to sustain it long enough for breakthroughs to happen.
In a recent collaborative study reported in the journal Science Advances by a group of institutions in Japan, researchers realised qubits at room temperature in a metal-organic framework (MOF).
A MOF is a network of repeated molecular arrangements where the repeating structure has a metal atom or ion with organic molecules attached to it like tentacles. Each tentacle attaches to another metal atom, and the structure repeats itself to make up the MOF.
Chromophores, the ‘colour molecules’
In the system studied by the Japanese team, zirconium is the metal component and an organic molecule containing the chromophore pentacene bridges the metal atoms. A chromophore is an organic molecule or a part of a larger molecule that absorbs light of some specific colour. An object containing such molecules thus appears to have some dominant colour. For example, the leaves of many plants appear green since the chromophore chlorophyll predominantly absorbs red and blue colours from sunlight. Since the presence of chromophores is responsible for the colouration, they are also called “colour molecules”.
When it absorbs light, the chromophore molecule jumps to a higher energy level (i.e. an excited state).
In its lowest energy state, or ground state, a chromophore molecule has a pair of electrons in a special configuration called a singlet. Every electron possesses a property called spin that is inherent to it. The spin of an electron can point in two opposite directions, each corresponding to a distinct quantum state.
In a singlet, the spins of the two electrons are pointing in opposite directions. If we say ‘pointing up’ is +1 and ‘pointing down’ is -1, we can say the spins in a singlet add up to zero. When the chromophore molecule absorbs some light, one of the electrons moves to a higher energy level while their respective spins still point in opposite directions.
Imagine the energy levels to be like steps, often unequally spaced, of a ladder. Excitation amounts to climbing up the ladder. If two electrons, one on a lower rung of the ladder and another on a higher rung, have their spins pointing in opposite directions, it is a singlet excited state. If the two electrons are on different steps of the energy ladder and have their spins in the same direction (say, +1 and +1), the configuration would be called a triplet excited state.
Role of singlet fission
An excited molecular system has a small but non-zero chance of releasing its extra energy in a process called deexcitation. The higher energy singlet excited state can deexcite to a lower energy triplet excited state. The energy released in the process will excite a neighbouring chromophore molecule in a singlet ground state to jump to a triplet excited state.
This process of generating two triplet excited chromophores from a singlet excited state chromophore is called singlet fission. This energy transfer happens as the two chromophores interact.
The MOF networks are very porous, like sponges, allowing the chromophores to rotate by a small degree. The rotation leads to a change in the interaction strength between two adjacent chromophores.
The triplet state of one of the chromophores involves two of its energy levels (recall the ladder-rung analogy), and that of the other chromophore involves two energy levels in its own energy ladder. The interaction between the chromophores prepares the two pairs of electrons in a superposition wherein each pair is in a triplet state. The rotation-induced modulation also seems to play a role in ensuring the superposition of triplet states generated by singlet fission is long-lived.
A necessary first step
In simpler terms, the interaction between the chromophores is strong enough to cause singlet fission but weak enough to not allow the coherence to be lost once the triplets form. In their experiment, the Japanese team found that even at room temperature, the coherence of the superposition of two four-electron states survived up to a fraction of a microsecond, which is a long duration in the current context. Other qubit systems require an extremely low temperature if coherence has to last this long.
It remains to be seen whether researchers can demonstrate how to achieve quantum gate operations on these qubits, assemble several qubits, and achieve controllability. Nevertheless, the availability of room-temperature qubits is a significant achievement that will invite many research groups to explore the system further.
S. Sivakumar is a member of the physics faculty at Krea University.
