Looking at Molecules for the Next Quantum Computer Material

Title: Room-Temperature Quantum Coherence and Rabi Oscillations in Vanadyl Phthalocyanine: Toward Multifunctional Molecular SpinQubits

Authors: Matteo Atzori, Lorenzo Tesi, Elena Morra, Mario Chiesa, Lorenzo Sorace, and Roberta Sessoli

Publication Info: J. Am. Chem. Soc., 2016, DOI: 10.1021/jacs.5b13408

Quantum computing is needed to enable the advanced computers of the future. They’re essential in realizing advanced cryptography, AI, and next generation computers for science exploration. It’s a long sought-after idea with lots of developed theory based in quantum mechanics.

While there are working quantum computers already made, they are extremely expensive, and require sensitive conditions to operate. If you’re looking to develop quantum computing of the future, it’s all about hardware now; being able to make stable quantum computers and components.

Figure 1. IBM Q is IBM’s initiative to build and advance quantum computing.

At the heart of quantum computing is the qubit. A qubit is the quantum version of a bit. Bits are the 0’s and 1’s that make up current computing technology. A qubit is also a two-state system, but since it’s quantum, it can be a superposition of both 0 and 1 at the same time. One commonly studied phenomena related to qubits, is the quantum spin states of electrons. They are up or down spin, or a superposition of both; perfect for qubits.

However, measuring this qubit, is not trivial. The signal of the electron’s state, vs the noise of the entire system is incredibly hard to measure in most materials. Also, most materials don’t host this quantum state at room temperature, which would be essential for quantum computing to be accessible  

The role of chemists in the field of quantum computing is synthesizing materials that can host observable qubits with measurable lifetime. Right now, the most common materials studied is nitrogen-vacancy pairs in diamonds.

The researchers in this work are looking past diamond, into the physical realization of molecular spin systems to host qubits. The goal, or benchmark is to achieve long-lifetimes of the quantum states at useable temperatures. This has not been achieved to standards required in previous studies of molecular systems.

To investigate potential molecular spin qubits, these researchers made vanadyl phthalocyanine (VOPc, figure 2). This system based on VIV hosts a single unpaired d1 electron in a well separated dxy orbital. This means, there is a single electron in a stable and isolated orbital, making it easy to observe the two levels (up and down spin) required for a molecular qubit. Importantly, the electron doesn’t have any other close by electrons to interact with, which would cause a short lifetime of the quantum state.

Figure 2. Left: A drawing of the VOPc complex showing the V1, vanadium center. Right: The lifetime of the quantum state, showing up to 1 µs lifetime at 300 K (room temp.).

Furthermore, researchers explored into the potential ways to stabilize the qubits in these relatively new class of materials. To probe the potential of VOPc, the researchers made pure VOPc as well as VOPc hosted in a solid titanium phthalocyanine (TiOPc). TiOPc, is titanium based, and shows no magnetism (due to not having any paired electrons). TiOPc is a perfect host material for VOPc, as TiOPc doesn’t interact electronically with the vanadyl compound and has a similar structure to VOPc enabling them to pack well into solids. This allows for “diluted” VOPc, which is important to prevent adjacent electron spins from interacting (which would be the case of two VOPc sitting next to each other). 

To measure a materials potential for quantum states, an important parameter is measuring the phase memory time (Tm). This parameter describes how long the quantum coherence, or quantum state related to encoded information exists. At this stage in research, the longer the quantum coherence the better.

The Tmof VOPc diluted in TiOPc at room temperature was ~1 µs. This is longer than previous molecular systems at room temperature, making this an exciting report. However, inorganic crystalline compounds, like the previously mentioned nitrogen vacancy diamond, at room temperature is as long as ~58 µs.

This work shows there is potential in using molecular systems for quantum computing. This field is still in its infancy, and reports such as these will guide the future directions to making useable materials for quantum computing.

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