• Physics 15, 168
A new quantum random access memory device reads and writes information using a chirped electromagnetic pulse and a superconducting resonator, making it significantly more hardware efficient than previous devices.
Random access memory (or RAM) is an integral part of a computer, acting as a bank of short-term memory from which information can be quickly recalled. Apps on your phone or computer use RAM so you can switch between tasks on the fly. Researchers working on building future quantum computers hope that such systems could one day work with analogous quantum RAM elements, which they believe could speed up the execution of a quantum algorithm. [1, 2] or increase the density of information that can be stored in a quantum processor. Now, James O’Sullivan of the London Center for Nanotechnology and his colleagues have taken an important step towards realizing quantum RAM, demonstrating an efficient hardware approach that uses chirped microwave pulses to store and retrieve quantum information in atomic spins .
Much like quantum computers, experimental demonstrations of quantum memory devices are in their infancy. One of the main chip-based platforms for quantum computing uses circuits made from superconducting metals. In this system, central processing is done with superconducting qubits, which send and receive information via microwave photons. Currently, however, there is no quantum memory device capable of reliably storing these photons for long periods of time. Fortunately, scientists have some ideas.
One such idea is to use the spins of impurity atoms embedded in the superconducting circuit chip. Spin is one of the fundamental quantum properties of an atom. It acts like an internal compass needle, aligning itself with or against an applied magnetic field. These two alignments are analogous to the 0s and 1s of a classical bit and can be used to store quantum information [4, 5]. If the chip contains many impurity atoms, the spins of the atoms can act as a “multimode” memory device, capable of simultaneously storing the information contained in many photons.
For atomic spins, information storage times can be orders of magnitude longer than for superconducting qubits. Researchers have shown, for example, that bismuth atoms placed inside silicon chips can store quantum information for times longer than a second . One might ask: why not use spin qubits instead of superconducting qubits? Indeed, there are research groups working on atom-based quantum computers, but controlling and measuring atomic spins presents its own unique challenges. A hybrid approach is to use superconducting qubits for processing and atomic spins for storage, but here the challenge has been how to transfer information between the two systems using microwave photons. While researchers have previously demonstrated the absorption and retrieval of information from microwave photons by an atomic spin ensemble, these demonstrations have required the use of strong magnetic field gradients or specialized superconducting circuits, which both add complexity to quantum memory hardware. [7, 8].
O’Sullivan and his colleagues propose an elegant solution for storing and retrieving microwave photon information that uses an efficient hardware approach. The team’s device consists of a superconducting resonator circuit that sits on a silicon chip embedded with bismuth atoms (Fig. 1). The team sent weak microwave excitations containing around 1000 photons into the resonator, which were absorbed by the spins of the bismuth atoms. They then hit the resonator with microwave electromagnetic pulses that increased in frequency over time, an effect known as chirping. Because of this, the quantum information contained in the photons is imprinted on the spins with a unique “phase” identifier, which captures the relative pointing positions of neighboring spins. The team then retrieved this information, transferring the photons to the superconducting circuit, by hitting the spin assembly with an identical pulse, which reversed this printed phase.
O’Sullivan and his colleagues show that their memory device is capable of simultaneously storing multiple photonic information in the form of four weak microwave pulses. Above all, they also demonstrate that information can be read back in any order, making their device a real RAM.
In this first demonstration, the team reports an efficiency of 3%, indicating that most information is lost by memory. Thus, their device is still far from the faithful storage and retrieval required for a future quantum computer. However, an analysis of the potential sources of this low efficiency indicates that it does not come from the transfer process but rather from potentially resolvable limitations of the device. The team believes that by increasing the number of turns, they could significantly improve the efficiency of the device.
In addition to storing information, quantum RAM elements could help increase the density of qubits in a quantum processor. In September, IBM introduced the Goldeneye project, a high-dilution refrigerator . This ultracool behemoth has a volume larger than three household refrigerators and will house IBM’s next-generation superconducting quantum computer. Today’s superconducting quantum computers have a qubit density of less than 100 per square millimeter – typical computer chips contain 100 million transistors per square millimeter – so it’s understandable why IBM needs such a large fridge. O’Sullivan and his colleagues’ spin-based quantum memory device could, in principle, store multiple qubit states in the space currently occupied by a single one, which could one day help alleviate this daunting problem. .
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- J. Biamonte et al.“Quantum Machine Learning”, Nature 549195 (2017).
- J.O’Sullivan et al.“Quantum Random Access Memory Using Chirped Pulse Phase Encoding,” Phys. Rev. X 12041014 (2022).
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- G.Wolfowicz et al.“Atomic Clock Transitions in Silicon-Based Spin Qubits”, Nat. Nanotechnology. 8561 (2013).
- H. Wu et al.“Storage of Multiple Coherent Microwave Excitations in an Electron Spin Ensemble,” Phys. Rev. Lett. 105140503 (2010).
- C. Grezes et al.“Multi-mode storage and retrieval of microwave fields in a set of spins”, Phys. Rev. X 4021049 (2014).
- P. Gumann and J. Chow, “IBM scientists cool the world’s largest quantum-ready cryogenic concept system,” IBM Blog, September 8, 2022.
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