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NQIT researchers demonstrate ‘hybrid’ logic gate as work towards quantum computer continues




NQIT researchers demonstrate ‘hybrid’ logic gate as work towards quantum computer continues

Since it began in 2014, the Networked Quantum Information Technologies Hub has been focusing on developing quantum technologies that could dwarf the processing power of today’s supercomputers and now a new paper by Oxford researchers, published in the journal Nature, demonstrates how the work of the Hub is progressing.

Professor David Lucas of Oxford’s Department of Physics, co-leader, with Professor Andrew Steane, of the ion trap quantum computing group, explains: ‘The development of a “quantum computer” is one of the outstanding technological challenges of the 21st century. A quantum computer is a machine that processes information according to the rules of quantum physics, which govern the behaviour of microscopic particles at the scale of atoms and smaller.

‘An important point is that it is not merely a different technology for computing in the same way our everyday computers work; it is at a very fundamental level a different way of processing information. It turns out that this quantum-mechanical way of manipulating information gives quantum computers the ability to solve certain problems far more efficiently than any conceivable conventional computer. One such problem is related to breaking secure codes, while another is searching large data sets. Quantum computers are naturally well-suited to simulating other quantum systems, which may help, for example, our understanding of complex molecules relevant to chemistry and biology.’

One of the leading technologies for building a quantum computer is trapped atomic ions, and a principal goal of the NQIT project is to develop the constituent elements of a quantum computer based on these ions.

Professor Lucas, of Balliol College, says: ‘Each trapped ion (a single atom, with one electron removed) is used to represent one “quantum bit” of information. The quantum states of the ions are controlled with laser pulses of precise frequency and duration. Two different species of ion are needed in the computer: one to store information (a “memory qubit”) and one to link different parts of the computer together via photons (an “interface qubit”).’

The Nature paper, whose lead author is Chris Ballance, a Junior Research Fellow at Magdalen College, demonstrates the all-important quantum ‘logic gate’ between two different species of ion – in this case two isotopes of calcium, the abundant isotope calcium-40 and the rare isotope calcium-43.

Professor Lucas says: ‘The Oxford team has previously shown that calcium-43 makes the best single-qubit memory ever demonstrated, across all physical systems, while the calcium-40 ion has a simpler structure which is well-suited for use as an “interface qubit”. The logic gate, which was first demonstrated for same-species ions at NIST Boulder (USA) in 2003, allows quantum information to be transferred from one qubit to another; in the present work, the qubits reside in the two different isotopes, stored in the same ion trap. The Oxford work was the first to demonstrate that this type of logic gate is possible with the demanding precision necessary to build a quantum computer.

‘In a nice piece of “spin-off science” from this technological achievement, we were able to perform a “Bell test”, by first using the high-precision logic gate to generate an entangled state of the two different-species ions, then manipulating and measuring them independently. This is a test which probes the non-local nature of quantum mechanics; that is, the fact that an entangled state of two separated particles has properties that cannot be mimicked by a classical system. This was the first time such a test had been performed on two different species of atom separated by many times the atomic size.’

While Professor Lucas cautions that the so-called ‘locality loophole’ is still present in this experiment, there is no doubt the work is an important contribution to the growing body of research exploring the physics of entanglement. He says: ‘The significance of the work for trapped-ion quantum computing is that we show that quantum logic gates between different isotopic species are possible, can be driven by a relatively simple laser system, and can work with precision beyond the so-called “fault-tolerant threshold” precision of approximately 99% – the precision necessary to implement the techniques of quantum error correction, without which a quantum computer of useful size cannot be built.’

In the long term, it is likely that different atomic elements will be required, rather than different isotopes. In closely related work published in the same issue of Nature, by Ting Rei Tan et al, the NIST Ion Storage group has demonstrated a different type of quantum logic gate using ions of two different elements (beryllium and magnesium).

The Oxford experiments were designed and carried out by graduate students Chris Ballance and Vera Schäfer, following a proposal of Jonathan Home and David Lucas, using an apparatus developed by all members of the research team.

See also Chris Ballance’s DPhil thesis, available online.

This article originally appeared on the Oxford University Science blog – read the original here.


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Hub job opportunity!




University of Cambridge

The Department of Engineering, University of Cambridge, seeks to appoint a Research Associate to work on Quantum Communications as part of the Quantum Communications Hub, until 30 November 2022, extendable for another 2 years.

The post holder will be located in the Electrical Engineering Building on the West Cambridge Site, Cambridge, UK.

The key responsibilities and duties are to maintain the network and introduce new systems for trial. This will involve design, construction and assessment of sub-systems. Examples of tests are the hybrid Continuous Variable (CV) QKD system, the new Quantum Alarm, and options for carrying out signal processing using CV techniques. Preference will be given to candidates with demonstrated quantum or photonic communications experimental aptitude in relevant areas of research and an ability to work within a team. Experience of DSP/FPGA programming would be an advantage.

The qualifications required to perform the role are to have obtained a PhD in Electronic Engineering, Physics, Applied Maths, Computer Science, or a related discipline. A good publication record would be an advantage.

Salary Ranges: Research Associate: £32,816 – £40,322

Fixed-term: The funds for this post are available until 30 November 2022 in the first instance.

For more information regarding this position follow this link.


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Quantum Algorithms for Simulating the Lattice Schwinger Model




Alexander F. Shaw1,5, Pavel Lougovski1, Jesse R. Stryker2, and Nathan Wiebe3,4

1Quantum Information Science Group, Computational Sciences and Engineering Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S.A.
2Institute for Nuclear Theory, University of Washington, Seattle, WA 98195-1550, U.S.A.
3Department of Physics, University of Washington, Seattle, WA 98195, U.S.A.
4Pacific Northwest National Laboratory, Richland, WA 99354, U.S.A.
5Department of Physics, University of Maryland, College Park, Maryland 20742, U.S.A.

Find this paper interesting or want to discuss? Scite or leave a comment on SciRate.


The Schwinger model (quantum electrodynamics in 1+1 dimensions) is a testbed for the study of quantum gauge field theories. We give scalable, explicit digital quantum algorithms to simulate the lattice Schwinger model in both NISQ and fault-tolerant settings. In particular, we perform a tight analysis of low-order Trotter formula simulations of the Schwinger model, using recently derived commutator bounds, and give upper bounds on the resources needed for simulations in both scenarios. In lattice units, we find a Schwinger model on $N/2$ physical sites with coupling constant $x^{-1/2}$ and electric field cutoff $x^{-1/2}Lambda$ can be simulated on a quantum computer for time $2xT$ using a number of $T$-gates or CNOTs in $widetilde{O}( N^{3/2} T^{3/2} sqrt{x} Lambda )$ for fixed operator error. This scaling with the truncation $Lambda$ is better than that expected from algorithms such as qubitization or QDRIFT. Furthermore, we give scalable measurement schemes and algorithms to estimate observables which we cost in both the NISQ and fault-tolerant settings by assuming a simple target observable–the mean pair density. Finally, we bound the root-mean-square error in estimating this observable via simulation as a function of the diamond distance between the ideal and actual CNOT channels. This work provides a rigorous analysis of simulating the Schwinger model, while also providing benchmarks against which subsequent simulation algorithms can be tested.

► BibTeX data

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Cited by

[1] Indrakshi Raychowdhury and Jesse R. Stryker, “Solving Gauss’s Law on Digital Quantum Computers with Loop-String-Hadron Digitization”, arXiv:1812.07554.

[2] Christopher David White, ChunJun Cao, and Brian Swingle, “Conformal field theories are magical”, arXiv:2007.01303.

[3] Anthony Ciavarella, “An Algorithm for Quantum Computation of Particle Decays”, arXiv:2007.04447.

[4] Minh C. Tran, Yuan Su, Daniel Carney, and Jacob M. Taylor, “Faster Digital Quantum Simulation by Symmetry Protection”, arXiv:2006.16248.

The above citations are from SAO/NASA ADS (last updated successfully 2020-08-12 00:44:15). The list may be incomplete as not all publishers provide suitable and complete citation data.

On Crossref’s cited-by service no data on citing works was found (last attempt 2020-08-12 00:44:14).


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Mapping graph state orbits under local complementation




Jeremy C. Adcock1, Sam Morley-Short1, Axel Dahlberg2, and Joshua W. Silverstone1

1Quantum Engineering Technology (QET) Labs, H. H. Wills Physics Laboratory & Department of Electrical & Electronic Engineering, University of Bristol, Merchant Venturers Building, Woodland Road, Bristol BS8 1UB, UK
2QuTech – TU Delft, Lorentzweg 1, 2628CJ Delft, The Netherlands

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Graph states, and the entanglement they posses, are central to modern quantum computing and communications architectures. Local complementation – the graph operation that links all local-Clifford equivalent graph states – allows us to classify all stabiliser states by their entanglement. Here, we study the structure of the orbits generated by local complementation, mapping them up to 9 qubits and revealing a rich hidden structure. We provide programs to compute these orbits, along with our data for each of the $587$ orbits up to $9$ qubits and a means to visualise them. We find direct links between the connectivity of certain orbits with the entanglement properties of their component graph states. Furthermore, we observe the correlations between graph-theoretical orbit properties, such as diameter and colourability, with Schmidt measure and preparation complexity and suggest potential applications. It is well known that graph theory and quantum entanglement have strong interplay – our exploration deepens this relationship, providing new tools with which to probe the nature of entanglement.

Graph states are ubiquitous representations of entanglement in quantum information science, and classify the most studied set of quantum states—clifford states—by the entanglement they possess.

However, many graph states are locally equivalent to one another, that is, they possess the same type of entanglement. Graph states which are locally equivalent can be transformed into one another by successive applications of the graph operation local complementation (example shown above). Using this operation, we can analyse only graph structure of the state, which is much simpler than analysing the exponentially large quantum state vector. This equivalence of graph states has been studied previously, with all graph states up to 12 qubits classified.

However, local complementation gives us more than sets of locally equivalent graphs: it also gives us an orbit (example shown above) which tells us how different graphs are related via local complementation. In this work we study these orbits, and relate their properties to properties of the entangled quantum states they contain. We find that orbit properties, such as colourability, correlate with entanglement properties, such as schmidt measure, and discuss applications of local complementation in quantum technology.

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