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From quantum memory to quantum money

Von Dr. Fernando Pastawski (08.07.2013)

Classical physics provides a foundation for classical information processing. Analogously, the defining trait of quantum information is to assume that the physical processing of information is governed by the postulates of quantum mechanics. Like the photon or electron which can simultaneously travel two paths before constructively interfering at a detector, quantum information can take a superposition of classical values while it is being processed into a final result. This opens the door to exciting possibilities such as secure private communication and computers which can solve relevant problems faster. 

While much of our understanding of classical information carries on to quantum information, it is the differences that make the younger field so promising but also so elusive. Indeed, if one were to pinpoint the single obstacle which makes quantum information processing so much more difficult to realize than classical processing, “decoherence” would most likely take the full culprit. Classically, the only possible noise processes transform a stored zero into a one or vice-versa. Decoherence is an additional form of noise which by its very definition affects only the quantum nature of information while leaving classical information unaffected. If allowed to act, it flattens the rich structure available to quantum states into a simplex associated to classical information. 

Three decades of increasing efforts have confronted researchers with overwhelming experimental difficulties in the quest for a scalable realization of a quantum computer. Progress in the precision and control of individual quantum systems has been made and theoretical results indicate that only technical difficulties stand in the way of this tantalizing goal. As evinced by the 3 million dollar fundamental physics prize awarded to Alexei Kitaev, topological quantum systems are currently seen as one of the most promising approaches to deal with the problem of decoherence, in both the context of quantum computing and memory.

[Bildunterschrift / Subline]: Figure 1: Currency is based on the assumption that it is impossible to duplicate bank notes and other objects which embody money. The uncertainty principle of quantum mechanics, attributed to Werner Heisenberg, may provide the ultimate key to making counterfeiting fundamentally impossible.

Quantum memories are a basic building block and necessary requirement for almost all quantum information technologies. As such they should be no harder to realize than the applications for which they are needed and hopefully more accessible than a full fledged quantum computer. The first part of my PhD thesis focuses on understanding the virtues and limitations of existing proposals to quantum memories and constructively extending such proposals. The realization of a qubit memory is in many ways analogous to the realization of a classical bit storage. It requires information to be distributed in such a way that it is not directly affected by ubiquitous sources of noise. In the case of quantum memory, this requirement also implies that intentional access will be logically more involved.

In order for a quantum memory to function when put in contact with its environment,    a mechanism which avoids the accumulation of errors must be present. This remains true, even if one aims at constructing topological quantum memories. For early proposals, this role was taken by periodic error correction procedures which “clean” the possible errors before they irrecoverably accumulate. In contrast, we advocate engineering the environment of the quantum memory to do this job. This approach shows promise, yet also raises many questions which need to be answered.

In contrast to the many-body topological approach to quantum memory, it is possible that sufficiently long memory times can be achieved by choosing to use a natural quantum degree of freedom which is very well isolated. In turn this isolation means that it will not be disturbed and only weakly affected by noise. We are faced with a common tradeoff between isolation and accessibility necessary for the system to be useful. Carbon-13 nuclear spins in diamond work quite nicely in this respect. They can be very well isolated from the environment yet allow access mediated by nitrogen-vacancy impurities. Our experimental research in this direction has led to record qubit storage times at room-temperature and adequate conditions for technological applications. Many techniques were combined in order to suppress the distinct sources of noise and ultimately extend the lifetime of the stored qubit to over a second.

Finally, a natural question that arises is what we would be able to technologically achieve if allowed access to good quality qubit memories. While the most common answer is to use these as a component for quantum computing, this seems to impose more requirements in terms of the coherent processing of information. Quantum money is a paragon application of a quantum memory. By exploiting the very nature of quantum information, it is possible to produce tokens which are impossible to counterfeit. This is the holy grail of the money printing press. Of course, by adopting quantum money, other problems arise. One of these is that decoherence leads to the degradation of the tokens and ultimately to their becoming unacceptable. My last project provides rigorous proof that a mild amount of decoherence can be tolerated while still excluding the possibility of counterfeiting. In other words, even if valid bill are subject to a moderate amount of wear and tear, it remains possible to distinguish them from illegitimate copies.

Wissenschaftlicher Werdegang
  • 2000-2008
  • Licenciado en Física (equivalent to Master in Physics) obtained at Universidad Nacional de Córdoba (UNC)
  • 2000-2005
  • Licenciado en Ciencias de la Computación (equivalent to Master in Computer Science) obtained at Universidad Nacional de Córdoba
  • 2008-2012
  • PhD in Physics (Summa Cum Laude) at the LMU Munich under the supervision of Prof. Ignacio Cirac. Dissertation titled “Quantum memory: design and applications”

Publikationen (Auszug)
  • * “Quantum memories based on engineered dissipation”, F. Pastawski, L. Clemente and J.I. Cirac, Physical Review A, 83(1), 012304. APS (2011).
  • * “Limitations of Passive Protection of Quantum Information” F. Pastawski, A. Kay, N. Schuch and J. I. Cirac, Quantum Information and Computation, 10(7&8), 0580-0618 (2010).
  • * “How Long Can a Quantum Memory Withstand Depolarizing Noise?”, F. Pastawski, A. Kay, N. Schuch and J. I. Cirac, Physical Review Letters 103, 080501-4 (2009).